[FFmpeg-user] Slowing ffmpeg down

[Shaun Procter]

> On 13 Aug 2020, at 2:35 am, Rich Andrews <stuart.r.andrews at gmail.com> wrote:
> 
> On Tue, Aug 11, 2020 at 11:54 PM Shaun Procter <shaun.procter at gmail.com <mailto: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!

No worries at all, am glad I could help. It’s definitely not silly and we all need to start somewhere. The main thing is you asked for help and that shows intelligence.

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