CD-Recordable FAQ - Section 2
- [2-1] How is the information physically stored?
- [2-2] What is XA? CDPLUS? CD-i? MODE1 vs MODE2? Red/yellow/blue book?
- [2-3] How do I know what format a disc is in?
- [2-4] How does copy protection work?
- [2-2] What is XA? CDPLUS? CD-i? MODE1 vs MODE2? Red/yellow/blue book?
- [2-4-1] ...on a data CD-ROM?
- [2-4-2] ...on an audio CD?
- [2-4-3] ...on an audio CD (Macrovision - SafeAudio)
- [2-4-4] ...on an audio CD (SunnComm - MediaCloQ)
- [2-4-5] ...on an audio CD (Midbar Tech - Cactus Data Shield)
- [2-4-6] ...on an audio CD (Key2Audio / Sony DADC)
- [2-4-7] ...on an audio CD (BayView Systems - Duolizer)
- [2-4-8] ...on an audio CD (Sanyo)
- [2-4-9] How does the Doc-Witness OpSecure CD-ROM work?
- [2-4-2] ...on an audio CD?
- [2-5] What's a multisession disc?
- [2-6] What are subcode channels?
- [2-7] Are the CD Identifier fields widely used?
- [2-8] How long does it take to burn a CD-R?
- [2-9] What's the difference between disc-at-once and track-at-once?
- [2-10] Differences between recording from an image and on-the-fly?
- [2-11] How does an audio CD player know to skip data tracks?
- [2-12] How does CD-RW compare to CD-R?
- [2-13] Can DVD players read CD-Rs?
- [2-14] Should I buy a DVD recorder instead?
- [2-15] What are "jitter" and "jitter correction"?
- [2-16] Where can I learn more about the history of CD and CD-R?
- [2-17] Why don't audio CDs use error correction?
- [2-18] How does CD-R compare to MiniDisc?
- [2-19] What does finalizing (and closing and fixating) do?
- [2-20] How are WAV/AIFF files converted into Red Book CD audio?
- [2-21] What does MultiRead mean? MultiPlay?
- [2-22] If recording fails, is the disc usable?
- [2-23] Why do recorders insert "00" bytes at the start of audio tracks?
- [2-24] How many tracks can I have? How many files?
- [2-25] Will SCMS prevent me from making copies?
- [2-26] Is a serial number placed on the disc by the recorder?
- [2-27] What's a TOC? How does it differ from a directory?
- [2-28] What's an ISO? A CIF? BIN and CUE? .DAT?
- [2-29] Why was 74 minutes chosen as the standard length?
- [2-30] Why is there a visibly unwritten strip near the CD-R hub?
- [2-31] What is "BURN-Proof"? "JustLink"? "Waste-Proof"?
- [2-32] Can playing CD-Rs in a DVD player hurt the discs?
- [2-33] Who *really* made this CD-R blank?
- [2-34] Can I make copies of DTS-encoded CDs?
- [2-35] Why 44.1KHz? Why not 48KHz?
- [2-36] What format are .CDA files in?
- [2-37] What are DD-R and DD-RW?
- [2-38] What's an ATIP?
- [2-39] What are "ML" discs and devices?
- [2-40] What's CD-MRW? Mount Rainier?
- [2-41] What's Audio Master Quality (AMQ) recording?
- [2-42] Can I draw pictures on a disc with the recording laser?
- [2-43] What are the gory details about how are 1s and 0s encoded?
- [2-6] What are subcode channels?
Subject:  CD Encoding
Subject: [2-1] How is the information physically stored?
From _The Compact Disc Handbook, 2nd edition_ by Ken Pohlmann, 1992 (ISBN 0-89579-300-8):
"Write-once media is manufactured similarly to conventional playback-only discs. As with regular CDs, they employ a polycarbonate substrate, a reflective layer, and a protective top layer. Sandwiched between the substrate and reflective layer, however, is a recording layer composed of an organic dye. .... Unlike regular CDs, a pre-grooved spiral track is used to guide the recording laser along the spiral track; this greatly simplifies recorder hardware design and ensures disc compatibility." Yes, it's real gold in "green" and "gold" CDs, but if you hold a CD-R up to a light source you'll notice that it's thin enough to see through (the gold layer is between 50 and 100nm thick). Something to bear in mind is that the data is closest to the label side of the CD instead of the clear plastic side that the data is read from. If the CD-R doesn't have a hard top coating such as Kodak's "Infoguard", it's fairly easy to scratch the top surface and render the CD-R unusable.
A pressed CD has raised and lowered areas, referred to as "lands" and "pits", respectively. A laser in the CD recorder creates marks in the disc's dye layer that has the same reflective properties. The pattern of pits and lands on the disc encodes the information and allows it to be retrieved on an audio or computer CD player. See section (2-43) for specifics.
Discs are written from the inside of the disc to outside. On a CD-R you can verify this by looking at the disc after you've finished writing. The spiral track makes 22,188 revolutions around the CD, with roughly 600 track revolutions per millimeter as you move outward. If you "unwound" the spiral, it would be about 3.5 miles long.
The construction of a CD-RW is different:
[optional] labelSee the net references section for pointers to more data (especially http://www.cd-info.com/). You can find some nice drawings at http://www.nswc.navy.mil/cosip/nov97/cots1197-2.shtml and http://www.pctechguide.com/09cdr-rw.htm.
[optional] scratch-resistant and/or printable coating
Upper dielectric layer
Recording layer (the part that changes form)
Lower dielectric layer
Polycarbonate substrate (the clear plastic part)
Subject: [2-2] What is XA? CDPLUS? CD-i? MODE1 vs MODE2? and Red/yellow/blue book?
This is a very deep topic. Some of the links in section (8-1) introduced the relevant information, particularly http://www.cd-info.com/, http://www.cdpage.com/, and the alt.cd-rom FAQ pages.
A quick summary of standards and commonly used identifiers:
- Red Book
- physical format for audio CDs (a/k/a CD-DA)
- Yellow Book
- physical format for data CDs
- Green Book
- physical format for CD-i
- Orange Book
- physical format for recordable CDs
- Part I
- CD-MO (Magneto-Optical)
- Part II
- CD-WO (Write-Once; includes "hybrid" spec for PhotoCD)
- Part III
- CD-RW (ReWritable; originall called CD-E)
- White Book
- format for VideoCD (often written "VCD")
- Blue Book
- CD Extra (occasionally used to refer to LaserDisc format)
- CD Extra
- a two-session CD, 1st is CD-DA, 2nd is data (a/k/a CD Plus)
- standard 2048-byte Yellow Book sectors, with error correction
- 2336-byte sectors, usually used for CD-ROM/XA
- eXtended Architecture; CD-ROM/XA MODE-2 defines two forms:
- 2048 bytes of data, with error correction, for data
- 2324 bytes of data, no ecc, for audio/video
- file layout standard (evolved from High Sierra format)
- Rock Ridge
- extensions allowing long filenames and UNIX-style symlinks
- Sony's incremental packet-writing filesystem
- industry-standard incremental packet-writing filesystem
- Philips' std for encoding disc and track data on audio CDs
See http://www.licensing.philips.com/ if you want to buy copies of the standards. They're not cheap! You can download some of them from http://www.ecma.ch/. ECMA-119 describes ISO-9660, and ECMA-130 sounds a lot like "yellow book" if you say it slowly.
For SVCD, see http://www.iki.fi/znark/video/svcd/overview/. The discs are a modified White Book format, using a 2x player and variable bit rate MPEG-2 instead of MPEG-1 at 1x like VCD.
For HDCD, see http://www.hdcd.com/. The discs are in Red Book format, but the low bit of the audio has additional information encoded in it. They sound good on a standard CD player, and better on a HDCD player.
SACD isn't really a CD format. It can have a Red Book compliant layer that is read by standard CD players, but to get the high-fidelity benefits you need a special player.
Subject: [2-3] How do I know what format a disc is in?
You can usually tell by looking at the packaging and/or the disc itself:
- CD-DA discs will have a "Compact Disc Digital Audio" logo.
- CD+G discs will print "CD Graphics" (and perhaps even CD-EG "Extended Graphics").
- CD-I discs will have a "Compact Disc Interactive" logo.
- VideoCD discs will have a "Compact Disc Digital Video" logo and/or print "VideoCD".
- PhotoCD discs will most likely print "Kodak PhotoCD" on them.
- SVCD discs have a "Super Video CD" logo ("Super Video" under the standard CD logo). The discs use one of the standard CD-ROM formats.
- DVCD discs print "DVCD"?? [ can't find much info about DVCD ]
- HDCD (High Definition Compatible Digital) have an "HDCD" logo. See http://www.hdcd.com/. The discs appear to use the standard Red Book format.
- SACD (Super Audio Compact Disc) is relatively new. The discs can have two layers, one of which is in Red Book audio format, the other in a DVD-like format offering higher fidelity.
- DTS (Digital Theater Surround) CDs are just like normal CDs, but use DTS encoding instead of PCM. See (2-34).
There are a few references to Compact Disc MIDI, or CD-MIDI.
See (4-46) for some comments on High Speed CD-RW.
Subject: [2-4] How does copy protection work?
Copy protection (sometimes erroneously referred to as "copyright protection") is a feature of a product that increases the difficulty of making an exact duplicate. The goal is not to make it impossible to copy -- generally speaking, that can't be done -- but rather than discouraging "casual copying" of software and music.
The goal is *not* to conceal information from prying eyes; see section (3-19) for information on encrypting data on a CD-ROM.
A separate but related issue is "counterfeit protection", which the publisher wants to make it easy to detect mass-produced duplicates. An example is Microsoft's placement of holograms on the hubs of their CD-ROMs. There are full CD pressing plants dedicated to creating counterfeit software (the worst offender being mainland China), so this is a serious concern for the larger software houses.
Copy protection on CD-ROMs used to be rare. But as the popularity of CD recorders grew, so did the popularity of copy protection. A large percentage of games released in the past few years have been protected.
A more recent innovation is copy protection for audio CDs, inspired by the rise of MP3 trading on the Internet. It is more difficult to do because the protection must allow correct behavior on a CD player but altered playback when being read by a CD-ROM drive. The best that can be accomplished is to force the user to play the music in an analog format, and then re-digitize it, resulting in an imperfect reproduction.
The article at http://news.cnet.com/news/0-1005-201-7320279-0.html is a nice introduction to the issues.
Some people have questioned whether copy protection is legal or not. In some countries, it may not be. In the USA, the law allows "fair use" of copyright material, but does not require that the content provided make it easy for you to do so. So making a copy of a song for your own private use may be legal. But the law can’t require the publisher to make the material available in an unprotected format. Copy protection has been debated for many years -- some of the schemes employed on the Apple II were remarkably elaborate -- and has never been challenged on legal principle.
Look http://overclockers.com/tips907/ for an article about why "fair use" is a legal right rather than a constitutional right in the USA, and what that means to you. The article has some interesting quotes from the courts regarding the DMCA and DeCSS, notably this one: "We know of no authority for the proposition that fair use, as protected by the Copyright Act, much less the Constitution, guarantees copying by the optimum method or in the identical format of the original." In other words, arguing that "fair use" means the publisher must allow you to make a perfect digital copy (as opposed to a lower-quality digital or analog copy) is without merit.
The next sections discuss data and audio individually.
Subject: [2-4-1] ...on a data CD-ROM?
There are several approaches. An article with a good overview of some popular protection technologies can be found at http://www.tomshardware.com/storage/02q2/020617/index.html. Another source is the "CD Protections" articles on http://www.cdmediaworld.com/hardware/cdrom/cd_protections.shtml.
For anyone interested in protecting their own discs: don't bother. Copy protection, on the whole, does not work. If you have a major application, such as a game or CAD package, you may want to consider one of the commercially licensed schemes listed later, or (heaven forbid) the use of a dongle. In general, though, if the disc can be read, then the contents can be copied. If you don't want somebody to make a copy of your stuff, then you'd better encrypt it (3-19).
