The Sound of Silence

Let's say you have a video file of some sort with no audio stream, and a device that plays video files but can't handle ones with no audio. And you want to use the command-line tool ffmpeg, because you already have a script set up to use it to do whatever transcoding, but for the slight problem of sound.

Maybe you don't, but I did. Know that ffmpeg can take multiple input and/or output files and shuffle them around, in addition to decoding/encoding them. Know also that ffmpeg is documented in a manner both voluminous and not terribly approachable.

The wrong thing to do is to try to figure out how to extract the length of the video track and generate that many audio samples of silence. But, if one hasn't noticed the right part of the man page, this may be what one tries to do. (I did.)

The right thing to do is to use the -shortest flag, which directs ffmpeg to stop whenever any input stream reaches its end, rather than when all inputs are done. Some of you may be (but probably none of you are) thinking of various versions of the Scheme procedure map and their behavior on lists of unequal and/or infinite length, recently a point of contention in the discussion leading up to R6RS. And what is Unix if not a platform for stream processing? (Don't answer that.)

Thus: -f s8 -i /dev/zero -shortest, placed either before or after the regular input file, depending on whether input that does have audio should be silenced or not, respectively.

It's a not entirely inelegant solution to a mildly ridiculous problem.