Cache and Zen

Buffers and Cache

All CD-ROM optical devices contain a small amount of cache memory located right on the drive's controller board. Most drives contain somewhere between 256KB to 1MB of cache, depending in the speed of the drive, and the manufacturer. The cache is used to buffer the data from the disc, stacking it up into larger blocks before sending it out to the computer for processing. This assures that the drive is able to send a steady stream of data for the computer to process.

Using the cache to buffer data allows the drive to read ahead and keep on top of the needs of the computer, and relieving the controller of a burden. Without the cache, the drive would be forced to attempt to synchronize a stream from the disc to the system. If the system hit a scratch that it could not read correctly the first time around, the results would be clearly visible as the user attempted to playback from the disc.

Consistency is the key

Because of the way that audio CD-ROMs are encoded, the highest transfer rates can only be attained on a standard CLV CD-ROM drive when reading from the outer edges. Near the inner edge of the disk, the drive must be spun at a slower rotational speed in order to assure that the data is read correctly. As the drive progresses to the outer tracks, more data can be read because more surface area is covered at the same rotational speed.

CD-ROM data is recorded from the inner to the outer edge. This means that most of the time, your CD-ROM is going to be reading slower than its maximum speed. To maintain a consistent reading speed, the disc spins faster while the inner edge of the disc is being read and slows down as the drive progresses to the outer edge, but data throughput is always higher near the outer edge.

Everyone needs a little Zen

True-X technology takes a different path from conventional CD laser lens units in an attempt to achieve maximum transfer rate regardless of where the data is on the disc. True-X technology was developed by Zen Research, who has licensed it to Kenwood Technologies. Kenwood is currently the exclusive manufacturer of True-X CD-ROM drives. Hi-Val was selling Kenwood drives under the Hi-Val brand name, but has since discontinued them.

Traditional CD-ROMs increase their speed by spinning the disc faster. True-X uses a multi-beam laser head, which splits the laser into several beams that read several tracks of data at once. True-X is capable of reading data at an overall higher rate. True-X can be employed to both CLV and CAV, but Zen chose to base True-X on CLV so that the disc speed can be varied as needed to attain the highest possible data throughput.