A simple and commonly seen technique is to increase the length of several files on the CD so that they appear to be hundreds of megabytes long. This can be accomplished by setting the file length in the disc image to be much larger than it really is. The file actually overlaps with many other files. So long as the application knows the true file length, the software will work fine. If the user tries to copy the files onto their hard drive or do a file-by-file disc copy, they will fail because the CD will appear to hold a few GB of data. (In practice this doesn't foil pirates, because they always do image copies. And, none of the standard software provides a way to create such discs.)
One possible implementation, given sufficient control over the reader and mastering software, is to write faulty data into the ECC portion of a data sector. Standard CD-ROM hardware will automatically correct the "errors", writing a different set of data onto the target disc. The reader then loads the entire sector as raw data, without doing error correction. If it can't find the original uncorrected data, it knows that it's reading a "corrected" duplicate. This is really only viable on systems like game consoles, where the drive mechanism and firmware are well defined. This can be defeated by doing "raw" reads.
A more sophisticated approach is to write special patterns of data to the disc. The stream of data that results, after EFM encoding, is difficult for some recorders to reproduce successfully because they don't choose correct values for the merging bits. This is often referred to on web sites as "writing regular EFM patterns" or "weak sectors". See section (2-43) for details on EFM.
A less sophisticated -- and no longer effective -- method is to press a silver CD with data out beyond a 74-minute CD can write. Copying the disc used to require hard-to-find CD-R blank discs, but now it's easy to use an over burned 80-minute disc (sections (3-8-1) and (3-8-3)).
The approach some PC software houses have taken is to use nonstandard gaps between audio tracks and leave index marks in unexpected places. These discs cannot be copied by most software, and it may be impossible to duplicate them on drives that don't support disc-at-once recording (see section (2-9)). With the right reader and software, this is not a problem at all.
A popular method is non-standard discs with a track which is shorter than 4 seconds. Most recording software will either refuse to copy a disc with such a track, or attempt to do so but fail. A protected application would check for the presence and size of the track in question. Some recorders may succeed, however, it isn't foolproof. (In one case, a recorder could write tracks that were slightly over three seconds, but refused to write tracks that lasted only one second. There may be a limit below which no recorder will write.) In such cases, the pirates need to remove the explicit check from the software itself.
Putting multiple data tracks interleaved with audio tracks on a CD will confuse some disc copiers. However, it's difficult to actually use the data on those additional tracks.
Sometimes the copy of a disc will have a different volume label. This usually happens with file-by-file copies, not disc image copies. So checking the disc name is marginally useful, but not very effective.
Modifying the TOC so that the disc appears to be larger than it really is will convince some copy programs that the source disc is too large.
Some of the fancier technologies use non-standard pit geometry that causes players to read the data differently on consecutive attempts. Sometimes the player sees a "1", sometimes a "0". If, when reading the track, the CD-ROM drive sees different data each time, the software knows that the disc is an original. A duplicate disc will return the same data reliably. (So will some CD-ROM drives... this technology is not perfect.)
Some programs will examine the disc to try to determine if it's a CD-R. This doesn't work on all readers, and it's possible to disguise discs, so this isn't very effective.
CloneCD (section (6-1-49)) can copy many copy protected discs without trouble, given the right combination of reader and writer. Its main feature is "raw" reads and writes, which not all drives support.
The Laserlok system from http://www.diskxpress.com/ claims to be able to prevent unauthorized disc duplication at a low cost. It can be copied by CloneCD.
An unrelated product called LaserLock, from MLS LaserLock International (http://www.laserlock.com/) has similar features. It can be copied by CloneCD.
TTR Technology's DiscGuard (http://www.ttr.co.il/ or http://www.ttrtech.com/ claims to be able to write a signature onto pressed CDs and CD-Rs that is detectable by all CD-ROM drives but isn't reproducible without special hardware. A program could use this for copy protection by checking for the presence of the signature, and refusing to run if it's not there.
Sony DADC is promoting a simliar product called Securom. Some information is at http://www.sonydadc.com/hotnews/secu_fra.htm.
Yet another variant is C-Dilla's SafeDisc. They were bought by Macrovision (http://www.macrovision.com/). Their more recent product, SafeDisc 2, was the first to feature "weak sectors".
Yet another variant is CD-Cops from Link Data Security (http://www.linkdata.com/).
Subject: [2-4-2] ...on an audio CD?
The challenge here is to create a disc that will play on a standard audio CD player but be difficult to copy or "rip" into an MP3. The techniques making headlines in mid-2001 were developed by Macrovision (2-4-3) and SunnComm (2-4-4).
The earliest form of audio CD copy protection was SCMS. This only works on recorders that support SCMS, specifically consumer-grade stand-alone audio CD recorders. "Professional" recorders, and recorders that attach to computers, do not support SCMS. See section (2-25).
Some CDs used a damaged TOC (Table of Contents -- see section (2-27)) that confused some CD-ROM drives and ripping software. More recent schemes attempt to modify the audio samples in ways that confuse CD-ROM drives into playing static. The next few sections describe these approaches in detail.
For a list of suspected copy-protected discs, and some tips on what you can do to let the industry know that the protection isn't appreciated, see http://www.fatchucks.com/corruptcds/.
Many forms of copy protection violate the CD-DA standard, and so the discs aren't allowed to use the official CD logo art. However, many CDs don't have the logo anywhere, so its absence doesn't prove anything.
A paper entitled "Evaluating New Copy-Prevention Techniques for Audio CDs" by J.A. Halderman (available only in PostScript format) can be found at http://crypto.stanford.edu/DRM2002/halderman_drm2002_pp.ps. The paper was submitted to the 2002 ACM Workshop on Digital Rights Management (http://crypto.stanford.edu/DRM2002/prog.html).
Incidentally, if you're convinced that record companies and artists are raking in huge piles of cash from every CD they sell, you might want to take a look at an Electronic Musician article that talks about where the money comes from and where it goes. See: http://industryclick.com/magazinearticle.asp?magazineid=33&releaseid=9554&magazinearticleid=132835&SiteID=15 (You may need to use IE; Netscape 4.7 for Linux couldn't view the site.)
Interesting figures: only about 16% of CDs sold make enough money for the publishers to break even. The ones that do make enough money have to pay for the rest. For the recording artists, only about 3% sell enough music to get any royalties. With figures like these, it's not surprising that the industry is taking steps to combat piracy.
For more news & commentary, see:
Subject: [2-4-3] ...on an audio CD (Macrovision - SafeAudio)
In the first part of the year 2000, TTR Technologies announced a product called MusicGuard (http://www.MusicGuard.com/) that claimed to prevent duplication of audio CDs. The product was withdrawn, but the technology has resurfaced in mid-2001 as a product called SafeAudio from Macrovision (http://www.macrovision.com/).
The basic idea is to create samples that sound like bursts of static, and scramble the ECC data around to make it look like an uncorrectable error. Audio CD players will interpolate the samples during playback, but CD-ROM drives doing digital audio extraction generally won't. The result is a disc that plays back correctly on a CD player, but won't "rip" or copy correctly on a CD-ROM drive.
Some relevant sites and news articles:
The digital path requires reading the "raw" audio samples off of the disc, possibly modifying the data (e.g. changing the byte ordering) into something appropriate for the sound card, and then writing them to the sound device. Until a few years ago, most CD-ROM drives did this very poorly, in part because the analog and digital data paths were logically distinct in the designers' minds. Audio CDs used the audio path, data CD-ROMs used the digital path, and while you *could* send audio over the digital path there didn't seem to be much value in doing so. (See section (2-15) for some additional notes.)
What Macrovision appears to be exploiting is the different handling of uncorrectable errors in audio samples on the digital path vs the analog path. When playing an audio CD in a CD player or CD-ROM drive, the analog path is used. This path deals with uncorrectable (E32) errors by examining the samples that come before and after the error, and interpolating between them. On a scratched-up CD, this means that, while you may not be hearing the exact samples that were originally recorded, you won't notice any glitches because they're smoothed over. This feature is definitely not something you'd want on a data CD-ROM -- interpolating pieces of your spreadsheet is not going to help you.
In most CD-ROM drives, reading an audio sector with digital audio extraction is handled the same way that reading a data sector is: uncorrectable errors are left alone. Instead of getting interpolated samples, you get to hear the original, scratched-up audio. This is why some CDs will play back just fine on your computer, but will come out all scratched up when you extract them with the same drive. The errors are there either way, but when using a desktop CD player the errors have been smoothed over by the logic in the analog output path.
Some drives may use interpolation during DAE at lower speeds. If so, it should be possible to "rip" a track from a copy-protected disc by reducing the extraction speed to 1x.
Some people have suggested that software could be used to perform the interpolation on extracted music, stripping out the bits that the music companies added in. The trouble with this approach is that, once the data has been extracted, the CIRC encoding is no longer visible. It may not be easy to tell where the glitches are. For example, it should be possible to create a low-level but rhythmic distortion that will be noticeable, annoying, and difficult to identify automatically.
(It's possible that any software specializing in defeating the copy protection would run afoul of the DMCA (Digital Millenium Copyright Act), and the authors subject to fines and criminal prosecution. Come to think of it, the preceeding discussion might be illegal. For more information about the DMCA, see http://www.eff.org/.)
How can you get a "clean" copy of a protected disc? There are four basic approaches, in order of least to most desirable:
(1) Record directly from the analog outputs of the drive, feeding the sound into a sound card or outboard A/D converter. Some fidelity will be lost when converting from digital to analog and back again, which is what the industry is counting on.
(2) It should be possible to play the disc on a CD player with an S/PDIF connector, and get error-interpolated digital output. If played into a digital sound card or a CD recorder with an S/PDIF input, it should be possible to capture an exact copy of the original. Of course, it has to be done at 1x, and the track breaks may have to be added manually, making it a potentially tedious affair. This might be avoidable on a CD-R "dubbing deck", but inexpensive models will add SCMS to the set of things to worry about. Don't forget that generation loss is possible with CDs, especially if you record from CD-Rs (due to their higher BLER rate), so copying to CD-R and then extracting from CD-R requires some care. See section (3-18).
(3) Some drives support an extension described in recent versions of the ATA/ATAPI and SCSI MMC specifications. This extension to the "READ CD" command returns a set of flags indicating which bytes in an audio block were not corrected at the C2 level (section (2-17). An audio extraction application with access to this information could do its own interpolation across errors. Some applications already make some use of this feature; see http://www.feurio.com/English/faq/faq_vocable_c2error.shtml. The "drive check" feature of cdspeed (section (6-2-11)) reports on whether or not a drive is capable of returning "C2 pointers".
(4) A CD-ROM drive with logic that interpolates uncorrectable errors during DAE would allow copying and ripping with no additional effort required.
The success or failure of audio CD copy protection hinges upon two factors: how effective is it at preventing "casual copying", and what sort of problems do the legitimate owners of audio CDs encounter when playing their discs? Macrovision claims that their "golden ear" listeners were not able to tell the difference, though the test might be biased if the folks with the shiny lobes were using high-end CD players that did an especially good job of concealing uncorrectable errors.
A legitimate technical concern is that the copy protection reduces the effectiveness of the error correction. Because some percentage of ECC is now required for proper playback on a *clean* disc, the odds of scratches and fingerprints causing audible degredation are increased. In practice, if the "static" samples are relatively few and far between, the difference would be statistically insignificant.
One last piece of advice: do not assume that any disc that doesn't extract cleanly is copy-protected. There have been many, many postings on message boards from people who think they have found a protected disc, or how some specific piece of software can defeat the protection. Start with the more common reasons: the disc is dirty, the disc was poorly made, your CD-ROM drive is not that great at audio extraction, you're using software that isn't the best. There are many reasons why ripping an audio track might fail. People have been having trouble getting clean audio for years. See section (3-3) for some advice if you're having trouble.
Certain web sites (notably cdfreaks.com) have been reporting that a replacement CDFS.VXD will fix everything. However, doing the audio extraction in a VXD instead of an EXE makes no difference. So far, none of the sites that have claimed victory list a single SafeAudio-protected disc that was copied, most likely because -- at the time this was written -- there weren't any discs known to use SafeAudio. (This phenomenon is not unheard-of; Sega's Dreamcast discs were widely reported to be copyable by a means that was quickly determined to be utterly ridiculous.) If the widely-touted CDFS.VXD is in fact a hijacked Plextor driver, then it may well use technique #3 mentioned above, but would only work on a drive that supported the extended READ CD feature.
