[Ffmpeg-devel] Re: [Ffmpeg-user] ffmpeg encoding, really slow video when recording with audio

[Erik Slagter]

On Thu, 2006-02-02 at 20:54 +0000, Karl Lattimer wrote:
>    I'm playing around with ffmpeg as it seems to be the only thing  
> around that can record from line input and a composite video feed  
> from a bt878 card. When I record video only I have no problem, the  
> video plays back with exceptional quality and is absolutely perfect.  
> However when I try and record audio from line-in also there seems to  
> be a problem.
> 
> I am using the following command line to record with audio
> 
> ffmpeg -vd /dev/video0 -r 25 -b 2048 -ad /dev/dsp -s cif video.mpg

It's probably reading from /dev/dsp in blocking mode. That will
guarantee a/v unsync, ffmpeg does not have special provisions for
recording from separate audio and video devices at the same time.
Several other tools do indeed.

Maybe this could be solved by making the audio reads non-blocking and
adding a timestamp, but reading non-blocking from a file or audio-only
recording is silly. Also the time at which an audio frame is read from
the device is not necessarily anywhere near the time the frame was
recorded :-(
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