Constant Linear Velocity

Electronics Technician Volume 06-Digital Data Systems
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Figure 11-2.—A cross section of CD-ROM with data areas defined. beam is over a land, the light is reflected back to a photodetector. When the laser beam is over a pit, the light is defused and not detected by the photodetector. The data on a CD-ROM is written in a continuous spiral, much like the groove of a phonograph record, and was adapted from the CD audio standard. The data track is 0.5 micrometers wide. The space between the turns of the track is 1.6 micrometers. This equates to a track density of 16,000 tracks per inch (tpi) and a maximum of 640 megabytes per disc. The actual capacity of a CD-ROM is dependent on the mode used to produce the disc. Two modes of recording data on a CD-ROM are currently in use. Mode 1 writes 2,048 data bytes per sector, followed by error connection codes. Mode 2 writes 2,336 data bytes per sector and eliminates the error correction codes. In chapter 2 of this manual, you saw that a disk is divided into tracks and sectors. The disk rotation speed is constant and data is accessed by defining the track and sector. On a CD-ROM disc, the data is also stored in sectors of 512 bytes. The size of the sectors on a CD-ROM disc remains the same, regardless of the physical location of the sector. The spiral increases in size as it winds toward the outer edge of the disc, thus the number of sectors per rotation increases. Constant Linear Velocity Constant linear velocity is the technique that the CD-ROM drive uses to access data from a disc. To properly read the data from the disc, the speed of the disc must decrease as the laser moves to the outer edge of the disc. Rotation speed of the disc while reading the inner tracks is approximately 500 rpm. As the read head moves to the outer edge of the disc, rotation speed decreases to 200 rpm. Sector addresses on CD-ROM are adaptations of the CD audio standard and are recorded on the disc in terms of minutes, seconds, and sector (minute:second:sector). To find a sector, the read head is slewed to the approximate position of the data, the rotation speed of the disc is adjusted, and the drive reads the position data in the header of the next sector to determine the location of the read head. The read head is then fine positioned to the desired location by repeating this procedure until the proper sector is found. This process can lead to access times of about 1 second. Once the proper sector is found, data transfer is 150 to 300 kilobytes per second, depending on the type of CD-ROM drive. These relatively slow access times and data transfer rates are among the biggest problems with CD-ROMs. Manufacturers are striving to improve these rates and have introduced double-speed, triple-speed, and higher multiple-speed drives. Eight-to-Fourteen Modulation The eight-to-fourteen modulation technique for encoding data on a CD-ROM disc was developed to increase the accuracy of the data read from the disc. Each byte has a corresponding 14-bit code. When the disc is manufactured, the data is recorded in the eight-to-fourteen code. When the data is read from the 11-3

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