Someone at Reddit inspires me to try this command:
ffmpeg -r 25 -i input.mp4 -c:v amv -vf scale=160:128 output.amv
Sadly, that produces an error:
Unable to find a suitable output format for 'output.amv' output.amv: Invalid argument
But if I change the command to produce output.avi rather than output.amv, and then rename the finished output.avi to be output.amv, I get a file that plays on the computer. In that case, MediaInfo reports that output.amv is in AVI format, and its video portion is in AMVF format. An attempt to play output.amv produces "Format error" on the portable device that requires AMV files, unfortunately.
Someone says the explanation is, "FFmpeg supports the amv codec but for some reason not the amv container." If that is true, is there a way to use ffmpeg to convert various (e.g., AVI, MP4) videos to AMV videos that will play on the portable device? Or if not, is there an alternative that is known to produce working AMVs? I'm using Windows 10 but could probably handle a fairly simple Linux solution.