Subject: [2-4-4] ...on an audio CD (SunnComm - MediaCloQ)
SunnComm (http://www.sunncomm.com/) has a product called "MediaCloQ". It was used to protect the album _A Tribute to Jim Reeves_ by Charley Pride in mid-2001. The results were inconclusive: clean versions of the tracks appeared on the net, but SunnComm claimed they came from an unprotected disc released on Australia. Their plan was to alleviate "fair use" concerns by allowing users to download MP3 versions of the songs after they registered the original. Some articles:
The idea behind this protection is to make it hard for CD-ROM drives to identify the disc as being an audio CD. The disc is multisession, and uses a hacked TOC, so track rippers and disc copiers have trouble dealing with it. SunnComm hasn't publicly stated any details.
In August 2001, SunnComm announced v2.0 of their product, but didn't provide specific details.
Some personal notes on SunnComm's protection of the Charley Pride disc, including the steps I took to get a clean copy:
The packaging is labeled with the SunnComm logo, and states, "This audio CD is protected by SunnComm(tm) MediaCloQ(tm) Ver 1.0. It is designed to play in standard audio CD players only and is not intended for use in DVD players." However, my DVD player was able to play the disc after overcoming some initial confusion.
The disc itself has an unusual construction. There is a heavy band at about the point where the music stops, and thin bands between tracks. These appear to be purely decorative (and, I'm told, increasingly common on non-protected discs). Some images are available on http://www.fadden.com/cdrpics/.
A computer running Win98SE with a Plextor 40max CD-ROM drive saw the disc as having two sessions and 16 data tracks. My CD player only saw 15 audio tracks. This feature alone makes the disc difficult to rip or copy, because the software doesn't see any audio tracks, and a CD-R copy would be full of tracks that even a CD player would see as data. Another machine, with a Plextor 12/20 and a slightly different set of software, seemed to have a lot of trouble figuring out what the disc was. It eventually sorted things out, but I get the sense the disc has been tweaked in ways that confuse the drive firmware.
I tried using "Session Selector" to select the first session and then access the tracks. This resulted in a Plextor 8/20 CD recorder becoming unusable until a reboot. I'd guess the firmware got confused.
The next thing I tried was to crank up CDRWIN v3.7a (section (6-1-7)), and extract some tracks using my Plextor 12/20. No dice -- the display showed 15 unselectable tracks and 1 MODE-2 data track.
Next, I tried the "Extract Disc/Tracks/Sectors" function, selected "Extract Sectors", chose "Audio-CDDA (2352)" for the data type, and entered a nice range (0 to 300000, where each audio sector is 1/75th of a second). This choked when trying to read starting at block 173394, so I tried again stopping at 173390. This resulted in a rather large WAV file, which I opened with Cool Edit -- revealing the entire contents of the disc, plain and clear. Playback revealed no audible defects.
I believe this worked because the sector extraction function ignores track and session boundaries, and just pulls the blocks straight off. Losing the track markers is annoying, but it's easy to add them back with something like CDWave (section (6-2-16)).
(FWIW, this same approach did *not* work for the _My Private War_ disc with the damaged TOC, described in (2-4-2). It would probably not be of help with a SafeAudio disc either.)
"zEEwEE" came up with a complicated but enlightening scheme for side-stepping the protection on discs with damaged second TOCs. It has the advantage of allowing you to use standard tools, such as Exact Audio Copy ( section (6-2-12)), which keeps the track breaks and can do fancy tricks to get the best extraction quality. See http://cdprot.cjb.net/. [ I'm told the disc used as an example was actually protected with Midbar Tech's Cactus Data Shield 100, not MediaCloQ. ] The method involves making the outer rim of the disc unreadable to the CD-ROM drive. It appears you can use dry erase markers instead of adhesive stickers for the procedure, which is good since an adhesive label might peel up and damage your drive. This method, first posted in August of 2001, resulted in a flurry of media attention in May of 2002.
Subject: [2-4-5] ...on an audio CD (Midbar Tech - Cactus Data Shield)
Midbar Tech Ltd (http://www.midbartech.com/) appears to have two different schemes under the "Cactus Data Shield" brand. (The web site shows three now: CDS100, CDS200, and CDS300.) The first uses a non-standard TOC. The position of the lead-out and the length of the last track were tweaked, resulting in a disc that appears to be only 28 seconds long. The alterations didn't confuse all CD-ROM drives, and it has been reported that some Philips CD players couldn't play the discs. BMG Entertainment reportedly tried it and abandoned it.
In late 2001, Midbar Tech announced a different approach. A US patent (http://www.delphion.com/details?&pn=US06208598__) describes the invention.
The approach appears to involve inserting frames of bogus control information into a relatively constant part of the CD audio stream. During playback, the extra frames are skipped. A disc copy or digital stream on an S/PDIF output will include the bogus frames, and when written to CD-R the extra control information won't be included. The result is bad samples that only appear in copies.
No specific disc titles have been announced, but Sony has reportedly released a few titles in eastern Europe that use this.
Some personal notes on the early version (CDS100?) of the Cactus Data Shield: I bought a copy of _My Private War_, by Phillip Boa & The Voodoo Club, from an online retailer. The disc is labeled "Kopiergesch¨¹tzte CD - nicht am pc abspielbar" which translates literally to "copy-protected CD - not at the PC playable". Supposedly this is one of the BMG discs that was protected with Midbar's first product.
The Plextor Plextools utility saw it as a single-session audio CD with 13 tracks, but when I asked it to play the disc it only saw the first 28 seconds of the first track, and stopped after playing just that much. My Panasonic CD "boom box" also thought the disc was only 28 seconds long, but it happily played past that point, and would let me select any track.
The page at http://uk.eurorights.org/issues/cd/docs/natimb.shtml has an analysis of the CD _White Lilies Island_ by Natalie Imbruglia.
http://www.cdrinfo.com/Sections/Articles/Specific.asp?ArticleHeadline=Cactus%20Data%20Shield%20200&index=0 has a very thorough examination of a CDS200 disc. Recommended reading.
Subject: [2-4-6] ...on an audio CD (Key2Audio / Sony DADC)
This was used to protect promotional copies of the Michael Jackson single "You Rock My World". See http://www.key2audio.com/ for product information.
Subject: [2-4-7] ...on an audio CD (BayView Systems - Duolizer)
The "Duolizer" system splits music into two pieces. The bulk of the music is on the CD, but a small but essential piece is streamed from a secure server over the Internet. The idea is to allow music publishers to distribute songs to the media and retail outlets ahead of scheduled releases. This was a response to songs appearing in MP3 form on the Internet before the CDs went into distribution.
See http://www.bayviewsystems.com/solutions/duolizer.htm for product info.
http://www.HighCriteria.com/). It will be reasonably effective for promotional copies of songs, though, where the goal is to prevent people from walking away with copies of the discs.
As an added bonus, because the music is streamed from a central location, it could have a digital watermark added. If (say) somebody at a radio station made an MP3 copy, it might be possible to trace the source of the MP3 file back to the source. There is nothing on the product pages to suggest that such a scheme is currently in place.
Subject: [2-4-8] ...on an audio CD (Sanyo)
Sany has joined the growing list of companies to announce CD copy protection. It's not clear if this is their own scheme or one licensed from another company.
Subject: [2-4-9] How does the Doc-Witness OpSecure CD-ROM work?
The disc has an embedded secure micro (like a smart card) that is activated when the laser light strikes a photodetector. The light is converted to electrical impulses, the impulses drive the chip, and if all goes well the results are presented to the drive via an embedded light-emitting diode.
Making an exact duplicate of the disc would be very difficult. It's unclear whether this technology actually makes it harder to get a working copy of the contents. The scheme seems to essentially be a combination of an "uncopyable" disc and a hardware dongle, both of which have been around for years (neither of which have brought an end to piracy).
The company's web site is http://www.doc-witness.com/.
Subject: [2-5] What's a multisession disc?
A session is a recorded segment that may contain one or more tracks of any type. The CD recorder doesn't have to write the entire session at once -- you can write a single track, and come back later and write another -- but the session must be "closed" before a standard audio CD or CD-ROM player will be able to use it. Additional sessions can be added until the *disc* is closed or there's no space left.
Multisession writing was first used on PhotoCD discs, to allow additional pictures to be appended. Today it's most often used with "linked" multisession discs, and occasionally for CD-Extra discs. These require a bit more explanation.
When you put a data CD into your CD-ROM drive, the OS finds the last closed session on the disc and reads the directory from it. (Well, that's how it's supposed to work. Depending on your operating system and CD-ROM drive, you may get different results.) If the CD is ISO-9660 format - which it almost certainly is unless it's a Macintosh CD written in HFS - the directory entries can point at any file on the CD, no matter which session it was written in.
Most of the popular CD creation programs allow you to "link" one or more earlier sessions to the session currently being burned. This allows the files from the previous sessions to appear in the last session without taking up any additional space on the CD (except for the directory entry). You can also "remove" or "replace" files, by putting a newer version into the last session, and not including a link to the older version.
In contrast, when you put an audio CD into a typical CD player, it only looks at the first session. For this reason, multisession writes don't work for audio CDs, but as it happens this limitation can be turned into an advantage. See section (3-14) for details. This limitation does *not* mean you have to write an entire audio CD all at once; see section (2-9) for an overview of track-at-once writing.
(Some audio CD players do seem to be able to recognize all of the tracks on a multisession audio disc. Most do not. The only way to know for sure is to try and see. If you are planning to give an audio CD you create to others, it would be wise to write it in a single session.)
Note that mixing MODE-1 (CD-ROM) and MODE-2 (CD-ROM/XA) sessions on a single disc isn't allowed. You could create such a thing, but many CD-ROM drives will have a hard time recognizing it.
See also http://www.roxio.com/en/support/cdr/multisession.html, which goes into more depth.
Discs written with packets are an entirely different story. See section (6-3).
Quick recap: if you want to write some data to a CD-ROM now, and some more later, you write a single data track in multiple sessions (or with packet writing). If you want to write some audio tracks to a CD now, and some more later, you write multiple audio tracks in a single session.
Subject: [2-6] What are subcode channels?
There are eight subcode channels (P,Q,R,S,T,U,V,W). The exact method of encoding is discussed in section (2-43), but it's really only important to note the data is distributed uniformly across the entire CD, and each channel can hold a total of about 4MB.
The P subcode channel identifies the start of a track, but is usually ignored in favor of the Q channel.
The Q subcode channel includes useful information, which can be read and written on many recorders. The user data area contains three types of subcode-Q data: position information, media catalog number (MCN), and ISRC code. Other forms are found in the lead-in, and are used to enable multisession and describe the disc TOC (table of contents).
The position information is used by audio CD players to display the current time, and has track/index information. This can be controlled when doing Disc-At-Once recording.
The ISRC (International Standard Recording Code) is used by the recording industry. It states the country of origin, owner, year of issue, and serial number of tracks, and may be different for each track. It's optional; many CDs don't use this. The media catalog number is similar, but is constant per disc. Note these are different from the UPC codes.
See http://www.ee.washington.edu/conselec/CE/reports/Group.1/matt_page_individual/subcode.html for some details on P and Q.
The R-W subcode channels are used for text and graphics in certain applications, such as CD+G (CD w/graphics, supported by SegaCD among others). A new use has been devised by Philips, called ITTS. It enables properly equipped players to display text and graphics on Red Book audio discs. The most recent result of this technology is "CD-Text", which provides a way to embed disc and track data on a standard audio CD.
Subject: [2-7] Are the CD Identifier fields widely used?
At present, not many manufacturers use them, and not all devices can read all of the fields.
Programs that identify audio CDs automatically compute an ID based on the quantity and positions of the audio tracks, measured down to 1/75th of a second. http://www.cddb.com/ has a collection of CD information.
Subject: [2-8] How long does it take to burn a CD-R?
It depends on how much data you're going to burn, and how fast your drive is. Burning 650MB of data takes about 74 minutes at 1x, 37 minutes at 2x, and 19 minutes at 4x, but you have to add a minute or two for "finalizing" the disc. Remember that single speed for CD-ROMs is 150KB/sec, double speed is 300KB/sec, and so on.
