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. Low decoder overhead is paramount as the S1 MP3 players have very low-end processors (a Z80 variant). 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. The video format is a variant of Motion JPEG, with fixed rather than variable quantisation tables. The container is a modified version of AVI. There are two different versions of this format: an older one for Actions chips, and a newer one for ALi’s M5661 chip, sometimes called ALIAVI. AMV (Actions Media Video) is a proprietary video file format, produced for MP4 players, as well as S1 MP3 players with video playback.
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