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Essential Concepts in CD Recording

A CD is only a medium on which to store information; you can think of it as a very large, write-protected floppy disk. The difference is that, while DOS floppy and hard disks are written using the DOS file format, CD-ROMs are written using a standard format called ISO 9660. This standard is so widely accepted that it can be read back on any computer platform including DOS, Macintosh, and UNIX. This is one of the advantages of ISO 9660.

In an ideal world, you shouldn't have to think about ISO 9660 at all when you write a CD; there should be an operating system command similar to the DOS COPY command which would simply copy files from hard disk to CD. The world of recordable CD-ROM isn't ideal quite yet, so you need an entire software package to do the job. However, we've done our best to make it easy for you.

What Kind of Information Can a CD Hold?

The simple rule of thumb is: whatever you have stored as a file on any other storage medium can also be stored as a file on an ISO 9660 CD-ROM. To ISO 9660, a file is a file, and ISO doesn't care whether the file contains pictures, text, or sound. There are a few special cases where you would want to record a file to CD in a special format. The most obvious is CD-DA audio: if you want to record an audio file to CD so that it could be played back by your home stereo, you need to write a CD-DA (Digital Audio) disc.

How Much Information Can a CD Hold?

When you copy data to a CD, you must take care that the your data does not exceed the capacity of the CD you are recording to. Due to the audio origin of CDs, the amount of information a CD can hold is measured in minutes:seconds:sectors. Each second contains 75 sectors, each of which can hold 2048 bytes (2 kilobytes) of Mode 1 user data. Recordable CDs come in 21-minute (80 mm diameter), 63-minute, and 74-minute sizes (both 120 mm diameter), which can contain:

21 min x (60 sec) x (75 sectors) x (2 kbytes) = 189,000 kilobytes = 184 megabytes

63 min x (60 sec) x (75 sectors) x (2 kbytes) = 567,000 kilobytes = 553 megabytes

74 min x (60 sec) x (75 sectors) x (2 kbytes) = 660,000 kilobytes = 650 megabytes

Factory-recorded CDs can hold up to 74 minutes of audio.

Using the CD Size command in the Edit menu, you can set the capacity of the CD you want to produce so that you don't exceed it. The status bar shows you how much space is used, how much is still free, and the percentages of each.

And Subtract...

Files on CD do not occupy a space exactly equal to their original size, but usually a bit more. This is because the minimum recordable unit on a compact disc is the logical block. In theory a logical block could be 512, 1024 or 2048 bytes in size (that is, that you could fit 1, 2, or 4 logical blocks into a sector). In practice, MSCDEX reads only the 2048- byte block size. This means that a file will occupy a space equal to the closest (higher) multiple of 2048 bytes. In ISO, just as in the DOS file system, directories are also files, and also take up space.

The Yellow Book (the standard defining the physical format of CD-ROM) specifies that the CD data starts after a pause of two seconds. This means that the first two seconds on a CD are not available for user data. So, from the theoretical capacity of any CD you must subtract: [(2 sec) x (75 sectors) x (2 kilobytes)] = 300 kilobytes Furthermore, the ISO 9660 file structure needs space for the following:

  • Root file: Minimum 1 sector
  • Path Tables: Minimum 2 sectors
  • Primary Volume Descriptor: 1 sector
  • Volume Descriptor Set Terminator: 1 sector
  • The first 16 sectors are also reserved for system use.

More sectors may be needed to store the Path Tables, or the root directory if its size exceeds one sector as files are added. All this adds up to some space which will not be available for your applications and files, a fact which you must keep in mind when determining how much information you can fit onto the CD.

For Further Information

If you really want to know all about CD-ROM in deep detail, we humbly suggest that you read our book, "Publish Yourself on CD-ROM" by Fabrizio Caffarelli & Deirdré Straughan. It explains all and more than you ever want to know about CD-ROM in all its incarnations, and is published by Random House Electronic Publishing, ISBN Number 0-679-74297-2.