The capacity of a 'normal' CD-R, like that of a normal CD or CD-ROM, is 650 MB of data or 74 minutes of audio. Sometimes, one runs across a source disc that seems to be larger or a CD-R that claims to have more recording capacity. Those phenomena may be real or illusory and they can be handled in various ways. This note is intended to provide guidance in copying the uncopy-able and similar issues. One suggestion I do make is: Don't break the law. Backing up a disc you own is apparently legal. Saving your own files onto CD-R is certainly OK. Otherwise, please be careful; I don't want to be an accessory to theft.
First, capacity of a disc is measured in megabytes - but how large is a megabyte? Is it a million bytes? a thousand kilobytes? or 2^20 bytes? A thousand kilobytes is 1,024,000 bytes or about 2.5% more than a million bytes; a true megabyte (1,048,576 bytes) is about 5% more than a million bytes. Since there is no enforcable standard for the word 'megabyte', manufacturers of CD-R's get away with claiming almost 5% more capacity than is 'really' there. It's an accounting trick that will not let you store one extra character on the disc.
There are CD-R's with more than 74 minutes' recording capacity. The maximum possible is a shade under 80 minutes. There are also 63-minute discs, but while they are said to be more reliable than 74's, they are again hard to find and no less expensive than the longer ones. Even the 80-mm blanks with about 15 minutes' capacity may reappear. (There are also recordable minidiscs, but this is complicated enough already!) The result is that under all reasonable circumstances, you will be recording on a 74-minute, 650-megabyte CD-R. If you need an 80-minute blank, you will need to hunt it down.
A data CD-R gives the impression that it is written with a FAT and structure just like that of a hard drive, but it isn't. As a result, there is information stored on the CD-R that takes away capacity in hidden ways. The amount 'wasted' depends on the type of recording being made. More to the point, CD-R software does not usually estimate the capacity of the disc precisely. A similar phenomenon occurs with audio CD-R's. As a result, you may be 'cheated' out of some tens of megabytes. Even worse, you may not know about it until you have burned the disc - and find that it cannot be closed.
The size of the Table of Contents (TOC) which consumes CD-R capacity depends on parameters you set when recording. To minimize the wasted space, record DAO. If that doesn't solve the problem, try writing an image to your HD. That image is a byte-level preview of what will be written to the CD-R, so the software need not estimate whether or not it will fit but can actually count bytes and sectors to prove it. Another test (not a perfect one) is to do a test write. Most cases of trying to write more than the disc will hold will be caught in the process and all you will have wasted is the time for the test. You may even run a test on an ISO - just to be certain. As usual, different writing software has different ways to do these things, so check your manual.
When you are preparing to write a CD-R, your software may estimate that you do not have enough room. Some programs simply refuse to proceed. Others inform you and blithely try to do the impossible when you tell them to. If you are near capacity, write the image, then test writing it to the disc you want to use. If that works, cross your fingers over a rabbit's foot and burn your copy. And remember what happened to the rabbit.
Although standard discs are marked 650 MB and 74 minutes, in fact they vary significantly in actual length. The true size the manufacturer designed in - as blocks or another measure - can be found with appropriate software such as the Disc Information of Easy CD Creator 3.x and CD Information of ECDC 4.x.
Overburning refers to stealing some of the space beyond the manufacturer's intent to hold data. The leadout/runout track is standardized at about 13 MB, but not all readers need the full length. With some hardware and software, you may use that space for data or audio - but you should be aware that that may leave the disc unreadable in some drives. This procedure is risky because it leads to apparent success and works often, then can let you down when the disc is most critical to you. I suggest that you look into other choices - such as an 80-minute blank - before overburning even if your system has the capability. Still, if you are in a critical situation and *must* find an extra couple of megabytes, it may be worth trying.
The manufacturer specifies in the ATIP the number of blocks which the disc is prepared to hold. Given the parameters of the spiral, that should take the write laser to the specification limit. In fact, the spiral probably goes somewhat farther than that and you can probably get away with a shorter runout track than specified - so it is possible to overburn.
Overburning has nothing to do with SCSI or EIDE. As I understand the limitation, some drives use the ATIP information to refuse burns beyond the block count and others leave enforcement to the writing software. There is no command to "overburn".
The software commands the drive to write specific information to specific blocks of the disc. That is not on a block-by-block basis because of positioning, but in principle, the software says: put this there - and the hardware does it. There are several cases of interest for a standard (650-MB) blank.