If you have half the data, it will finish in (about) half the time. If you record the same thing twice as fast, it will finish in (about) half the time.
Most CD recording speeds are linear, i.e. recording at 12x is twice as fast as recording at 6x. If the drive uses a PCAV mechanism (see section (5-22)) the speed varies depending on which part of the disc you're recording. If a "20x" drive uses PCAV to get 12x at the start of the disc and 20x near the outside, you know that burning 60 minutes of audio will take somewhere between about 5 minutes and about 3 minutes.
Subject: [2-9] What's the difference between disc-at-once and track-at-once?
There are two basic ways of writing to a CD-R. Disc-at-once (DAO) writes the entire CD in one pass, possibly writing multiple tracks. The entire burn must complete without interruption, and no further information may be added.
Track-at-once (TAO) allows the writes to be done in multiple passes. There is a minimum track length of 300 blocks (600K for typical data CDs), and a maximum of 99 tracks per disc, as well as a slight additional overhead associated with stopping and restarting the laser.
Because the laser is turned off and on for every track, the recorder leaves a couple of blocks between tracks, called run-out and run-in blocks. If done correctly, the blocks will be silent and usually unnoticeable. CDs with tracks that run together will have a barely noticeable "hiccup". Some combinations of software and hardware may leave junk in the gap, resulting in a slight but annoying click between tracks. Some drives and/or software packages may not let you control the size of the gap between audio tracks when recording in track-at-once mode, leaving you with 2-second gaps even if the original didn't have them.
Many recorders, starting with the venerable Philips CDD2000, allow "session-at-once" (SAO) recording. This gives you disc-at-once control over the gaps between tracks, but allows you to leave the disc open. This can be handy when writing CD Extra discs (see section (3-14)).
There are some cases where disc-at-once recording is required. For example, it may be difficult or impossible to make identical backup copies of some kinds of discs without using disc-at-once mode (e.g. copy-protected PC games). Also, some CD mastering plants may not accept discs recorded in track-at-once mode, because the gaps between tracks will show up as uncorrectable errors.
The bottom line is that disc-at-once recording gives you more control over disc creation, especially for audio CDs, but isn't always appropriate or necessary. It's a good idea to get a recorder that supports both disc-at-once and track-at-once recording.
For further commentary, see http://www.yangkun.com/CDR/redbook.shtml.
Subject: [2-10] Differences between recording from an image and on-the-fly?
Many CD-R creation packages will give you a choice between creating a complete image of the CD on disk and doing what's called "on-the-fly" writing. Each method has its advantages.
Disc image files are sometimes called virtual CDs or VCDs (not to be confused with VideoCD). These are complete copies of the data as it will appear on the CD, and so require that you have enough hard drive space to hold the complete CD. This could be as much as 650MB for CD-ROM or 747MB for an audio disc when using 74-minute blank discs. If you have both audio and data tracks on your CD, there would be an ISO-9660 filesystem image for the data track and one or more 16-bit 44.1KHz stereo sound images for the audio tracks.
(On the Mac, you might instead use an HFS filesystem for the data track. You can create the image with Mac CD recording software, or create it as a DiskCopy image file and then burn the data fork under a different OS.)
On-the-fly recording often uses a "virtual image", in which the complete set of files is examined and laid out, but only the file characteristics are stored, not the data. The contents of the files are read while the CD is being written. This method requires less available hard drive space and may save time, but increases the risk of buffer underruns (see (4-1)). With most software this also gives greater flexibility, since it's easier to add, remove, and shuffle files in a virtual image than a physical one.
A CD created from an image file would be identical to one created with on-the-fly recording, assuming that both would put the same files in the same places. The choice of which to use depends on user preference and hardware capability.
Subject: [2-11] How does an audio CD player know to skip data tracks?
There are subcode flags in the Q channel for each track:
- If set, the track contains data; if not, the track contains audio.
- Digital Copy Permitted
- Used by SCMS. Set to allow copies, clear to prevent them.
- Four-Channel Audio
- The Red Book standard allows four-channel audio, though very few discs have ever been made that use it.
- Set if the audio was recorded with pre-emphasis.
Subject: [2-12] How does CD-RW compare to CD-R?
CD-RW is short for CD-Rewritable. It used to be called CD-Erasable (CD-E), but some marketing folks changed it so it wouldn't sound like your important data gets erased on a whim. The difference between CD-RW and CD-R is that CD-RW discs can be erased and rewritten, while CD-R discs are write-once. Other than that, they are used just like CD-R discs.
Let me emphasize that: they are used just like CD-R discs. You can use packet writing on both CD-R and CD-RW, and you can use disc-at-once audio recording on both CD-R and CD-RW. Some software may handle CD-RW in a slightly different way, because you can do things like erase individual files, but the recorder technology is nearly identical.
CD-RW drives use phase-change technology. Instead of creating "bubbles" and deformations in the recording dye layer, the state of material in the recording layer changes from crystalline to amorphous form. The different states have different refractive indicies, and so can be optically distinguished.
These discs are not writable by standard CD-R drives, nor readable by most older CD readers (the reflectivity of CD-RW is far below CD and CD-R, so an Automatic Gain Control circuit is needed to compensate). Most new CD-ROM drives do support CD-RW media, but not all them will read CD-RW discs at full speed.
A few older audio CD players and many new ones can handle CD-RW discs, but many can't. If you want to create audio CDs on CD-RW media, make sure that your player can handle them.
All CD-RW recorders can write to CD-R media, so the only reason not to buy a CD-RW recorder is price. Some Internet sites like to put the devices in completely separate categories, calling them "CD recorders" and "CD ReWriters", but the differences between them don't really merit such a distinction. Think of a "CD ReWriter" as a CD recorder that can also make use of CD-RW media.
Oddly enough, it may be easier for a DVD drive to read CD-RW discs than CD-R discs, because of the way the media is constructed.
CD-RW media is more expensive than CD-R, but recent price reductions have narrowed the gap considerably. There is a limit to the number of times an area of the disc can be rewritten, but that number is relatively high (the Orange Book requires 1000, but some manufacturers have claimed as much as 100,000). Of course, this is under laboratory conditions. If you don't handle the disc carefully, you will add scratches, dirt, fingerprints, and other obstacles that make the disc harder for the drive to read.
It appears that CD-RW discs have speed ratings encoded on them, so discs that are only certified for 2x recording can't be written to at 4x (or, for that matter, 1x). To make things more complicated, different media is required for high-speed CD-RW drives (those that exceed 4x). See http://www.emediapro.net/EM2000/writer11.html for an explanation.
If you're trying to decide if you want a drive that supports CD-RW, see section (5-16).
Subject: [2-13] Can DVD players read CD-Rs?
The only discs that a DVD player is guaranteed to read are DVD discs. Support for CD-ROM, CD-R, and CD-RW may be included, but is by no means guaranteed.
CD-R was designed to be read by an infrared 780nm laser. DVD uses a visible red 635nm or 650nm laser, which aren't reflected sufficiently by the organic dye polymers used in CD-R media. As a result, many DVD players can't read CD-R media. Some DVD players come with two lasers so that they can read CD-R. For a technical discussion, see http://www2.osta.org/osta/html/cddvd/intro.html and http://www.emedialive.com/EM1998/bennett3.html.
CD-RW discs have a different formulation, and may work even on players that can't handle CD-R media. If CD-R media doesn't work, try copying the disc to CD-RW instead (assuming your recorder supports CD-RW).
Some DVD-ROM drives may be unable to read multisession discs. In general, though, DVD-ROM drives (as opposed to DVD players) are able to read CD-R media.
If the box doesn't say that something is supported, assume that the feature isn't. Look for the MultiRead or MultiPlay logos, which indicate that the DVD player or DVD-ROM drive can read CD-R and CD-RW.
See also "Is XXX compatible with DVD" in the DVD FAQ:
Subject: [2-14] Should I buy a DVD recorder instead?
Perhaps, but it's best if you can get a "combo" drive that records on CDs as well.
CDs are starting to pass the venerable 3.5" floppy disk as the most universal physical media. If you want to be able to exchange music or data with someone else, CD and CD-ROM are your best bet. DVD-ROM drives and DVD players haven't been as successful as some in the industry had hoped. Near the end of 2000, one of the major computer sellers was offering an "upgrade" on their systems from DVD-ROM drives to CD recorders.
DVD-R recorders and media are still fairly expensive compared to CD-R, though they're finally down to consumer levels. An example: http://www.electroweb.com/product/hard.htm was, as of early February '98, selling a Pioneer CDVR-S101 DVD-Recordable Drive for US$18K. In June '99, the same site had a Pioneer CDVR-S201 for US$5100. In October 2001 the Pioneer DVR-A03PK was on sale for $699, and the price of media had fallen from $50 to $15 per disc.
In mid-2001 Apple started selling a drive with high-end Macintoshes that wrote to both CD-R and DVD-R. If you can afford it, being able to write either format is valuable.
Writers for related formats like DVD-RAM and DVD+RW are available for less, but aren't widely compatible with current DVD players. HP and several other companies are promoting the DVD+RW format, which is compatible with DVD players and is rewritable. See http://www.dvdplusrw.org/.
Subject: [2-15] What are "jitter" and "jitter correction"?
The first thing to know is that there are two kinds of jitter that relate to audio CDs. The usual meaning of "jitter" refers to a time-base error when digital samples are converted back to an analog signal; see http://www.digido.com/jitteressay.html for a discussion. The other form of "jitter" is used in the context of digital audio extraction from CDs. This kind of "jitter" causes extracted audio samples to be doubled-up or skipped entirely. (Some people will correctly point out that the latter usage is an abuse of the term "jitter", but we seem to be stuck with it.)
"Jitter correction", in both senses of the word, is the process of compensating for jitter and restoring the audio to its intended form. This section is concerned with the (incorrect use of) "jitter" in the context of digital audio extraction.
The problem occurs because the Philips CD specification doesn't require block-accurate addressing. While the audio data is being fed into a buffer (a FIFO whose high- and low-water marks control the spindle speed), the address information for audio blocks is pulled out of the subcode channel and fed into a different part of the controller. Because the data and address information are disconnected, the CD player is unable to identify the exact start of each block. The inaccuracy is small, but if the system doing the extraction has to stop, write data to disk, and then go back to where it left off, it won't be able to seek to the exact same position. As a result, the extraction process will restart a few samples early or late, resulting in doubled or omitted samples. These glitches often sound like tiny repeating clicks during playback.
On a CD-ROM, the blocks have a 12-byte sync pattern in the header, as well as a copy of the block's address. It's possible to identify the start of a block and get the block's address by watching the data FIFO alone. This is why it's so much easier to pull single blocks off of a CD-ROM.
With most CD-ROM drives that support digital audio extraction, you can get jitter-free audio by using a program that extracts the entire track all at once. The problem with this method is that if the hard drive being written to can't keep up, some of the samples will be dropped. (This is similar to a CD-R buffer underrun, but since the output buffer used during DAE is much smaller than a CD-R's input buffer, the problem is magnified.)
Most newer drives (as well as nearly every model Plextor ever made) are based on an architecture that enables them to accurately detect the start of a block.
An approach that has produced good results is to do jitter correction in software. This involves performing overlapping reads, and then sliding the data around to find overlaps at the edges. Most DAE programs will perform jitter correction.
Subject: [2-16] Where can I learn more about the history of CD and CD-R?
Some information about "the goode olde days" can be found in Robert Starrett's "The History of CD-R" article, currently available from http://www.roxio.com/en/support/cdr/historycdr.html.
The first CD player was available in Japanese stores on October 1, 1982. CD-Recordable technology wasn't introduced until 1988. For a timeline, see http://www.oneoffcd.com/info/historycd.cfm.
Back in the late 1980s, CD recorders cost thousands of dollars, and were part of systems the size of a washing machine. Disks cost US$100.00 each.
Things started to get better in 1995, when Yamaha released the CDR100 (the first 4x recorder) for a mere US$5000.00. In September of 1995, HP released the 4020i (a 2x recorder based on the Philips CDD2000) for just under US$1000.00. Media was down to about US$8.00, though 80-minute discs were extremely rare and expensive (US$40.00 each, if you could find them at all).
