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Chip's CD Media Resource Center:
CD Media Background & Basics

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Some CD Media Basics

Understanding the properties of CD media requires some basic understanding of how the discs are constructed and how the various forms (CD audio, CD-ROM, CD-R and CD-RW) differ, both physically and logically. We'll examine each of these forms in turn. There are other CD variants such as CD-i and PhotoCD which I won't bother with for now.

The basic concept of a CD is very much analogous to the old phono discs (also known as "records" or "vinyl") which they replaced. Like a phonograph record, the information on a CD is recorded in a single continuous track that spirals across an entire side of the disc. Phonograph records gradually wore out due to the pressure of the needle or stylus during playback, and a number of serious efforts have been made through the years to invent non-contact physical playback methods, usually involving some sort of optical reading and tracking of the groove. The CD does exactly that, using a laser to read what's on the disc.

But what is on the disc? Rather than driving the playback laser to read out an analog waveform to match the original recorded material (like a phono pickup, or even a LaserDisc), the "groove" on a CD records a stream of bits. The stream of 1's and 0's is represented by various sizes of pits in the reflective CD surface. As the laser beam travels over the edges of the pits, its reflection flickers across a photo-detector. These flickers encode the digital information. The actual encoding depends on the length of the pits and of the lands (a "land" is simply an area where there is no pit).

Unlike a phono disc, the "groove" on a CD runs from the inner part (near the label) to the outer part (near the edge). So your fingerprints around the edges will only affect the last tracks on an audio CD.

CAV and CLV

Phono discs are spun at a constant RPM and therefore have a Constant Angular Velocity (CAV). CAV discs are mechanically simple to play back, but have the unfortunate property that the needle travels at a much higher speed (linear velocity) across the outer grooves than across the inner grooves.

For example, on Side 1 of my Carlos Santana Zebob! LP, track 1 (Changes) starts 14.5cm from the center of the disc. At 33 1/3 RPM, the needle is travelling across the vinyl at the start of track 1 at

Linear Velocity = pi * R * 2 cm/rev * 33 1/3 rev/min / 60 sec/min
                = 3.14 * 14.5 * 2 * 33.33 / 60
                = 50.6 cm/sec

By contrast, the innermost track (Winning) starts at exactly half the radius: 7.25cm. The linear velocity at the start of Winning is therefore only half what it was at the start of Changes: 25.3 cm/sec.

Besides causing more wear on your records, CAV results in a significant loss of information density. The first second of Changes required 50 centimeters of groove, but the first second of Winning required only 25 centimeters of groove. So, roughly, if we could have slowed the record down to half speed for the first track, it would have taken only half as much space on the disc, while retaining the same fidelity as the last track. Ideally, we'd like to continuously adjust the rotational speed of the disc so that the linear velocity of the stylus remained constant. This technique alone would increase the total capacity of the disc by about 50%.

Laserdisc afficionados will recognize the distinction between (CAV) and Constant Linear Velocity (CLV), since Laserdiscs were produced in both formats. CLV LaserDiscs actually have double the capacity of CAV LaserDiscs, for reasons that I don't feel like investigating right now. CDs are always CLV devices.


Last Updated Monday October 15, 2001 17:58:06 PDT