AMV is a proprietary video file format, produced for MP4 players, as well as S1 MP3 players with video playback. There are now two different MTV formats: the older one for the Actions chip, and a newer one for ALi’s M5661 chip. This format for ALi one was ALIAVI.
The container is a modified version of AVI. The video format is a variant of Motion JPEG, with fixed rather than variable quantisation tables. The audio format is a variant of IMA ADPCM, where the first 8 bytes of each frame are origin (16 bits), index (16 bits) and number of encoded 16-bit samples (32 bits); all known AMV files run sound at 22050 samples/second.
Low decoder overhead is paramount as the S1 MP3 players have very low-end processors (a Z80 variant). Video compression ratio is low — around 4 pixels/byte, compared with over 10 pixels/byte for MPEG-2 — though as the files are of low resolution (96×96 up to 208×176) and frame rate (10, 12, or 16 frame/s), file sizes are small in bytes per second. With a resolution of 128×96 pixels and a framerate of 12 frame/s, a 30-minute video will be compressed into 80 MB.
Documentation for this format is not publicly available, but Dobrica Pavlinušić reverse engineered the format to produce a Perl-based decoder and Pavlinušić, Tom Van Braeckel and Vladimir Voroshilov produced a version of FFmpeg that works on AMV files. The AMV code has been sent upstream to the main FFmpeg project and the mainline version of FFmpeg now decodes AMV.