Subject: [2-17] Why don't audio CDs use error correction?
Actually, they do. It is true that audio CDs use all 2352 bytes per block for sound samples, while CD-ROMs use only 2048 bytes per block, with most of the rest going to ECC (Error Correcting Code) data. The error correction that keeps your CDs sounding the way they're supposed to, even when scratched or dirty, is applied at a lower level. So while there isn't as much protection on an audio CD as there is on a CD-ROM, there's still enough to provide perfect or near-perfect sound quality under adverse conditions.
All of the data written to a CD uses CIRC (Cross-Interleaved Reed-Solomon Code) encoding. Every CD has two layers of error correction, called C1 and C2. C1 corrects bit errors at the lowest level, C2 applies to bytes in a frame (24 bytes per frame, 98 frames per sector). In addition, the data is interleaved and spread over a large arc. (This is why you should always clean CDs from the center out, not in a circular motion. A circular scratch causes multiple errors within a frame, while a radial scratch distributes the errors across multiple frames.)
If there are too many errors, the CD player will interpolate samples to get a reasonable value. This way you don't get nasty clicks and pops in your music, even if the CD is dirty and the errors are uncorrectable. Interpolating adjacent data bytes on a CD-ROM wouldn't work very well, so the data is returned without the interpolation. The second level of ECC and EDC (Error Detection Codes) works to make sure your CD-ROM stays readable with even more errors.
It should be noted that not all CD players are created equal. There are different strategies for decoding CIRC, some better than others.
Some CD-ROM drives can report the number of uncorrected C2 errors back to the application. This allows an audio extraction application to guarantee that the extracted audio matches the original. The Plextor UltraPlex 40 is one such drive.
See http://www.cdpage.com/dstuff/BobDana296.html for an overview of error correction from the perspective of media testing. If you really want to get into the gory technical details, try http://www.ee.washington.edu/conselec/CE/kuhn/cdmulti/95x7/iec908.htm.
Subject: [2-18] How does CD-R compare to MiniDisc?
MiniDiscs, or MDs, are small (64mm) discs that hold about 140MB of data or 160MB of audio. By using sophisticated compression techniques they are able to compress audio by a 5:1 ratio, allowing a capacity of 74 minutes with little or no audible difference in quality. As with CD recorders, there are MD recorders that connect to your computer and MD recorders that connect to your stereo.
There are stamped MDs that are similar to CDs in construction, and rewritable MDs that use magneto-optical technology. Audio MD recorders are generally more convenient than stand-alone audio CD recorders, because the playback mechanism allows a more flexible layout of audio data, so it's possible to delete a track from the middle of the MD and then write a longer one that is recorded in different places across the disc. The current generation of MD technology is unlikely to replace CD-R or DAT, however, because the lossy compression employed is disdained by audio purists. MD is more often positioned as a replacement for analog cassette tape, which it matches in portability and recordability, and surpasses in durability and its ability to perform random accesses.
Computer-based MD recorders can write data, but may not be able to record audio. Check the specifications carefully.
A wealth of information is available from http://www.minidisc.org/. If you want to transfer CD to MD or MD to CD-R, check there for more information. (It used to be item #37 in the FAQ, but doesn't seem to be now.)
Subject: [2-19] What does finalizing (and closing and fixating) do?
A disc that you can add data to is "open". All data is written into the current session. When you have finished writing, you close the session. If you want to make a multisession disc, you open a new session at the same time. If you don't open a new session then, you can't open one later, which means that it's impossible to add more data to the CD-R. The entire disc is considered "closed".
The process of changing a session from "open" to "closed" is called "finalizing", "fixating", or just plain "closing" the session. When you close the last session, you have finalized, fixated, or closed the disc.
A single-session disc has three basic regions: the lead-in, which has the Table of Contents (or TOC); the program area, with the data and/or audio tracks; and the lead-out, which is filled with zeroes and provides padding at the end of the disc. An "open" single-session disc doesn't yet have the lead-in or lead-out written.
If you write data to a disc and leave the session open, the TOC -- which tells the CD player or CD-ROM drive where the tracks are -- is written into a separate area called the Program Memory Area, or PMA. CD recorders are the only devices that know to look at the PMA, which is why you can't see data in an open session on a standard playback device. CD players won't find any audio tracks, and CD-ROM drives won't see a data track. When the session is finalized, the TOC is written in the lead-in area, enabling other devices to recognize the disc.
(Something to try: write an audio track to a blank CD, and leave the session open. Put the disc in a CD player. Some players will deny the existence of the disc, some will spin the disc up to an incredible speed and won't even brake the spindle when you eject the disc, others will perform equally random acts. The TOC is important!)
If you close the current session and open a new one, the lead-in and lead-out of the current session will be written. A TOC will be written in the current lead-in that points to the eventual TOC of the next session. This process is repeated for every closed session, resulting in a chain of links from one lead-in area to the next. Typical audio CD players don't know about chasing TOC links, so they can only see tracks in the first session. Your CD-ROM drive, unless it's broken or fairly prehistoric, will know about multisession discs and will happily return the first session, last session, or one somewhere in between, depending on what the OS tells it and what it is capable of.
Some CD-ROM drives, notably certain early NEC models, are finicky about open sessions, and will gag when they try to read the lead-in from a still-open session. They follow the chain of links in the lead-ins of each session, but when they get to the last, they can't find a valid TOC and become confused. Even though these drives support multi-session, they require that the last session be closed before they will read the disc successfully. Fortunately, most drives don't behave this way.
If you use disc-at-once (DAO) recording, the lead-in is written at the very start of the process, because the contents of the TOC are known ahead of time. With most recorders there is no way to specify that more than one session should be created in DAO mode, so creating a multisession disc with DAO recording isn't generally possible. Such discs must be created with track-at-once (TAO) or session-at-once (SAO) recording.
If you're using certain versions of Windows, the Auto Insert Notification feature will "discover" the CD-R as soon as the TOC is written. This can cause the write process to fail, which is why Windows software automatically enables and disables AIN as needed. Otherwise, if recording in track-at-once mode, it will fail during finalization; in disc-at-once mode, it will fail near the beginning of the write process. In both cases, test writes will succeed, because the TOC doesn't get written during a test pass.
Packet-written discs follow the same rules with regard to open and closed sessions, which is why they have to be finalized before they can be read on a CD-ROM drive. The "Packet Writing - Intermediate" document in the primer at http://www.mrichter.com/cdr/primer/primer.htm goes into a little more detail on this subject. (Some people like to refer to packet writing as "PAO", for packet-at-once.)
There are gory details beyond what is written here. For example, the lead-in on a CD-R actually has a pre-recorded TOC that specifies physical parameters of the recording layer, such as required laser recording power, and information about the disc, like how many blocks can be written (the "ATIP" discussed in section (2-38)). You don't usually need to worry about such things though.
Subject: [2-20] How are WAV/AIFF files converted into Red Book CD audio?
There is absolutely nothing special about the audio data encoded on a CD. The only difference between a "raw" 44.1KHz 16-bit stereo WAV file and CD audio is the byte ordering.
It isn't necessary to convert a WAV or AIFF file to a special format to write to a CD, unless you're using a format that your recording software doesn't recognize. For example, some software won't record from MP3 files, or from WAV files that aren't at the correct sampling rate. Similarly, you don't have to do anything special to audio extracted from a CD. It's already in a format that just about anything can understand.
Just put your audio into the correct format -- uncompressed 44.1KHz, 16-bit, stereo, PCM -- and the software you use to write CDs will do the rest. All of the fancy error correction and track indexing stuff happens at a lower level.
Don't get confused by programs (such as Win95 Explorer) that show ".CDA" files. This is just a convenient way to display the audio tracks, not a file format unto itself. See section (2-36).
Subject: [2-21] What does MultiRead mean? MultiPlay?
The MultiRead logo indicates that a CD or DVD drive can read all existing CD formats, including CD-ROM, CD-DA, CD-R and CD-RW. See the description at http://www.osta.org/specs/multiread.htm The presence of this logo on a CD-ROM drive does *not* mean that the drive can read DVD.
MultiPlay does essentially the same thing, but is meant for consumer CD and DVD players. See http://www.osta.org/specs/multiplay.htm.
Subject: [2-22] If recording fails, is the disc usable?
That depends on what was being recorded, how it was being recorded, and how far along in the process things were.
If it failed while writing the lead-in, before any data was written, the disc probably isn't usable. Some drives, notably certain Sony models, have a "repair disc" option that forcefully closes the current session. This would allow you to add extra data in a second session on the disc, but anything written in the first session will be unavailable.
Failures when finalizing the disc may be correctable. Sometimes the TOC gets written before the failure, and the disc can be used as-is. Sometimes you can use a "finalize disc" option from a program menu that will do the trick. Other times the recorder will refuse to deal with a partially-finalized disc, and you're stuck.
Failures in the middle of writing result in a CD-ROM that probably isn't worth trusting. Some of the data will be there, some won't. The directory for the disc may show more files than are actually present, and you won't know which are actually there until you try to read them.
Audio CDs recorded in disc-at-once mode are a special case. Because the TOC is written up front, the disc is readable in a standard CD player even if the write process doesn't finish. You will be able to play the tracks up to the point where the recording failed.
If you were using a packet writing program like DirectCD, the experiences of people on Usenet suggest that you are either 100% okay or 100% screwed. The ScanDisk utility included with DirectCD 2.5 may help though.
Subject: [2-23] Why do recorders insert "00" bytes at the start of audio tracks?
This phenomenon is familiar to users who have attempted to extract digital audio from a CD-R. Very often the result of copying an audio CD is an exact copy of the original audio data, but with a few hundred zero bytes inserted at the front (and a corresponding number lost off the end). Since this represents the addition of perhaps 1/100th of a second of silence at the start of the disc, it's not really noticeable.
The actual number of bytes inserted may very slightly from disc to disc, but a given recorder usually inserts about the same number. It's usually less than one sector (2352 bytes).
According to a message from a Yamaha engineer, the cause of the problem is the lack of synchronization between the audio data and the subcode channels, much like the "jitter" described in section (2-15). The same data flow problems that make it hard to find the start of a block when reading also make it hard to write the data and identifying information in sync. According to the engineer, no changes to the firmware or drive electronics can fix the problem.
Making copies of copies of audio CDs would result in a progressively larger gap, but it's likely to be unnoticeable even after several generations.
Subject: [2-24] How many tracks can I have? How many files?
You can have up to 99 tracks. Because the track number is stored as a two-digit decimal number starting with "01" (BCD encoded, in case you were wondering), it's not possible to exceed this.
Tracks must be at least 4 seconds long, according to the standard. In practice, CD recorders have different notions of how short a track can be, but most recorders will refuse to write a track shorter than one second.
The maximum number of files depends on the filesystem you're using. For ISO-9660, you can (in theory) have as many as you want. In practice, DOS or Windows will treat the disc internally as a FAT16 filesystem, so you are limited to about 65,000 files if you want broad compatibility.
Subject: [2-25] Will SCMS prevent me from making copies?
SCMS is the Serial Copy Management System. The goal is to allow consumers to make a copy of an original, but not a copy of a copy. Analog recording media, such as audio cassettes and VHS video tape, degrades rather quickly with each successive copy. Digital media doesn't suffer from the same degree of generation loss, so the recording industry added a feature that has the same net effect.
SCMS will affect you if you use consumer-grade audio equipment. Professional-grade equipment and recorders that connect to your computer aren't restricted. See section (5-12) for more about the differences between these types of devices.
The system works by encoding whether or not the material is protected, and whether or not the disc is an original. The encoding is done with a single bit that is either on, off, or alternating on/off every five frames. The value is handled as follows:
- Unprotected material: copy allowed. The data written is also marked unprotected.
- Protected material, original disc: copy allowed. The data written will be identified as a duplicate.
- Protected material, duplicate: copy not allowed.
If you're using a consumer audio CD recorder, SCMS will prevent you from making copies of copies of protected material. It will not prevent you from making a copy of an original disc you have purchased, and it won't stop you from copying unprotected discs.
Subject: [2-26] Is a serial number placed on the disc by the recorder?
