A look at ACT-L2

This is yet another video codec from the 90s used for streaming and completely forgotten now. But since I had nothing better to do I decided to look at it as well.

Essentially it is another H.263 rip-off with a twist. From H.263 it took overall codec design (I/P-frames, 8×8 DCT, DC prediction, OBMC) but the data coding is special. For starters, they don’t use any codebooks but rather rely on fixed-width bitfields. And those bit values are not written as they occur but rather packed together into separate arrays. There’s a way to improve compression though: those chunks can be further compressed using binary arithmetic coder with an adaptive model to code bytes (i.e. you have 256 states and you select state depending on which bits you have already decoded).

Additionally it has somewhat different method of coding block coefficients. Instead of usual (zero-run, level, end-of-block) triplets assigned to a single code it uses bit flags to signal that certain block areas (coefficients 0-3, 4-7, 8-11 and 12-63) are coded and for the first three areas it also transmits bit flags to signal that the coefficient is coded. And only the last area uses zero-run + level coding (using explicit bitfields for each).

Overall it’s an interesting idea and reminds me of TM2 or TM2X since those codecs also used data partitioning (and in case of TM2 data compression as well).

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