[FFmpeg-user] Slowing ffmpeg down

[Rich Andrews]

On Tue, Aug 11, 2020 at 11:54 PM Shaun Procter <shaun.procter at gmail.com>
wrote:

> > On 12 Aug 2020, at 9:01 am, Rich Andrews <stuart.r.andrews at gmail.com>
> wrote:
> >
> > Is there any trick to slow down ffmpeg on a simple stream mapping?
> >
> > Stream mapping:
> >  Stream #0:0 -> #0:0 (pcm_s16le (native) -> pcm_s16le (native))
> >
> > Given an input file that is looped and output with a command such as:
> >
> > ffmpeg-4.2.1-armhf-static/ffmpeg -y -t 200 -stream_loop -1 -i
> HoldMusic.wav
> > -map 0:a:0 -f wav output.wav
> >
> > ffmpeg can move the data really quickly!
> >
> > bitrate=1411.2kbits/s speed= 262x
> >
> > Any way to get that speed down to ~1x?  My purpose is that ffmpeg is
> being
> > used as a child subprocess and I would like to be able to test its
> behavior
> > in-situ.  There is a lot of timing in my  application between fork and
> join
> > and having ffmpeg be slow would be really helpful.
> >
> > Push comes to shove, I'll just use another program to more slowly copy,
> but
> > that wouldn't be as good as using ffmpeg itself since it has a host of
> > behaviors as a child process regarding signaling and return codes. Come
> to
> > think of it, since I built ffmpeg, maybe I can just build a variant that
> > sleeps frequently?
> >
>
> You can use the -re flag like this,
>
> ffmpeg-4.2.1-armhf-static/ffmpeg -re -y -t 200 -stream_loop -1 -I
> HoldMusic.wav -map 0:a:0 -f wav output.wav
>
> “The FFmpeg's "-re" flag means to "Read input at native frame rate. Mainly
> used to simulate a grab device." i.e. if you wanted to stream a video file,
> then you would want to use this, otherwise it might stream it too fast (it
> attempts to stream at line speed by default). My guess is you typically
> don't want to use this flag when streaming from a live device, ever.” -
> https://trac.ffmpeg.org/wiki/StreamingGuide
>
> Hope that helps.
>
> Shaun
>
>
I feel silly for having looked at that very switch and didn't bother trying
it thinking that the input file doesn't have a framerate.  '-re' works
perfectly.  Thank you for the perfect solution!