In general, no, but it appears that some of the newer consumer audio CD recorders write one. The Recorder Unique Identifier (RID) is a 97-bit code recorded every 100 sectors. It is composed of a brand name identifier, a type number, and a drive serial number. Recorders such as the Philips CDR870 write the RID to discourage distribution of copyrighted material.
Windows will show something like "Volume Serial Number is 4365-0FED". There does not appear to be any way to control this. Some have suggested that the serial number is generated based on data found on the disc, similar to the way that audio CDs can (mostly) be uniquely identified by the number and durations of the tracks.
On floppy disks and hard drives, the "serial number" is generated based on the date and time when the disk is formatted. The four bytes are:
- month + seconds
- day + hundredths of a second
- high byte of the year + hours
- low byte of the year + minutes
Subject: [2-27] What's a TOC? How does it differ from a directory?
The TOC (Table Of Contents) identifies the start position and length of the tracks on a disc. The TOC is present on all CDs. If it weren't, the disc would be unreadable on a CD player or CD-ROM drive. CD recorders write the TOC as part of "finalizing the disc. (Section (2-19) has some more details about finalizing discs.)
A "directory" is a list of files. If you're a Mac user, you're probably used to the term "folder". It's part of a filesystem, such as the ISO-9660 or HFS filesystem present on most CD-ROMs. Audio tracks don't have files, so they don't have directories either.
There's nothing stopping you from writing a FAT16 or Linux ext2 filesystem directly onto a CD-ROM. Whether or not you can read such a disc is a different matter. (The Linux "mount" command should allow you to mount just about anything read-only, but Windows may not be so willing.) The CD specification defines the TOC, and there are well-defined standards for certain filesystems, but [AFAIK] nothing in the CD spec requires that you fill a data track with a certain kind of data.
Subject: [2-28] What's an ISO? A CIF? BIN and CUE? .DAT?
In common use, an "ISO" is a file that contains the complete image of a disc. Such files are often used when transferring CD-ROM images over the Internet. Depending on who you're talking to, "ISO" may refer to all disc image files or only certain kinds.
Going by the more restrictive definition, an "ISO" is created by copying an entire disc, from sector 0 to the end, into a file. Because the image file contains "cooked" 2048-byte sectors and nothing else, it isn't possible to store anything but a single data track in this fashion. Audio tracks, mixed-mode discs, CD+G, multisession, and other fancy formats can't be represented.
To work around this deficiency, software companies developed their own formats that *could* store diverse formats. Corel developed CIF, which is still in use by Roxio's Easy CD Creator. (What does CIF mean? Nobody knows, though "Corel Image Format" is as good a definition as any.) Jeff Arnold's CDRWIN created them as "BIN" files, with a separate "cue sheet" that described the contents. You can unpack a BIN/CUE combo with "binchunker", which is now integrated into Fireburner (section (6-1-50)).
A ".DAT" file could be most anything, but usually it's a video file pulled off of a VideoCD. A program at http://www.vcdgear.com/ can convert .DAT to .MPG, and recording programs like Nero can record them directly.
A ".ISO" file that contains an image of an ISO-9660 filesystem can be manipulated in a number of ways: it can be written to a CD-ROM; mounted as a device with the Linux "loopback" filesystem (e.g. "mount ./cdimg.iso /mnt/test -t iso9660 -o loop"); copied to a hard drive partition and mounted under UNIX; or viewed with WinImage (section (6-2-2)). There is no guarantee, however, that a ".ISO" file contains ISO-9660 filesystem data. And it is quite common to hear people refer to things as "ISO" which aren't.
A ".SUB" file appears to contain subchannel data. Some programs pass these around in addition to one of the above formats.
We now have many different file extensions, including ISO, BIN, IMG, CIF, FCD, NRG, GCD, PO1, C2D, CUE, CIF, CD, and GI. Smart Projects' IsoBuster, from http://www.isobuster.com/, can open and manipulate just about any disc image format.
(The rest of this section is a philosophical rant, and can safely be skipped. This is intended to be more illustrative than factual, and any relation to actual events is strictly coincidental.)
The term "ISO" is ostensibly an abbreviation of "ISO-9660 disc image", which is itself somewhat suspect. ISO-9660 is a standard that defines the filesystem most often used on CD-ROM. It does not define a disc image format. "ISO-9660 filesystem image" would be more appropriate.
When you capture or generate a CD-ROM image, you have to call it something. When a CD-ROM was generated from a collection of files into an ISO-9660 filesystem image, it was written into a file with an extension of ".ISO". This image file could then be written to a CD-ROM. As it happens, the generated image files were no different in structure from the images that could be extracted from other CD-ROMs, so to keep things simple the extracted disc images were also called ".ISO".
(Some programs used the more appropriate ".IMG", but unfortunately that was less common.)
This meant that, whether you extracted a data track from a disc written with the HFS filesystem or the ISO-9660 filesystem, it was labeled ".ISO". This makes as much sense as formatting a 1.4MB PC floppy for HFS, creating an image, and calling it a "FAT12 disk image" because such floppies are usually formatted with FAT. It didn't really matter though, because no matter what was in the file, the software used the same procedure to write it to CD-R.
As a result of this filename extension convention, any file that contained a sector-by-sector CD-ROM image was referred to as an "ISO file". When CD recorders hit The Big Time and many people started swapping image files around, the newcomers didn't know that there was a distinction between one type of disc image and another, and started referring to *any* sort of disc image as an "ISO".
These days it's not altogether uncommon to see messages about "making an ISO" of an audio CD, which makes no sense at all.
Subject: [2-29] Why was 74 minutes chosen as the standard length?
The general belief is that it was chosen because the CD designers wanted to have a format that could hold Beethoven's ninth symphony. They were trying to figure out what dimensions to use, and the length of certain performances settled it.
There are several different versions of the story. Some say a Polygram (then part of Philips) artist named Herbert von Karajan wanted his favorite piece to fit on one disc. Another claims the wife of the Sony chairman wanted it to hold her favorite symphony. An interview in the July 1992 issue of _CD-ROM Professional_ reports a Mr. Oga at Sony made the defining request. (This is almost certainly Norio Ohga, who became President and COO of Sony in 1982 and has been a high-level executive ever since.)
The "urban legends" web site has some interesting articles for anyone wishing to puruse the matter further. The relationship of Beethoven's ninth to the length is noted "believed true" in the alt.folklore.urban FAQ listing, but no particular variant is endorsed.
Searching the net will reveal any number of "very reliable sources" with sundry variations on the theme.
Subject: [2-30] Why is there a visibly unwritten strip near the CD-R hub?
You haven't closed the session yet. The lead-in area, which includes the TOC (section (2-27)), isn't written until the session is closed. A space is left for it that is large enough to see. Read section (2-19) for more details on what happens when you close a disc.
You will see the narrow unwritten strip if you:
- write a disc, telling the program to leave the disc and session open.
- eject a packet-written disc without having closed it in ISO-9660 mode.
- have a failure during recording in track-at-once mode.
If you use disc-at-once recording, the lead-in area is written right away, so after a failure you won't see the gap.
Subject: [2-31] What is "BURN-Proof"? "JustLink"? "Waste-Proof"?
BURN-Proof (or BurnProof) is an unfortunate abbreviation of "Buffer-Under-RuN Proof". The technology allows you to avoid buffer underruns by suspending and restarting the write process when the recorder's buffer is about to empty. (See section (4-1) if you're not familiar with buffer underruns.)
Ideally, the results of interrupted and uninterrupted writes would be identical. In practice, there may be a small glitch at the point where writing was suspended. Sanyo recommends 4X or higher speed CD-ROM drives and audio equipment made in 1995 or later for playback.
The general concensus is that these technologies are effective and do not result in noticeable glitches.
There are several different, competing technologies. Here's a sample of what's out there (note that many of the names are trademarked):
- BURN-Proof (Sanyo)
- Buffer-Under-RuN Proof. The first. Can restart the laser after a buffer underrun. For details, see http://www.sannet.ne.jp/BURN-Proof/faq/.
- JustLink (Ricoh)
- Can restart the laser after a buffer underrun. Data gap length is less than two microns. See
- ExacLink (Oak Technology)
- Can restart the laser after a buffer underrun. See http://www.oaktech.com/products/optical/cdrw/exaclink.html.
- ExactLink (Mitsui)
- Appears to be the same as ExacLink. Mitsui's pages refer to "Oak Technology's ExactLink(tm)".
- SMART-BURN (Lite-On IT)
- Smart Monitoring & Adapting Recording Technology for BURNing. Can restart the laser after a buffer underrun, and will reduce the recording speed if it thinks the media can't be written safely at the requested speed. See http://www.liteonit.com/english-s-tech-7.htm.
- Waste-Proof (Yamaha)
- Waste-Proof Write Strategy. Does some extra work to prevent the buffer underrun from happening in the first place, but won't save you if one actually happens.
- SafeBurn (Yamaha)
- Can restart the laser after a buffer underrun, and will reduce the recording speed it if thinks the media can't be written safely at the requested speed. Data gap length is less than one micron. See http://www.yamaha.ca/computer/cp_safeburn.asp.
- Just Link (AOPen)
- Can restart recording after a buffer underrun. Data gaps are less than 2 microns. http://www.aopen.com/products/cdrw/JustLink.htm.
- Seamless Link (BenQ)
- Unclear whether this is a trade name or just a way of referring to one of the other variants.
- SafeLink (Waitec)
- Another one (no details available?).
- Power Burn (Sony)
- And another one.
Nearly all CD recorders announced in or after 2001 featured some variation of buffer underrun protection.
Some related technologies:
- Just Speed (AOpen)
- Reduces the record speed if it doesn't think the media can handle it. See http://www.aopen.com/products/cdrw/justspeed.htm. Combined with Just Link.
- Smart Speed (BenQ)
- See above.
Subject: [2-32] Can playing CD-Rs in a DVD player hurt the discs?
There appear to be three kinds of DVD players:
- Those that can play CD-Rs.
- Those that can't play CD-Rs.
- Those that damage the discs.
If playing CD-R discs in your DVD player is important, make sure the player can handle them before you buy a player. See section (2-13).
It's a little unclear what the player is doing to damage the CD-R media. The playback laser would have to be operated at a wavelength and intensity that caused a change in the recording dye layer.
There are no known instances of DVD-ROM drives that damage discs.
Subject: [2-33] Who *really* made this CD-R blank?
Many of the "big name" media manufacturers don't actually make their own media. Instead, they buy from other manufacturers and stamp their logo on the discs. Generally speaking, this isn't a bad thing, because the discs were certified good enough that the Big Brand was willing to put the company name behind the product.
If you have a picky recorder or player, though, it helps to be able to try several different pieces of media. If you buy several different brands, and they're all coming from the same manufacturer, chances are they'll all behave the same way, and your time and money will be wasted.
So... how do you tell who really made a piece of media? The short answer is: you don't.
It's tempting to believe that CD-R media identifier applications (e.g. section (6-2-9)) will give you the answer you need. Unfortunately, the data you get is unreliable at best. Charles Palmer, from cd-recordable.com, had this to say about the manufacturer identification:
"Two components that many users of these programs always take as gospel are Media Manufacturer and Dye Data. These two readings are next to worthless.The only reliable piece of information in the "ATIP" region is the disc length. See section (2-38) for further remarks.
The reason for this is that many CD-R manufacturers (like CD- Recordable.com) purchase their stampers (the nickel die that all CD-R substrates are molded from) from 3rd party sources. These 3rd party sources (either other disc manufacturers, or mastering houses) encode the data that these 'Identification' programs read, at the time that the original glass master is encoded. The 'Manufacturer' information that is encoded is usually the name of the company that made the master. Since stampers made from that master will be sold to disc manufacturers the world over, all of discs that those manufacturers produce from those stampers will contain the same 'Manufacturer' information. Information which is obviously quite erroneous and irrelevant. Very seldom will the 'manufacturer' information encoded on a CD-R actually tell you anything other than who made the original master. [...]
The second piece of data (the dye type) is also dubious. Because most master/stamper configurations are designed to be matched to specific dye types (Phthalocyanine, Cyanine, Azo, Etc), the 'Dye' information that is encoded when the master is produced indicates the type of dye that the master was designed for. This of course, does not assure that the manufacturer that buys and uses this stamper will be using it with the dye that it has been designed for. It is quite possible that a stamper/dye combination is used by a CD-R manufacturer that contradicts the 'dye' information encoded on the master. Therefore that information becomes as potentially misleading as the 'Manufacturer' data discussed earlier."
