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The average access time specification for CD-ROM drives has long been seen as one of the keys to evaluating performance. But the average access time, with its definition taken from hard drive performance measurements and adopted by CD-ROM drive manufacturers, is not guaranteed to have a consistent assessment methodology. Obviously, each manufacturer's interest centers on presenting the best specifications, but the CD-ROM consumer would benefit from a standard testing procedure.
For hard disk drives, access time defines the time between the request for data and the reading of that data, measured in milliseconds; more specifically, access time equals seek time plus latency plus translation time. Seek time consists of the time necessary to locate the track/sector of the data. Latency refers to the delay for the disk rotation--the wait for the requested data to arrive at the read/write head. Translation time measures the time to read and transfer the sector of data. Access time always appears as an average since the information requested is typically found at many locations on the hard disk drive.
CD-ROM AND ACCESS RATES
CD-ROM manufacturers adopted the average access time specification established for hard disk drives. The prominent feature impacting this specification is rotation, and spinning the disc by double and quadruple speeds decreases the latency because the data sector comes around again sooner. However, this is only part of the rotation influence because compact discs have one continuous, clockwise spiral track (starting in the center or inside of the disc) that is divided into sectors that remain the same size and contain the same quantity of data. This scheme enables more sectors of data to be stored.
Since the data must be read at a constant rate, sectors on the outside of the disc would spin too fast for the read head if the disc spun at a constant speed. The solution to this problem is variable rotation rates that speed down the disc to read outside sectors and speed up the disc to read inside sectors. This functionality, named Constant Linear Velocity (CLV), must be efficient to achieve a low average access time.
Hard disk drives...