CD-ROM CAV

CLV remained the dominant CD-ROM technology until Pioneer, the first with quad-speed when most only had single, skipped six- and eight-speed entirely in favour of the new ten-speed speed technology employed by its DR-U10X drive, launched in mid-1996. The clever part was that the drive operated not only in conventional CLV (constant linear velocity) mode, but also CAV (constant angular velocity) mode, transferring data at a variable rate while the drive spins at a constant rate, like a hard disk.

Access time has a big influence on overall performance. As the speed of a CLV drive increases, access times often suffer as it becomes harder to perform the abrupt changes in spindle velocity needed to maintain a constant high data transfer rate, due to the mass inertia of the disc itself. CAV maintains a steady spin speed resulting in increased data transfer rates and reduced seek times as the head moves towards its outside edge. While early CLV drives had average access times greater than 500 ms, modern CAV drives typically have average access times less than 100 ms.

Pioneer’s revolutionary design allowed operation exclusively in either CLV or CAV mode, or in a mixed mode. In the latter, CAV was used for reading close to the centre of the disc while the drive switched to CLV mode for reads closer to the outer edge. The Pioneer drive signalled the end of the line for CLV-only designs and the so-called Partial CAV drives assumed the mantle of the leading CD-ROM technology.

This remained the case until the advent of a new generation of DSP (digital signal processing) chips capable of handling faster than 16-speed data rates and, in the autumn of 1997 Hitachi launched the first CD-ROM drive to use Full CAV technology. This overcame many of the problems inherent in Partial CAV designs, obviating the need to monitor the head location and step the motor speed up and down to maintain a steady DTR and making access times more consistent because there is no waiting for the spindle speed to settle between transitions.

Most of the 24-speed full CAV CD-ROMs on the market by the end of 1997 spun discs at a constant 5,000rpm, achieving a DTR of 1.8 MBps at the centre increasing to 3.6 MBps at the outer edge. By the summer of 1999 outer track DTRs had been increased to 48-speed, or 7.2 MBps, as a result of increasing spindle speeds to an incredible 12,000 rpm – comparable to the rotation speed of many high-performance hard disk drives.

However, one of the main problems with spinning CD-ROMs at such high speeds is excessive noise and vibration, often including loud hissing noise caused by air being forced out of the drive casing by the spinning CD-ROM. Because the CD-ROM is clamped at its centre, the most severe vibration occurs at the outer edges of the disc – at exactly the point where the decoding circuits have to handle the highest signal rate. Also, since few CD-ROMs actually have data stored on their outer edges these high spin-rate drives rarely achieve their theoretical maximum DTR in real-life.

CD-ROM CAV

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