The underlying technology of CD-ROM is markedly similar to that used in Philips’ first optical video disc system demonstrated in Eindhoven as early as 1972. This research and development led to LaserVision. CD-ROM is a reflective light system, where laser light is shone against turns of track which are encoded with digital data using pits and areas of land. Pits scatter the laser light, while areas of land produce reflected light. Reflected light is diverted to a photodetector that produces a series of electrical pulses corresponding to encoded data.

A CD-ROM disc rotates at a speed to provide a constant average read rate of around 75 blocks/sec. The disc must spin faster when the read head converges on its center (vice versa ). This is quite unlike traditional  vinyl audio discs which rotate at a constant angular velocity (CAV). The required constant linear velocity(CLV) is achieved by coordinating the position of the read with speed of rotation using a simple feedback system.

Like all random-access, storage devices, encoded information has to be organized into manageable data blocks so as it may be located and retrieved. The amount of user data that CD-ROM data blocks provide is a function of Mode of operation. A Mode 1 disc, for example, will yield just 2 Kbytes (2048 bytes) user data, while Mode 1’s lower user data is due to demands imposed by error detection and correction codes. Conventional CD-DA(Compact disc-Digital Audio) data blocks comprise 2336 bytes of digital audio.

The addition of error detection and correction codes makes Mode1 discs more suited to data highly sensitive to error, such as computer software, in multimedia, Mode 2 is commonly used for the storage of video and audio data, as its accuracy is not so critical.

The average user data transfer rate of a disc can be calculated by applying the following simple formula:

Average user data transfer (Kbytes/sec)= blocks/sec * user block data (Kbytes)

Mode 1:

Average user transfer rate = 75*2

                                          = 150 Kbytes/sec

Turning to Mode 2 discs, we have a noticeable improvement brought about through increased user block data:

Average user data transfer rate = 75*2.28

                                                  = 171 Kbytes/sec

CD-ROM Addressing and User Data Capacity

        CD-ROM addressing is carried out using measurement of time and data blocks read. Minutes, second and blocks provide enough information to locate any item of information. (Note, these are inherited directly from CD-DA). For example, considering a

One hour CD-ROM, absolute addressing would take the following form:

            Minutes (M): 0-59

            Seconds (S) : 0-59

            Blocks   (B) : 0-74

A track beginning mid way through the CD-ROM, for instance, is addressed 29:29:37 (M:S:B).

            The length of a CD-ROM in minutes and the data block size in terms of user data, can be used to calculate user data capacity with pinpoint accuracy. For example, consider the following simple formula:

User data capacity =discs length (seconds)* blocks read/second * usable block data

            Applying this formula to calculate the precise data capacity of one hour, Mode 1 disc gives:

User data capacity = 3600 * 75 * 2048 bytes

                              = 552960000 bytes

                              = 527 bytes

Similarly, the capacity of a one hour, Mode 2 disc:

User data capacity = 3600 * 75 * 2336 bytes

                              =  630720000 bytes

                              = 602 Mbytes

            Once again, it is important to recognize the difference between ‘data capacity’ and ‘user data capacity’, of which the former will always be significantly larger, and even more so using a Mode 1 disc. Drive specification sheets consistently fail to differentiate between the two. It should be noted that, in most instances Mode 2 data blocks have to coexist with Mode1 data blocks on the same disc. Such mixed mode discs are necessary where both software (retrieval system, for example) and video and audio are same disc. It can be assumed, therefore, that technically a solely Mode 2 disc is not possible.