[FFmpeg-user] "Invalid sample rate" and "Guessed Channel Layout" warnings from -i /dev/zero

Nicolas George nicolas.george at normalesup.org
Tue Mar 12 23:34:03 CET 2013

Le duodi 22 ventôse, an CCXXI, gheine at mathnmaps.com a écrit :
> Goal is to create a silent 4-second audio that can be merged with a
> static 4-second video to create a simple menu for a DVD (NTSC
> format).  Entering
> ffmpeg -f s16le -i /dev/zero -ac 2  -ab 224k -ar 48000 -t 4 -acodec
> ac3 -y tmp.ac3
> gives the warnings
> [s16le @ 0xaf96560] Invalid sample rate 0 specified using default of
> 44100
> [s16le @ 0xaf96560] Estimating duration from bitrate, this may be
> inaccurate
> Guessed Channel Layout for  Input Stream #0.0 : mono

You put your -ac and -ar options after -i: that means they apply to the
output, and therefore, the sample rate and channel count for the input is
not specified. That has no consequence here, since silence at 44100 Hz in
mono is just the same as at 48000 Hz in stereo, but it would be a better
practice (and negligibly more efficient) to set the options to the input,
i.e. before -i.


  Nicolas George
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