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About Laserdiscs, part 1

Laserdiscs come in two varieties: CAV (constant angular velocity) and CLV (constant linear velocity).

As its initials imply, a CAV laserdisc spins at the same speed no matter where on the disc it is being read from. Since laserdiscs play back at a fixed data rate, this implies that data is packed more tightly near the center of the disc than it is toward the outer edges.

If you remember your basic geometry, the circumference of a circle is directly proportional to the radius (C = K × r). Let's say the disc spins just fast enough so that one rotation holds one video frame. So if you encode one frames' worth of data at r=5, you have to pack it into a linear distance of 5K. If you encode the same amount of data at r=8, you have much more room (8K) to store it in.

In contrast, a CLV laserdisc packs its data at the same density regardless of its location on this disc. This means that in order to read it, a laserdisc player must adjust the rotation speed depending on how far away from the center of the disc it is reading. Compact discs are CLV devices and you probably recognize the disc speed changing as you seek back and forth.

Back to the geometry, in CLV discs data is read at a constant rate (R) per unit of circumference (rate = R × C). Since the circumference is proportional to the radius, this makes the data rate proportional to the radius as well (rate = R × (K × r) = RK × r). This means that at r=5, the disc stores 5RK units of data, and at r=8, the disc stores 8RK units of data.

It's pretty clear that CLV discs pack the data more efficiently than CAV discs. This is because with CLV discs, you can pack the data at the maximum rate the player is capable of reading consistently across the entire disc. Whereas with CAV discs, the data can be efficiently packed at the inner part of the disc, but as you move farther away from the center, it gets increasingly less densly packed. In reality, CAV discs maxed out at about 30 minutes (54,000 frames) of playback per side, while CLV discs generally got about 60 minutes per side.

Given these facts, why on earth would you ever create a CAV disc, when a CLV disc allows you to pack the data more efficiently? Well, because the simplicity of finding information on a CAV disc enabled many special features: still frames, reverse play, slow motion, and — most importantly for laserdisc-based video games — direct access to any frame on the disc by index.

With a CAV disc, each video frame happens to corresponds to one rotation of the disc. Thus, if you want to advance 100 frames ahead, you simply moved the laser up the appropriate amount and read the data there. On the other hand, with a CLV disc, advancing 100 frames involves knowing how fast the data is coming and computing where to seek and how much to adjust the rotation speed to find the target, a much more complicated maneuver.

When it comes to movies, most laserdiscs were produced as CLV, to minimize the number of times you need to flip or change discs. A few special edition and high-end versions of movies were released as CAV, enabling access to nice still frames and other effects.

When it comes to videogames, however, it's all CAV. In the next article, I'll ignore CLV discs entirely, and talk about tracks, frames, and VBI data on CAV discs.