[Ffmpeg-devel] Re: [Ffmpeg-user] ffmpeg encoding, reallyslowvideo when recording with audio

Luca Abeni lucabe72
Fri Feb 3 15:53:03 CET 2006

Hi Erik,

On Fri, 2006-02-03 at 15:19 +0100, Erik Slagter wrote:
> Just to make sure we're on the same track here:
> The saa7134 (on which the cinergy cards are based) delivers both video
> and audio, the latter one via it's own, more or less oss compatible
> interface.
Ah, ok... I misunderstood you. I am capturing audio through a "regular"
audio card (using oss emulation over alsa).

In case I get a saa7134-based card, how can I setup the drivers to
capture both video and audio from the card, so that I can reproduce the

>  If everything is alright, it will try and succeed in keeping
> audio and video in sync; it is able to vary the samplerate of the audio
> slightly to compensate for drifts, which works very nicely. But if the
> crystal is wrong, it doesn't work.
If the audio coming from the card is available through the OSS
interface, I think libavformat will associate timestamps to it based on
gettimeofday(), and ffmpeg will do the samplerate adjustments by
resampling the audio (if you specify the async option).
So, I suspect ffmpeg can work around this hw/driver's bug. But I am just
speculating ;-)

> BTW it's a shame oss does not feature timestamps on audio frames, like
> v4l2 does on video frames.
Does alsa provide timestamps? (BTW, what about explicit alsa support in
libavformat? No, I will not have time for working on it, but if someone
else steps in... ;-)

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