Subject: [2-34] Can I make copies of DTS-encoded CDs?
Yes. CDs encoded with DTS (Digital Theater Sound) follow the Red Book standard for the most part. The chief difference is that the audio is encoded with DTS rather than 44.1KHz 16-bit stereo PCM. If you put one into an audio CD player, it will recognize the tracks and try to play them, resulting in a hissing noise.
You can copy DTS CDs the way you would any other audio CD. Attempting to convert them to MP3 is a bad idea though -- they're already in a compressed format.
A common way to play DTS-encoded CDs is with a DVD player connected to a DTS-capable receiver. The DVD player passes multichannel audio to the receiver over an S/PDIF connection. Many DTS CDs are encoded in 5.1 surround sound, which is kinda neat.
Subject: [2-35] Why 44.1KHz? Why not 48KHz?
The "Red Book" specification for audio CDs chose 44100 samples per second, where each sample is 16-bit stereo PCM. PCM is a fine choice for encoding audio, stereo is widely recognized and supported, and it's very easy to manipulate data in 16-bit quantities with existing hardware and software.
Why 44100? Why not make it a round decimal value like 44000, or a round binary quantity like 44032? Why not 32KHz or 48KHz?
In general, the human ear can hear tones out to about 20KHz. According to a smart fellow named Nyquist, you have to sample at twice that rate. Because of imperfections in filtering, you actually want to be a little above 40KHz.
According to John Waktinson's _The Art of Digital Audio_, 2nd edition, page 104, the choice of frequency is an artifact of the equipment used during early digital audio research. Storing digital audio on a hard drive was impractical, because the capacity needed for significant amounts of 1 Mbps audio was expensive. Instead, they used video recorders, storing samples as black and white levels. If you take the number of 16-bit stereo samples you can get on a line, and multiply it by the number of recorded lines in a field and the number of fields per second, you get the sampling rate. It turned out that both NTSC and PAL formats (the video standards used in US/Japan and Europe, respectively) could handle a rate of 44100 samples per second. This rate was carried over into the definition of the compact disc.
The sampling rate for "professional" audio, 48KHz, was chosen because it's an easy multiple of frequencies used for other common formats, e.g. 8KHz for telephones. It also happens to be fairly difficult to do a good conversion from 48KHz to 44.1KHz, which makes it harder to, say, copy an audio CD with a "consumer" DAT deck. (Well, okay, some consumer DAT decks can do 44.1KHz now, but initially only "professional" decks could handle the lower frequency.)
There is relatively little difference in audible quality between 44.1KHz and 48KHz, since the slight increase in frequency response is outside the range of human hearing. Some inaudible tones produce "beats" with audible tones and thus have a noticeable impact, but the improvement from 44.1 to 48 is marginal at best.
Subject: [2-36] What format are .CDA files in?
Actually, .CDA files aren't really files at all. Windows shows the tracks on an audio CD as ".CDA" files for convenience. For example, you can create a file association for ".CDA" and invoke an audio CD player when you double-click on a track.
The tracks themselves are in a format almost identical to a common WAV or AIFF file. See section (2-20).
Subject: [2-37] What are DD-R and DD-RW?
DD-R and DD-RW are Sony standards for "double-density" recordable and rewritable discs. The discs hold 1.3GB of data, and are relatively inexpensive, but aren't compatible with current CD or DVD players. You can only read the discs in a DD-R/DD-RW drive.
The recorders form a middle ground between CD-R and DVD-R in terms of storage capacity and price, but the lack of compatibility reduces their usefulness. On the bright side, the drives are expected to be able to record on CD-R and CD-RW media.
Subject: [2-38] What's an ATIP?
ATIP is an acronym for Absolute Time In Pregroove. It's a pre-written section of the disc that specifies disc characteristics, including the number of blocks on the disc (which is determined by the length of the pre-formed groove on the disc, hence the name), and some information about the disc's construction and manufacturer.
See section (2-33) for some comments about the usefulness of the ATIP fields. http://www.orangeforum.or.jp/e/reference/index.htm used to have ATIP information, but the "Disc Identification Method" link is now password-protected.
Subject: [2-39] What are "ML" discs and devices?
"ML" is short for "MultiLevel". Devices and media constructed by Calimetrics (http://www.calimetrics.com/) boast 3x the storage capacity and 3x the recording speed of conventional CD-R and CD-RW media.
CD technology works by measuring the light reflected from the surface of the disc. Traditional discs only have two levels ("pit" and "land"), ML discs have more than one. By increasing the effective bit density of the media, you can write 3x as much data in one revolution of the disc, improving both the storage capacity and the recording speed.
The technology requires minor changes to existing hardware, and requires discs optimized for ML recording. Discs written with ML devices will not be compatible with existing CD players and CD-ROM drives. However, ML recorders are expected to be able to record on CD-R/CD-RW media as well, so ML support could be a low-cost bonus feature on new drives.
Subject: [2-40] What's CD-MRW? Mount Rainier?
CD-MRW is the working name for a CD-RW storage format under development by the Mount Rainier Working Group (http://www.mt-rainier.org/). The Mount Rainier group is creating specifications for native OS support of CD-RW and DVD+RW, with the eventual goal of replacing floppies and similar formats (e.g. Zip disks).
This new standard is being promoted by Compaq, Microsoft, Philips, and Sony. The web site claims support by "over 40 industry leaders", including OS vendors and PC OEMs.
What this means to you: 500+MB of reasonably fast storage that doesn't require long formatting delays or the installation of special software.
Yamaha's CRW3200 claims to be the first CD-RW drive with CD-MRW support.
Subject: [2-41] What's Audio Master Quality (AMQ) recording?
Yamaha developed Audio Master Quality Recording to compensate for higher "jitter" in recorded CDs. This is not the kind of jitter addressed by "jitter correction" in CD rippers (2-15). This is the "jitter" that people selling fancy stereo equipment talk about.
Jitter is time-base error. It's not a corruption of the digital '1's and '0's, it's a distortion of the timing in which the '1's and '0's arrive at their destination. This doesn't affect extraction of audio, so you don't need to worry about this kind of jitter when reading a CD or ripping to MP3. You do need to worry about it when listening to a CD.
The digital signal is read from a CD via an analog process: bouncing a laser off of "pits" and "lands" on a CD. Various factors can prevent the signals from arriving at the right place at exactly the right time. High-end CD players can correct these anomalies, but many don't.
AMQ extends the length of the pits and lands on the CD in an attempt to produce a more stable signal. This reduces the recordable length of the CD -- a 74-minute disc only holds 63 -- but produces noticeably improved audio (says Yamaha). The process works because CD players automatically adjust the rotation speed.
Yamaha's explanation: http://www.yamaha.ca/computer/cp_AudioMQR.asp
The canonical jitter discussion: http://www.digido.com/jitteressay.html
Subject: [2-42] Can I draw pictures on a disc with the recording laser?
If you've ever looked at a recorded CD-R, you've probably noticed that the recorded and unrecorded areas have a different appearance. This is usually visible as a slight change in color. By controlling the write laser it's possible to mark the disc in a way that is meaningful to the human eye rather than to a CD player. Unfortunately, the level of control required to do this isn't achievable without firmware support.
In mid-2002, Yamaha announced "DiscT@2" (disc tattoo). This allows moderate-resolution (approx. 250dpi) graphics to be drawn in the parts of the disc that weren't recorded. Yamaha claims to get 256 shades of color (green, blue, or whatever color the disc happens to be), though it works best on dark blue azo discs. For more details and some pictures, see:
This section is for people who really want to know what's going on inside. You absolutely do not need to understand any of this to successfully record a CD. You will come away with a greater appreciation for CD players, and also may better understand how some forms of copy protection function.
The sections are written from the perspective of reading a disc. Generally speaking, the process is simply reversed when writing.
I tried to find a balance between not presenting enough information and presenting too much detail. My hope is that, when you are done reading this, you will have a broad understanding of how a CD player turns a lumpy piece of plastic into music, and will know exactly where to look if you need further details. If you want the kind of detail found in a textbook, there are some good ones listed in section (2-43-6).
Subject: [2-43-1] How does the laser read or write a disc?
CD players use a near-infrared 780nm laser. The visible light spectrum is generally considered to be 400nm to 700nm; few people can see anything past 720nm.
The drive shines a laser through the polycarbonate (plastic) on the "bottom" of the disc. This bounces off the reflective layer, passes back through the polycarbonate, and is read by a photosensor in the drive head. The index of refraction for polycarbonate is about 1.55, so laser light bends when it enters, allowing a much finer focus for the laser (from 800um at the bottom of the polycarbonate down to about 1.7um at the metal surface). This minimizes the effects of dust and scratches, because the effects of any surface gunk are reduced as the laser's focus width is reduced. A 400um-wide piece of dust on the surface of a CD would completely block a laser focused down to 200um at the surface, but has little effect on a CD player.
If the photosensor sees a strong beam -- the CD standard requires the signal strength to be at least 70% when fully reflected -- it knows it's travelling over a "land". If it sees a weaker response, it's travelling over a "pit". Technically, it's travelling "under" a pit or land, so from its perspective a "pit" is actually a bump. The height of the bump is 1/4 of the laser's wavelength in polycarbonate, so that light reflected from the bump has a phase difference of one-half wavelength. The light reflected from the pit and from the surrounding land thus cancel each other out. (The geometries are actually such that a "pit" reflects about 25% of the intensity rather than 0%. For example, pits are 0.5um wide, or about 1/3 of the focused width of the laser.)
There are a lot of optical tricks involving polarization of light and the action of diffraction gratings going on. For example, the read head uses a three-beam auto-focus system that keeps the laser properly aligned on the spiral track and at the correct distance from the bottom of the disc. It's also worth mentioning that, because light travels more slowly in polycarbonate, the wavelength of the laser inside the CD is closer to 500nm.
CD-R and CD-RW discs do not have pits and lands. On CD-R media, the write laser heats the organic dye to approximately 250 degrees Celsius, causing it to melt and/or chemically decompose to form a depression or mark in the recording layer. The marks create the decreased reflectivity required by the read laser. On CD-RW media, the write laser changes the material between crystalline (25% reflectivity) and amorphous (15% reflectivity) states. This is done by either heating it above its melting point (500C to 700C) and letting it cool rapidly to convert it to amorphous form, or heating it to its transition point (200C) and letting it cool slowly to return it to the more stable crystalline state. The lower reflectivity of CD-RW makes the discs unreadable on most older players.
The rest of this discussion refers to "pits" and "lands", but applies equally to pressed CDs, CD-Rs, and CD-RWs.
Subject: [2-43-2] How do pits and lands turn into 1s and 0s? What's EFM?
The pits and lands on a CD do not directly correspond to 1s and 0s. The start and end of a pit (i.e. the pit edges) each correspond to 1s, and all other areas -- both in pits and on lands -- correspond to 0s. The number of zeroes between pit edges is determined through careful timing. This is an efficient approach that produces an easy to handle electrical signal (it's NRZI -- NonReturn to Zero Inverted -- which converts easily to NRZ where 1s are high voltage and 0s are low voltage).
The careful timing is possible because CDs are essentially self-clocking. Suppose you have a clock that ticks once per second. Plug your ears and count seconds to yourself, trying to keep the same pace as the clock. After ten seconds, unplug your ears. If you've drifted slightly, you can readjust to the clock without worrying that you've too far off. You might be missing the beat by a quarter of a second, but you can adjust forward or backward a fraction of a second and still be sure that both you and the clock got to 10 seconds at about the same time. Now try the same experiment for 10 minutes. When you unplug your ears you can get back in sync with the clock's timing, but unless you have a very good internal timer it's unlikely you will reach 10 minutes on the same tick. With your ears plugged for so long, you are likely to be off by several seconds.
