Since compact discs store audio in a digital format they are suitable for storing other digital information. In 1984, Philips and Sony released the Compact Disc Read Only Memory (CD-ROM) specification, known as the Yellow Book.

The physical parameters of CD-ROMs are identical to those defined in the Red Book. The use of the data it contains is different. While audio CDs can be played at only one speed, CD-ROM drives exist with a range of speed options from normal (1x) to four times normal speed (4x) and so on and now, 52x. As the speed increases the access time decreases.

CD-ROM discs differ from CD audio discs in two important ways.

The data on a CD-ROM disc are divided into sectors which contain both user data and other data for control and (additional) error protection. The data on a CD-ROM are contained in files. All CD-ROMs therefore, need a file system to enable the computer to access the required file easily and quickly.

Generally, the user need to alter the sector structure but should be aware of the file structure.

A CD-ROM has several advantages over other forms of data storage, and a few disadvantages. It can hold 700 megabytes (MB) of data, the equivalent of nearly 500 high-density floppy disks. The data on a CD-ROM can be accessed much more quickly than data on a tape, but CD-ROMs are considerably slower than hard discs. Like audio CDs you cannot write to a CD-ROM (but CDRW recordable discs do exist), so they are only used for prerecorded data.

CD-ROM Input Specification

The CD-ROM specification is defined in the Yellow Book which is based on the CD audio format but for computer data. The salient parameters are shown below:

Parameter	Value	Comments
Data capacity	682 MB	74 minutes
Raw data bit rate	1.41Mb/s	all bytes
User data rate	150 kB/s	at normal speed
Block (sector) size	2352 bytes
User data per sector	2048 bytes full error correction
Sector rate	75 sect/s	at normal speed
Sector Modes	1 or 2
Sector Forms	1 or 2	only with Mode 2

You must use "Disk at Once Mode" when burning your data onto CD-R. Do not attempt to send us masters using "Track Incremental" or "burn session". Disc at once mode finalizes the disc and prevents further additional data recording.

Storage capacity of our CD-ROMs

682 Mbytes* of user data in CD-ROM mode 1 (2048 bytes/record) OR
up to 78 minutes of digital audio @44.1Khz OR a combination of the above (enhanced CD).

CD-ROM Sectors & Modes

Data stored on a CD-ROM disc are divided into sectors which are equivalent to the audio frames for a CD audio disc. At normal playback speed 75 sectors are read every second. For double speed CD-ROM drives this increases to 150 sectors per second and so on. Seek times, while the disc rotates to the required starting position, will also reduce as speeds increase.

Because CDs were designed primarily for audio, their use for computer data requires the addition of header data and error correction codes which are included in every sector. There are two different types of sectors defined in the CD-ROM specification, mode 1 and mode 2 (the latter being used for CD-ROM XA discs).

Mode 1 Sectors

Mode 1 sectors are intended for the storage of computer data. They are comprised of:

• Header which consists of Minutes, Seconds, Sectors (frames) and Mode (= 1).
• 2048 bytes of user data.
• Error detection and correction codes to give the CD- ROM data extra protection.
• While CD audio players are able to conceal errors that are not correctable which may result from a scratched disc, CD-ROM
  data (especially application code) needs to be error free.
• Mode 1 sectors are the simplest type and are used for most CD-ROM based formats which follow Yellow Book guidelines.
  

CD-ROM Based Formats

The first CD-ROM discs contained Mode 1 sectors designed for general computer storage on CD. Although not specifically designed for multimedia there are a number of important multimedia formats which use mode 1 CD-ROM as the basic method for storing data on CD.

Format	File system	Comments
PC CD-ROM	ISO 9660	Most common format
MPC 1 & 2	ISO 9660	Specifies PC configuration
Mac CD-ROM	HFS	Based on Mac operating system
3DO	Proprietary	Data on disc is encrypted
CD32	ISO 9660	Not 100% ISO 9660
Mixed Mode	ISO 9660	Audio and Data combination

Test your CD-ROM - It is vital that you test your CD-ROM one-off master on as many different machines as possible. If you follow the formats listed in this section then you should have success. While we at Bizemedia try vary hard to ensure your satisfaction with project, we cannot be held responsible for discs that fail due to improper pre-mastering.

A good suggestion is to load your CD-R master on a freshly formatted PC without any additional software or drivers. You can then test your program on a "common base" machine with the knowledge that any missing drivers or utilities will result in an error, allowing you to add the drivers to the master or make adjustments.

All CD masters are scanned for 3rd party software to prevent anti-piracy and you must supply an IPR Copyright form with each master.

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Some of this material has been copied from a number of CD definition websites - we can't thank them all individually but we do acknowledge their assistance and we hold no copyright on the text or information.