Add mp3/wav as album in USB Disk mode (eg for reference track)

I wanted to use a mp3 as a reference track on my op-1 to help with sound design and mixing. I figured that rather then adding it as a sample or a track (which would require me to disable master effects settings) - the best way would be to add it as a side_b on the album.

But USB disk mode did not accept me just dragging in a side_b.aif audio file - as soon as I left DISC mode it would recreate side_b.aif track and overwrite my uploaded file.

… After some experimentation I think I found a reliable way to add audio as an album file in USB disk mode and have op-1 accept it (instead of recreating it).

OP-1 requires album files to be exactly 6 minutes (63.5) mb, and be encoded with pcm_s16le

Using the following FFmpeg command, I was able to do this:

ffmpeg \
    -i source.mp3 \
    -af "apad=whole_len=15876000" \
    -acodec pcm_s16le \

This assumes the mp3 is 44100 Hz stereo.

Hopefully this is helpful for anyone looking to do the same.