|[ Team LiB ]|
The medium provides space to store information, but a complete storage system requires something more. It needs a standardized method of organizing that information. That organization may take several levels—a physical level and one or more logical levels. The physical level or format determines how densely information fits onto the medium and how it is accessed. The logical level determines how it is addressed and used.
The physical format of the Compact Disc and Digital Versatile Disc shares a characteristic with hard disks that's the same with most disc-based media—the information is arranged around the disc in long arcs. But whereas hard disks and floppy disks make those arcs into concentric tracks, the CD and DVD turn the arc into a long spiral. As a result, CDs don't have tracks like hard disks. They have one continuous track. DVDs have a multiple tracks but only one on each layer.
The difference is that hard disks are meant to be a random-access medium. CDs and DVDs are designed for sequential access. The CD was optimized for storing music, which usually is a continuous and uninterrupted stream of data. The DVD was primarily optimized for video, which also requires a continuous and uninterrupted stream. In scanning a single spiral track, the reading mechanism never has to pause to jog to a new position. As long as it follows the track, it's where it needs to be.
CDs and DVDs also differ from hard and floppy disks in that they do not spin at a constant rate. Drives adjust the speed of the spin so that they read the same number of data pits of the same size in any given period. Technically speaking, hard and floppy disks spin with constant angular velocity. In any given period (say, a millisecond), the disk always rotates by the same-size angle. In contrast, CDs and DVDs are a constant linear velocity medium. In any given period, they spin the same linear length of track past the reading mechanism. As a result, near the center of a CD or DVD, where the diameter is small, the disc must spin faster to present the same length of track to the reading mechanism.
The actual linear velocity used by the original Compact Disc system was 1.2 meters per second. As a result, the spin varied from about 400 revolutions per minute (RPM) at the inner diameter to about 200 RPM at the outside edge. Higher-speed drives, discussed later, spin at multiples of this rate. Under the basic Digital Versatile Disc standard, a single-layer disc spins at a constant linear velocity of 3.49 meters per second. As a result, a disc's spin rate must vary from 600 RPM at the outer edge to about 1200 RPM at the inner edge of the recordable area. Multilayer discs spin somewhat faster, 3.68 meters per second. Although DVD drives in computers may spin at faster rates, the DVD players you connect to your video system are locked to this speed.
With each spin of a disc, the track advances outward from the center of the disc, a distance called the track pitch. In the case of CDs, the track pitch is 1.6 micrometers. The individual pits on the track that encode data bits are at least 0.83 micrometers long. The raw data rate at the basic speed of the CD system is 4.3218 megabits per second. In original form as an audio playback device, the actual data throughput was 150 kilobytes per second (1.2 megabits per sec) after demodulating, decoding, error-correcting, and formatting the raw signal.
DVD storage is considerably denser. Most significantly, all measurements can be smaller because the DVD system uses a shorter wavelength laser (650 to 680 nanometers compared to 780 nanometers used by CDs). The standard DVD track pitch measures 0.74 microns. On a single-layer disc, each data pit is about 0.40 micron long. On double-layer discs, pits measure 0.44 micron. The basic data rate for DVD is 26.16 megabits per second. Subtract out the overhead, and the net rate is about 11.08 megabits per second. Table 18.2 summarizes the physical format of CDs and DVDs.
Both CD-R and CD-RW blank discs (as well as their DVD equivalents) have a physical format already laid down on them. Each has one continuous spiral track at the standard pitch (1.6 microns for CDs). In addition, this track wobbles with a period of 22.05 kilohertz and an excursion of 0.3 microns. That is, the smooth spiral actually acts as a superimposed S-shape. The CD drive detects this wobble and uses it to control the rotation rate of the disc. The wobble is frequency-modulated by a one kilohertz signal that provides an absolute timebase for the drive.
|[ Team LiB ]|