CLV and CAV

Data Track

CD-ROM data is recorded in a single spiral track that starts from the inner surface the disk all the way to the outer edge. This is almost exactly the same way that standard LP vinyl records are recorded except that LPs are recorded from the outer edge in. Also note that CD-ROM data is digital, while a vinyl LP is analog.

When you first insert a CD-ROM disc into the drive or player, the drive positions the read head near the center of the disc and looks for a "Table of Contents" that will tell it what mode type and form the disc is. From there it will load in and display to the user the track information.

All of the information about the number of tracks on the disc, and the length (either in time or data capacity, depending on the type of data the disc holds) of each track is stored in the table of contents. The table of contents allows the drive to determine the start and end positions of each track and can randomly seek each track as a result.

Constant Linear Velocity (CLV)

All standard CD-ROMs use "Constant Linear Velocity," or CLV for short. Because CDs were originally designed to read audio data, the information was designed to be read at a constant rate. This means that the data track always has to be moving past the laser lens head at a consistent speed.

Since the inner tracks are shorter than the outer tracks, the disc has to be spun faster when reading the inside tracks to ensure a consistent data stream is being presented to the player. This is achieved by varying the rotational speed of the disc. The disc spins quickly when reading from the inner tracks, and the rotational speed of the disc gradually decreases as the drive advances towards the outer tracks.

Constant Angular Velocity (CAV)

While CLV recording maintains that the data be moving past the laser lens at a constant linear speed, CAV recording maintains that the disc itself be spun at a constant speed In this manner, data on the outer tracks is passing by the laser lens head at a faster rate than the inner tracks. This scheme would be impractical for audio data, since audio information needs to be sent to the player at a constant rate to maintain uninterrupted playback.

The benefit to CAV is that since the disc spins at a constant rate, there's less wear and tear on the disc spin motor, and CAV drives are also quieter as a rule. All current CD-ROM drives are CAV type drives, and are capable of reading back CLV recorded data. These drives are commonly referred to as "P-CAV" or "Partial-CAV."

The first generation Laser Disc players were CAV types. At that time, the technology gained a reputation as being unreliable. Head mis-tracking and skipping was commonplace because, over the course of time, the laser head would lose its alignment and malfunction. This was especially a problem with the popular Laser Disc games of the early 80s (ala Dragon's Lair).

These players were in constant operation. Combine that with the shocks they had to endure from "aggressive" users, and you have a couple failure prone drives. Later refinements were made to improve the reliability of the laser disc player, but this did nothing to stave off the impending death of the LD format at all.