CDs work the same way. Every pit edge represents an audible clock tick, while the insides of pits and lands represent inaudible ticks. If a pit or land is too long, the drive's clock will drift too far and possibly get out of sync. (This is why "blank" recordable discs aren't entirely blank: they have a pre-cut spiral groove with a "wobble" that the recorder can use as a timing signal. A clock accurate enough to produce a stable, reliable signal at these frequencies is too expensive to incorporate into a cheap consumer product.)
To guarantee pits of specific lengths, the CD standard requires that there are at least 2 and at most 10 zeroes between every 1. This is achieved by converting every 8-bit byte into a 14-bit value, a process called Eight to Fourteen Modulation (EFM).
The shortest possible pit (or land) thus represents 3 EFM bits (100), and the longest 11 EFM bits (10000000000). If a single bit requires time T to pass under the read head, then pits of these lengths can be referred to as 3T pits and 11T pits. If after seeking to a new location, the drive sees a pit shorter than 3T or longer than 11T, then it immediately knows that the disc is not spinning at the rate it was expecting, and can make appropriate adjustments.
Between each 14-bit EFM word there are 3 "merging bits". Because CDs aren't allowed to have runs shorter than 3T or longer than 11T, it is sometimes necessary to follow an EFM code with a 1 or 0. Suppose, for example, that an EFM code ending in 1 were immediately followed by an EFM code starting with 1. The merging bits also serve to prevent the frame synchronization pattern from appearing where it isn't supposed to (see next section).
If there is more than one possible arrangement of merging bits that satisfy the restrictions for run length and sync pattern, then a pattern is chosen that minimizes the low-frequency components of the signal. This is done by minimizing the Digital Sum Value (DSV), computed by adding one to a counter for every T after a transition to a land, and subtracting one for every T after a transition to a pit. Adding a 1 to the merging bits inverts the signal by causing a transition from a pit to a land or vice-versa. Minimizing the DSV is important because low-frequency signals can interfere with the operation of tracking and focusing servos.
With EFM there are more bits to encode, but the highest frequency possible in the output signal is decreased. The ratio of the number of bits transmitted to the number of transitions on the medium is high, making this an efficient way to store the data while still being able to recover the clock. It's also worth noting that a 3T pit is 0.833um long, while the laser spot is just over twice that length at 1.7um. If 2T or 1T pits were allowed, the laser would have a hard time detecting them. This is why it's important that transitions not occur too frequently: the laser is good at computing the time between transitions, but isn't so good at noticing transitions if they follow each other too quickly. Making the transitions more obvious requires making the pits and lands longer, which reduces the amount of data that will fit on the disc. (See the description of AMQ in section (2-41).)
Subject: [2-43-3] What's a frame? CIRC encoding? How does ECC work?
EFM encoding is applied to a series of bytes called a "frame". Some people refer to a CD sector as a "frame", but that's incorrect. A frame holds 24 bytes of user data, 1 byte of subcode data, and 8 bytes of parity (error correction), for a total of 33 bytes.
When read from the disc, each frame is preceded by a 24-bit synchronization pattern and 3 merging bits. The sync data has a unique pattern not found elsewhere on the disc, and it ensures the read head correctly finds the start of the frame. (The pattern is 100000000001000000000010, three transitions separated by 11T, which can't occur otherwise because the merging bits are specifically chosen to prevent it.) If you don't understand why having a sync field is important, remember that every time the read head seeks to a new part of the disc or is confused by a scratch, it has to start reading in the middle of a stream of 1s and 0s and try to make sense of what it's reading. Until it sees a synchronization pattern, it has no idea if it's reading the start or middle of a frame, or even if it's at the start or middle of an EFM word.
The rest of the 33-byte frame is read as 14-bit EFM values followed by 3 merging bits. This means there are 588 (24 + 3 + (14+3)*33) "channel bits" in a frame. This 588-bit structure is called a "Channel Frame".
Once EFM is decoded and the merging bits discarded, we are left with an "F3 Frame". The subcode byte is removed, and the remaining data (now an "F2 Frame") is passed into the CIRC (Cross-Interleave Read-Solomon) decoder. The decoder is an important part of the reason why CDs and CD-ROMs work.
The raw error rate from a CD is around 1 error per 100K to 1 million bits. That's pretty good, but at 4 million bits per second (588 channel bits per frame x 98 frames per sector x 75 sectors per second = 4.3218Mbps), the errors add up quickly. CIRC encoding takes the 192 bits (24 bytes) of data and 64 bits (8 bytes) of parity, shuffles it around, and performs some weird math involving Galois Fields. The bits are processed by two error correction stages, referred to as C1 and C2. The efficacy of the results can be expressed as a set of error counts.
Errors are noted with a two-digit number that indicates the number of errors with the first digit and the CIRC decoder stage with the second digit. The E11 count indicates the number of single-symbol (correctable) errors in the C1 decoder. E21 indicates double-symbol (correctable) errors in C1, and E31 indicates triple-symbol (uncorrectable at C1) errors in C1. The sum of these counts is the Block Error Rate (BLER), a measure of correctable and uncorrectable errors. The CD standard sets the acceptable limit to 220 BLER errors per second, averaged over a 10-second stretch.
The E12 count indicates the number of single-symbol (correctable) errors in the C2 decoder. Because the data is interleaved after the C1 pass, one E31 error can generate up to 30 E12 errors, so a high error count here is not problematic. E22 counts double-symbol (correctable) errors, which are a bad sign. The sum of E21 and E22 form a burst error count (BST), which can be used to identify physical defects on a disc.
Any E32 errors, representing triple-symbol (uncorrectable) errors in the C2 decoder, result in damaged data. For an audio CD interpolation is performed, for a CD-ROM the damaged data must be repaired at a higher level. (This, incidentally, explains how some forms of audio CD copy protection work. The CD author introduces deliberate uncorrectable errors to the CD. An audio player will inaudibly interpolate across them, but a CD-ROM performing digital audio extraction will simply return the bad bits.)
The 24 bytes that comes out of the CIRC decoder are referred to as an "F1 Frame".
It's worth noting that the subcode channels are not CIRC-encoded, and hence are the least-reliable storage directly accessible to the user. The EFM encoding provides some protection against single-bit errors, because only 256 of the 16,384 possible combinations are valid, but without any parity bits the best the drive can do is tell you that it failed to read the data correctly. The Q subcode channel, which can hold vital information about the disc, has a 16-bit CRC.
Subject: [2-43-4] What's in a sector?
98 frames of 24 bytes are combined to form a 2352-byte sector and 98 bytes of subcode data. The sector is assembled from F1 Frames, which are byte-swapped, shuffled, and run through a descrambler. The purpose of the scrambler is to reduce the likelihood that regular bit patterns will induce a large digital sum value.
It's worth pointing out that the 2352-byte sector is the smallest unit most CD-ROM drives will allow software to manipulate. It's only after all of the above that low-level CD-ROM operations, like "RAW DAO-96" reads and writes, begin. This is why making a "bit-for-bit" copy of a disc is tricky.
A sector on an audio CD holds 2352 bytes of data. 16-bit stereo samples require 4 bytes per sample, so there's 2352/4 = 588 samples per sector. At 75 sectors per second, that's 44100 samples per second (44.1KHz).
A sector on a CD-ROM holds 2048 bytes of user data, leaving 304 bytes for other purposes. Every data sector begins with a 16-byte header:
- 12-byte sync field (00 ff ff ff ff ff ff ff ff ff ff 00)
- 3 byte address (minute, second, fraction (1/75th) of a second)
- 1 byte mode
The mode byte determines what the remaining 2336 bytes in the sector looks like:
- Mode 0: null data; serves no practical purpose for CD recording
- Mode 1: the typical CD-ROM layout
- 2048 bytes of user data
- 4 bytes of EDC (Error Detection Code, a 32-bit CRC)
- 8 bytes of reserved space, set to zeros
- 172 bytes of "P" parity
- 104 bytes of "Q" parity
- Mode 2: 2336 bytes of user data, usually used for CD-ROM/XA (see below)
CD-ROM/XA (eXtended Architecture) Mode 2 extends the definition of a Mode 2 CD-ROM. Form 1 looks like a slight rearrangement of a Mode 1 sector, with the 8 bytes of space moved ahead of the user data and filled with a sub-header. Form 2, intended for compressed audio/video data, has the 8-byte sub-header, 2324 bytes of data, and an optional 4-byte EDC code. The sub-header contains some channel and data type flags.
A CD session must be written in a single mode, but the XA spec allows the form to change. Using CD-ROM/XA Mode 2 allows you to choose between extended error correction and increased data capacity, and also change your mind several times in a single track.
There are 8 subcode channels, labeled P,Q,R,S,T,U,V,W, or sometimes "P-W" for short. (The ECMA-130 standard refers to subcode bytes as "Control bytes".) Every frame contains one byte of subcode data, and each byte holds 1 bit of P, 1 of Q, and so on. The bytes from 98 consecutive frames are combined to form a subcode "section". The first two bits in each channel are used for synchronization, leaving 96 bits of useful data per channel (which is where RAW DAO-96 gets its name).
The P and Q channels are defined by the CD audio standard. (They are unrelated to the P and Q parity fields.) The P channel can be used to find the start of a track, but in practice most devices use the more sophisticated Q channel. Q contains four chunks of information: control (4 bits), address (4 bits), Q data (72 bits), and an EDC (16-bit CRC).
The control bits determine whether the track holds audio or data, the number of audio channels (stereo or quadraphonic), and specifies the Digital Copy Permitted and Pre-emphasis flags. The address bits determine the format of the Q data section. Address mode 1 holds information about tracks, mode 2 holds a catalog number (such as a UPC code, constant for an entire disc), and mode 3 contains the ISRC (International Standard Recording Code, constant for a given track but may change with each track).
A disc has three main regions: the lead-in area, the program area, and the lead-out area. Subcode Q mode 1 data in the lead-in is used to hold the table of contents (TOC) for the disc. The TOC is repeated continuously in the lead-in area in case of damage (remember, no CIRC encoding on subcode channels). In the program and lead-out area, mode 1 contains track numbers, index numbers, time within the current track, and absolute time. Index 0 marks the start of a pregap (pause) before the audio in a track begins, index 1 marks the start of the music, and indexes 2 through 99 are usually not set but can be added if desired.
The ability to specify track and index markers when writing a Red Book audio CD is often referred to as "PQ editing" because that information is contained in the P and Q subcodes.
Subcode channels R through W are not defined by the CD standard, except to say that they should be set entirely to zero if not used. They're currently used for CD+G (e.g. Karaoke) discs, CD-Text, and some forms of copy protection.
It is interesting to note that, while bytes from 98 consecutive frames are used to create a subcode "section", those frames don't have to be from a single sector. It's possible for a subcode section to start in one sector and end in the next.
Subject: [2-43-6] I want even more details
Try _The Compact Disc Handbook, 2nd edition_ by Ken Pohlmann, 1992 (ISBN 0-89579-300-8), chapter 3. Or better yet, Pohlmann's mammoth _Principles of Digital Audio, 4th edition_ (ISBN 0-07-134819-0), chapter 9. Chapter 5 of the latter is recommended for anyone wishing to understand CIRC.
Another good book is _The Art of Digital Audio_, 2nd edition, by John Watkinson, Focal Press, 1994 (ISBN 0-240-51320-7).
See http://www.ee.washington.edu/conselec/CE/kuhn/cdmulti/95x7/iec908.htm for more details on CIRC.
See http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm for a nice explanation of disc construction and optics, especially the three-beam autofocus.
The page at http://www.tc.umn.edu/~erick205/Papers/paper.html provides some background information on sampling, aliasing, dither, DACs, and other relevant topics.
You can get a copy of ECMA-130 from http://www.ecma.ch/. This document describes the format of a CD-ROM, including physical dimensions and optical characteristics, as well as sector formats and Q-channel specs. It also features some interesting annexes:
- Annex A: Error correction encoding by RSPC
- Annex B: Scramble (a description of the pre-EFM scrambler)
- Annex C: Error correction encoding by CIRC
- Annex D: 8-bit to 14-Channel bit conversion (has the full table)
- Annex E: Merging bits (algorithm for computation)
If you want source code for the CIRC, RSPC, EDC, and scramble functions, look for Heiko Eissfeldt's edc_ecc.c (and related files). The code is part of Mode2CDMaker, CDRDAO, and possibly others.