Last-modified: 2000/09/21
Version: 2.22
CD fundamentals.
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."
Your basic CD-R is layered like this, from top to bottom:
[optional] labelYes, 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, not 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.
[optional] scratch-resistant and/or printable coating
UV-cured lacquer
Reflective layer (24K gold or a silver-colored alloy)
Organic polymer dye
Polycarbonate substrate (the clear plastic part)
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 have 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.
Discs are written from the inside of the disc outward. On a CD-R you can verify this by looking at the disc after you've written to it. 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/). http://www.pc.be.philips.com/cdrw/general.html has some nice pictures, as does http://www.nswc.navy.mil/cosip/nov97/cots1197-2.shtml.
[optional] scratch-resistant and/or printable coating
UV-cured lacquer
Reflective layer
Upper dielectric layer
Recording layer (the part that changes form)
Lower dielectric layer
Polycarbonate substrate (the clear plastic part)
See the net references section for pointers. The "CD Related Info" section on http://www.cdarchive.com/ has useful data, as does http://www.cd-info.com/ and the alt.cd-rom FAQ pages.
A quick summary of standards:
If you're not entirely put off by all this, pay a visit to http://www.emediapro.net/AprEM/parker4.html.
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/.
You can usually tell by looking at the packaging and/or the disc itself:
There are several possibilities; as yet no developers have come forward and announced what they're doing (for obvious reasons).
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.
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 is 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, the attempt 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, no, 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.
A less sophisticated but nonetheless effective method is to press a silver CD with data out beyond where a 74-minute CD can write. Copying the disc would then require special CD-R blanks, moving the data and hacking the disc to compensate, or pressing silver discs with the pirated data. If taken too far, though, the disc can become unreadable on some drives. An overburned 80-minute blank (sections (3-8-1) and (3-8-3)) can hold about as much as you can reliably fit on a disc anyway.
The approach PC software houses have taken lately is to use nonstandard gaps between audio tracks and leave index marks in unexpected places. These discs are uncopyable 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)).
Another method gaining popularity is non-standard discs with a track shorter than 4 seconds. Most recording software, and in fact some recorders, will either refuse to copy a disc with such a track, or will attempt to do so and fail. A protected application would check for the presence and size of the track in question. Some recorders may succeed, however, so this isn't foolproof. (In one case, a recorder could write tracks that were slightly over three seconds, but refused to write tracks that were only one second. There may be a limit below which no recorder will write.)
Putting multiple data tracks interleaved with audio tracks on a CD will confuse some disc copiers.
Sometimes the copy of a disc will have a different volume label. This usually only 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.
The Laserlok system from http://www.diskxpress.com/ claims to be able to prevent unauthorized disc duplication at a low cost. (VOB's CD-Wizard, listed in section (6-1-35), claims to be able to copy them.)
A recent innovation is TTR Technology's DiscGuard (http://www.ttr.co.il/). They claim 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[tm]; see http://www.c-dilla.com/ for more.
On the other side of the table, CloneCD (section (6-1-49)) claims to be able to copy most or all of these discs given the right combination of reader and writer.
All of the above applies to CD-ROM discs. For audio CDs, a TTR product called MusicGuard (http://www.MusicGuard.com/) claims to do the trick. (The information seems to have been withdrawn.)
See also the "CD Protections" frame on http://www.esware.net/empire/hardware/cdrom/cd.htm.
A separate but related issue is "counterfeit protection", where the
publisher wants to make it easy to detect mass-produced duplicates. An
example of this is Microsoft's placement of holograms on the hub of their
recent CD-ROMs.
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.adaptec.com/support/faqs/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.
There are eight subcode channels (P,Q,R,S,T,U,V,W). The exact method of encoding is beyond the scope of this FAQ, but it should be noted that the data is distributed uniformly across the entire CD, and each channel can hold a total of about 4MB.
The P subcode channel can be controlled with the JVC/Pinnacle recorders, but apparently isn't used for much.
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, and ISRC code. Other forms are found in the lead-in, and are used to enable multisession and describe the 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.
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.
The other subcode channels are generally inaccessible and unused.
For more details, see the book by Pohlmann mentioned in section (2-1); _Principles of Digital Audio_, 3rd edition, by Ken Pohlmann, McGraw-Hill, 1995 (ISBN 0-07-050469-5); or _The Art of Digital Audio_, 2nd edition, by John Watkinson, Focal Press, 1994 (ISBN 0-240-51320-7).
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 lengths of the audio tracks. http://www.cddb.com/ has a collection of CD information.
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 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.
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.
A few recorders, such as the Philips CDD2000, allow "session-at-once" (SAO) recording. This gives you disc-at-once control over the gaps between tracks, and allows you to write in more than one session. 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 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.
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 blanks. 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. The DiskCopy method doesn't seem to allow you to create bootable CD-ROMs though.)
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.
There are subcode flags for each track:
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).
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).
For an *excellent* description of the technology, see http://www.emediapro.net/cdrompro/0996CP/bennett9.html
A technical discussion along with some handy drawings and graphs is available from http://www.pc.be.philips.com/cdrw/general.html.