Case 1 is not overburning at all - it is the normal condition. Case 2 means that the hardware precludes overburning Case 3 means that the software precludes overburning Case 4 means that overburning is attempted - leading to four additional cases:
4b-d may also be successful burns since most readers will be happy with less than a full runout track. Any of them is more likely to have created a coaster and, depending on the design of the drive, 4c and 4d may do physical damage.
There are two situations of interest to add to the above when the ATIP indicates a blank longer than 74 minutes. Obviously, no drive will accept an arbitrarily large block number - the maximum value will certainly correspond to a blank shorter than 7400 minutes. There are old drives which were designed with a 74-minute maximum; they will not burn 80-minute blanks at all and the software will not - or, at least, should not - allow you to write beyond that limit. A special case is one group of Sony designs used in some Sony and HP drives. They simply ignore commands beyond a certain point, corresponding to about 78:30 on an audio disc.
In all of this, you need to remember that the reader has only a very limited vocabulary when talking to the computer. It does not shake hands on each transaction or report regularly on the status of its operation. When it knows that something specific has failed, it says so; otherwise, it runs open-loop, trying to implement the commands it receives. It does not report the power calibration it achieved, only that it failed to find one. It does not report the level of its internal buffer, only the fact that it has emptied. And so on.
There are two distinct cases here: a source CD or CD-ROM which is too big to copy, and a set of files under your control which exceeds the capacity of the CD-R. In fact, there's a middle ground of considerable interest.
First, some CD-ROMs are effectively copy protected and cannot be copied at all. (More precisely, some can be copied but the copy won't run.) The mechanisms include physically altering the medium as well as creating software conditions that some or all CD-R software will not handle. If you wish to backup such a CD-ROM and if you have reason to believe that the protection is effected only in the data (not by the physical medium), try other writing software to see if it can do what your preferred program won't. That may cost you some coasters, but you must expect to pay a price for circumventing the vendor's efforts. (Incidentally, there may be information in a newsgroup or mailing list that will be of assistance.)
Next, there are tricks that may be played by the CD-ROM publisher with files and directories that make the amount stored appear to be larger than it is - sometimes by a factor of two or more. If you make an image of the source disc and find that it fits onto your CD-R, you're home free. Incidentally, those tricks may be used for purposes other than to inhibit copying, so paranoia may not be appropriate.
In the category of files under your control, the solution is obvious: Control the files. Save some for another disc. If you are archiving, consider zipping text, database and other low-density types. Resize images or use a bit more compression. For audio, drop a verse or a chorus or repitch the selection. 10 MB is about 1.5% of the capacity of a CD-R. If everything you did were 98.5% efficient, you wouldn't be likely to complain. So, don't do it here, either.
Finally, if you have a disc that is stubbornly just a bit too big to copy, don't waste your time trying to figure out how the publisher did it. Retrieve all the files to your HD and remove the read-only attribute on the data. Now you have control and can apply the same tricks you would if you had created the files in the beginning. Perhaps there's a directory full of information for a platform or OS you don't use; kill it. There may be 500 graphics files you don't care about; scan for any worth having and dump the rest. Use your imagination and your good judgement. (One useful trick if you have a Jaz drive or a blank HD partition: create your reduced disc on that drive and run it. If it works, burn it; if not, make the appropriate adjustments.)
It should not be a surprise to you that Microsoft knows their operating systems - and how to break the rules that they impose.
Much of MS's software is cross-platform: it runs on different systems with only moderate changes depending on the host and its configuration. An easy way to develop that code is to have the common elements in one set of folders developed by one group and have the parts particular to a specific platform developed by a group of specialists in that platform. For ease of control, the platform developers are probably given read-only access to the common code in folders that appear to belong to them. In effect, the developer for a DEC system builds her own CD-ROM using the common code and her own interface to the Alpha. She can then create a CD-R for the Alpha and test it. However, before release the Alpha and Intel (I386) packages need to be combined onto a single CD-ROM for distribution.
It is easiest to keep all the references to common code in the platform-dependent parts unchanged. To accomplish that, MS can and apparently does fake the references in the TOC. That is, the common code appears to be in both the Alpha and the I386 folders. It is actually present only once, but the table which references it gives the appearance of providing two copies. In many cases, that results in a disc which seems to have much more than 650 MB on it. Note that this illusion is developed as a programming convenience, not to provide 'copy protection'. You can probably back up a MS disc which appears to be oversize either by dropping unneeded platforms from your copy or by making an image and burning from there.
E-mail me at cdrecording@mrichter.com
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