[davidferguson]

Indeed it is! Films come on 20 minute reels, and from the late 80s onwards were spliced together onto one big reel (or platter) for playback. However the splice often wasn't perfect, and the reel ends are most likely to contain dust/dirt/scratches, which could knock out the digital audio and revert playback to the analog track. This obviously isn't ideal, so Dolby had the idea of encoding the area at the end of each reel throughout the entire reel too, which is stored in a cache in the processor. Then, when the end of the reel is reached (you can tell this because each block has a sequence number), the processor uses the cached data to ensure a nice changeover.