Some interesting articles on CD-RW - including an editorial critical of the new technology - can be found at:
http://www.emediapro.net/JanEM/news1.html#eraseIf you're trying to decide if you want a drive that supports CD-RW, see section (5-16).
http://www.emediapro.net/FebEM/writer2.html
http://www.emediapro.net/AprEM/parker4.html#bennett
The only discs that a DVD drive 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. Some DVD players come with two lasers so that they can read CD-R.
CD-RW discs have a different formulation, and may work even on players that can't handle CD-R media.
Some DVD drives may be unable to read multisession discs.
If the box doesn't say that something is supported, assume that the feature isn't. Look for the MultiRead logo, which indicates that the DVD player can read all existing CD formats, including CD-ROM, CD-DA, CD-R and CD-RW.
[ I verified that the Sony DVP-S3000 can read CD-R media, though the manual didn't say anything about them. It plays audio CDs and VideoCDs off CD-R just fine. I'm told that some models include two lasers -- called Dual Discrete pickup in the fancier Sony models -- but most players don't. I'm not planning on starting a compatibility list, so if you plan to buy a DVD player and CD-R compatibility is important, bring an audio or VideoCD CD-R with you to the store and try to play it before you buy anything. ]
Probably not. The recorders are still very expensive, and the format wars have made a mess of things.
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. The prices on hardware and software will eventually reach consumer levels.
As mentioned in section (0-2), this FAQ will not be expanding to cover DVD recorders. See http://www.dvddemystified.com/dvdfaq.html instead.
Writers for formats like DVD-RAM are available for less, but may not be compatible with current DVD players.
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.)
Some CD-ROM drives, e.g. most of the Plextor models, include special circuitry 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.
The CD-Information Center web site now has a small history section, http://www.cd-info.com/CDIC/History.html. Some of the items in the bibliography there might also be of interest.
http://www.cdpage.com/ has a CD-Recordable museum on their web page.
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.
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 block). 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.)
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, hence the need for additional ECC and EDC (Error Detection Codes).
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.
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.)
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 doesn't have anything meaningful in it. An "open" 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 your 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 of the closed session will include a link to the lead-in of the next. The CD player in your car or stereo system doesn't know about chasing from one lead-in to the next, so it 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 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 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 an additional session should be added, so creating a multisession disc with DAO recording isn't generally possible.
If you're using Win95 or WinNT, the Auto Insert Notification feature will "discover" the CD-R as soon as the TOC is written. This usually causes the write process to fail. Many of the current CD recording software packages will automatically disable AIN for this reason. 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://resource.simplenet.com/primer/primer.htm goes into a little more detail on this subject.
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. You don't usually need to worry about such things though.
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 some proprietary coding (like MP3 compression) that doesn't have a system-recognized codec. 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 -- 44.1KHz, 16-bit, stereo, uncompressed (a/k/a 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.
Some people get confused by programs (such as Win95 Explorer) that show ".CDA" files. This is just a convenient way to refer to the audio tracks. It's not a file format unto itself. You can't drag-and-drop CD audio tracks unless you're using software that understands what you're trying to do, such as a CD recording program or Plextor's "AudioFS".
The MultiRead logo indicates that a CD or DVD player can read all existing CD formats, including CD-ROM, CD-DA, CD-R and CD-RW. See the press release at http://www2.osta.org/osta/html/press/pr150797.html. The presence of this logo on a CD-ROM drive does *not* mean that the drive can read DVD.
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.
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.
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.
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.
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:
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.
Related sites:
http://www.oade.com/tapers/scms1.html
http://www.sfb.net/scms.htm
http://www.xs4all.nl/~jacg/dcc-faq.html
In general, no, but it appears that some of the newer stand-alone audio CD recorders write one. See http://www.sfb.net/scms.htm for a blurb about RID (Recorder Unique Identifier).
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.
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. (Section (2-19) has some more details.)
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.
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 Adaptec'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 ".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. "ISOBuster", from http://www.ping.be/vcd/isobuster.html, can work with some non-ISO-9660 formats, including .BIN.
(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.
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 diameter 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.
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.
http://www.urbanlegends.com/misc/cd/cd_length_skeptical.html http://www.urbanlegends.com/misc/cd/cd_length_karajan.html http://www.urbanlegends.com/misc/cd/cd_length_origin.html
Searching the net will reveal any number of "very reliable sources" with sundry variations on the theme.
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:
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.
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.
For more information, see http://www.sannet.ne.jp/BURN-Proof/faq/.
Ricoh has independently developed their own version of the technology, called "JustLink". If you speak Japanese, see http://ext.ricoh.co.jp/cd-r/drive/justlink_06.html.
There appear to be three kinds of DVD players:
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.
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. http://www.orangeforum.or.jp/e/reference/index.htm used to have ATIP information, but the "Disc Identification Method" link is now password-protected.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."
FAQ Copyright © 2000 by Andy McFadden. All Rights Reserved.