[1] The video format is a variant of Motion JPEG, with fixed rather than variable quantisation tables.
[1] Low decoder overhead is paramount as the S1 MP3 players have very low-end processors (a Z80 variant).
Documentation for this format is not publicly available, but Dobrica Pavlinušić reverse engineered the format[1] to produce a Perl-based decoder[3] and Pavlinušić, Tom Van Braeckel and Vladimir Voroshilov produced a version of FFmpeg that works on AMV files.
These cloned devices are sold with an AMV video converter software in a CD.
These AMV video converter software are also available for free online (freeware) such as the MPxConverter by Bytessence.