>>82691062First, assuming this is a Windows command line (if it's Linux or something the NUL might not work), run this, replacing <input> with the input filename:
ffmpeg -y -i <input> -c:v libvpx -preset veryslow -b:v 860k -pass 1 -an -f webm -y NUL
That should generate a ffmpeg 2-pass file that it can use for the second go around. After that file's been generated, do this, replacing both <input> and <output>:
ffmpeg -i <input> -c:v libvpx -preset veryslow -b:v 860k -pass 2 -an -f webm -y <output>
Rounding down to 860k to make sure it stays under the limit, and I replaced "-b:a 64k" with "-an" because this would have to be a soundless webm for this board.
