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About Laserdiscs, part 2

All known laserdisc-based videogames use CAV (constant angular velocity) laserdiscs, primarily because these discs could be manipulated in advantageous ways. Players of the era could seek to specific frames, play at different speeds, still frame, play backwards, and perform all sorts of other operations that were not possible with CLV discs.

The reason this was possible is that on a CAV disc, each track corresponds exactly to one frame of video. There are approximately 54,000 tracks on one side of a CAV laserdisc, giving 54,000 frames of video per side. Now, believe it or not, laserdiscs are actually analog devices, meaning that the signal that is stored on them is analog, and is described exactly by the video transmission data standard of its intended country (NTSC or PAL, depending on where you live). Thus, once the data is read from the laserdisc, it is basically identical to the signal that you would receive over the airwaves for watching regular broadcast TV.

Let's step back a moment and think about how laserdiscs of movies work. A movie is traditionally filmed at 24 frames per second. In contrast, the NTSC broadcast standard (which describes how video is transmitted in North America) describes the TV signal as running at 30 frames per second. Furthermore, each frame is built up of two "fields", which are drawn one after another at a slight offset. This is known as interlacing.

Since there are two fields each frame, and the frames run at 30 per second, the result is that the individual fields are transmitted at twice that rate, or 60 times per second (in reality, it is 29.97 frames/second and 59.94 fields/second, but we'll stick to round numbers).

So the question becomes, how do you reproduce a film, running at 24fps, within the NTSC standard, which runs at 30fps (or more precisely, 60 fields/second)? If you transferred one frame of film to one frame (two fields) of NTSC video, the result would play back at 30fps, or 25% too quickly. If you duplicated each film frame onto two frames (four fields) of NTSC video, the result would play back at 15fps, or 38% too slowly.

The solution that engineers came up with many years ago was to realize that 60 / 24 = 2.5. That is, when you look at it in terms of fields instead of frames, each film frame should conver exactly 2.5 fields of NTSC video. However, since you can't split the video midway within a field, they chose instead to alternate between using 3 fields per film frame and 2 fields per film frame. So the first film frame will be duplicated onto the first 3 fields, while the second film frame will be duplicated onto the next 2 fields, and so on. This is known as 3:2 cadence.

The important thing to realize out of all of this is that there was not always a 1:1 relationship between film frames and NTSC frames (though there often was, for example, when the laserdiscs contained video that was originally produced for broadcast TV). That is, if you wanted to view frame #1000 of the actual film, you could not necessarily just seek to NTSC frame #1000 and end up where you wanted. Instead you actually had to go figure out which NTSC frame corresponded to film frame #1000.

When the laserdisc was designed, this discrepancy was actually taken into account. The designers realized that being able to seek to a given NTSC frame was actually not nearly as useful as being able to seek to a particular film frame, and so they devised a way to encode information about the frame numbers on the laserdisc itself.

Which brings us back around to games. For years, laserdisc emulation has relied on "frame files" and "conversion equations" to determine how to map the film frames (which is what all the seek commands target) to NTSC frames (which is what you get when you capture video). But the information about which NTSC frame corresponds to which film frame was present all along in the video signal itself; we were just not aware of it, nor aware of how to capture it.

In the next article, I'll talk about how this information is encoded.