This is Side A of the National Disc, although, as with the Community Disc, the actual contents written onto the disc surface are those of side B. You may notice that the disc surface looks different to the Community Disc. This is because side B of the National Disc contains CLV data instead of CAV.

The difference between CAV and CLV discs is the way the video content is encoded. With CAV discs, video is stored as a set of individual picture frames, played back at 25 frames per second. This allows higher quality pictures and special playback effects like reverse play, slow motion, and individual picture search and freeze-frame, but reduces the capacity of the disc. All the sources I have read give a maximum capacity for a CAV disc of 54,000 frames, or 36 minutes at 25 frames per second, but I do have a couple of CAV discs with over 55,000 frames.

By contrast, a CLV disc cannot perform the special playback effects and has the content time-coded rather than numbered by picture. The capacity of a CLV disc is higher, officially being able to hold 60 minutes of video per side, although I have quite a few discs with over an hour; the maximum I have come across so far is about 63 and a bit minutes.