CDROM The Granddaddy of Optical Media

The CD-ROM is the oldest of the major optical media types, and it's still in common use today. All the more advanced drive types described in this chapter can read CD-ROM media, so you need not discard existing CD-ROM discs when it comes time to upgrade to a more capable drive type.

CD-ROM Design

The compact disc (CD) was developed in the 1980s as a medium for storing music. This fact had certain consequences that have influenced the way the CD medium was subsequently adapted as a computer data storage device. Most importantly, a music CD requires that data

Chapter 7

come off the disc at a constant rate. Two ways to accomplish this goal using a circular medium are in common use, assuming data are stored in sectors of fixed size:

• Fixed number of sectors per rotation Consider a medium that is broken down into sectors and cylinders (or tracks), as shown in Figure 7.1. Suppose the disc is spun at a constant rate, no matter what portion of the disc is being read. This design is known as constant angular velocity, and it causes data to be read at a constant rate. Most floppy disks use this approach, as do some older hard disks and one format used by video laserdiscs. In some sense, so do old-style LP records, although they use a single spiral track rather than multiple cylinders, as depicted in Figure 7.1. The drawback to this 7

approach is that it doesn't make optimal use of the recording medium. In order to record data reliably on the inner cylinders, the data density on the outer cylinders is much lower pT

than the medium is capable of maintaining. Packing more data into the outer cylinders CA

allows the storage of more data on the medium, but when the disc is spun at a constant D

angular velocity, the data transfer rate becomes variable from one cylinder to the next. §

This trade-off is acceptable for hard disks, but not for an audio recording medium. S

• Fixed linear sector size In order to pack more data onto a circular medium, it's possible to vary the number of sectors per cylinder. This approach is taken with modern hard disks. For absolutely optimum data storage capacity, it's useful to adjust the number of sectors for each cylinder. In CDs, in fact, the concept of a cylinder is eliminated, in favor of a single spiral track. Imagine a long track wound around a central core, as shown in Figure 7.2. This is the approach used by CDs. The difficulty with this design is that, to play music from the CD, it's necessary to spin the disc at a variable speed (in terms of rotations per minute), faster for reading the inner areas and slower for the outer areas. This requirement increases the complexity of the drive's design.

This spiral track design works well for audio CDs, but it's less than optimal for use as a more general-purpose data recording medium. The reason is that it can be difficult to locate specific tracks without precisely defined cylinders and sectors that begin at precisely-defined positions in the medium's rotation. For this reason and to add error-correction codes for increased reliability, data storage formats for CD-ROMs devote some space used for data in audio CD formats to other purposes. Audio CDs use a sector size of 2352 bytes. For data CDs, this 2352-byte sector is broken up into 2048 bytes of data plus 304 bytes of error correction and other codes.

Part II

Figure 7.1

Sectors on outer cylinders are physically larger in size with a fixed number oof sectors per cylinder, but sectors retain the same capacity on inner and outer cylinders.

Variable Sector Size

Figure 7.1

Sectors on outer cylinders are physically larger in size with a fixed number oof sectors per cylinder, but sectors retain the same capacity on inner and outer cylinders.

A standard 4.72-inch CD can hold 74 minutes of audio data as an audio CD or 650MB of data as a CD-ROM. It's also possible to wind the tracks slightly more tightly than normal in order to increase the CD's capacity, but CDs that can store more than 700MB, or 80 minutes, are quite rare. Increasing the disc's capacity past 650MB runs the risk that the disc won't be readable on some CD players or CD-ROM drives.

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