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CD-ROM CLV

The first generation of single-speed CD-ROM drives were based on the design of audio CD drives, employing constant linear velocity (CLV) technology to spin a disc at the same speed as an audio CD which, with error correction, meant 150 KBps.

Since there are more sectors on the outside edge of the CD than in the centre CLV uses a servo motor to slow the spin speed of the disc towards the outer tracks in order to maintain a constant data transfer rate over the laser read head. The internal memory buffer of the drive controls this by using a quartz crystal oscillator to clock the data out of the buffer at a specific rate and maintain it at a 50% capacity while data is being read into it. If the data is being retrieved too fast, the 50% capacity threshold will be breached, so a command is sent to slow the drive motor down.

While audio discs have to be read at single-speed, there is no limit for the CD-ROM. Indeed, the faster the data is read off, the better. As CD-ROM technology progressed, speeds increased at regular and seemingly ever-shorter intervals until, by early 1998, the fastest drives were capable of 32-speed, a data transfer rate of 4.8 MBps.

A quad-speed drive employing CLV technology, for example, span the disc at around 2,120rpm when reading the inner tracks and 800rpm when reading the outer tracks. Variable spin is also necessary to cope with audio data, which is always read at single-speed (150 KBps), whatever the transfer rate for computer data. The key issues with spin speed, then, are the quality of the spindle motor that rotates the disc and of the software that controls its operation at varying rotational velocities, as well as the ability of the read head positioning system to move quickly and accurately into position to begin accessing data. Unless the drive’s engineering and electronics are up to the job, merely increasing the basic spin speed won’t deliver the performance boost which might otherwise be expected.

A related factor is the level of utilisation of the host PC’s CPU: as the spin speed, and therefore the data transfer rate, increases, so does the amount of the CPU attention required to process the data streaming off the CD-ROM. This means that if other tasks are accessing the processor at the same time, then there’ll be less processing power available for the CD-ROM drive, and transfer rates will fall. A well-designed CD-ROM drive system should seek to minimise the CPU utilisation for a given spin speed and data transfer rate. If it doesn’t a fast CD-ROM drive might have inferior overall performance than a slower-speed model.

The size of data buffer is another widely-quoted statistic; a big on-drive buffer – 1MB as opposed to 128K, say – is certainly desirable for boosting burst data throughput, but unless this is controlled by high-quality firmware, the resulting small performance increase will hardly justify the cost of the extra buffer memory.

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