[Libav-user] Using libav to insert silence at the beginning of an audio clip

Chris Hennes chennes at pioneerlibrarysystem.org
Tue Mar 21 17:28:47 EET 2017

The software I am writing allows a user to combine multiple potentially-overlapping audio clips by specifying an in and out point for each clip, plus when in the final stream that clip starts. I'm trying to string together a single filter graph that will do this and am struggling with the start point bit. My thought was to take each of the audio files and construct a filter chain for them that inserts silence of the required length at the beginning, then the atrim'ed audio clip, then an apad to the end of the required length, so (for just a single audio file), something like:

(some kind of silence-generator here)[delay0];[in0]atrim=start=1:end=3,apad [end0];[delay0][end0]concat=n=2,v=0,a=1[final0]

I can't seem to figure out how to make the concat work, though. No matter what technique I use to generate the silence (or non-silence, I've tried using sine and anoisesrc), I am getting an "invalid argument". I've tried playing around with the ffmpeg exe on the command line, but still can't make heads or tails of concat. What am I missing? Or is there a better way to do what I am describing?
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