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

Erik Slagter erik
Fri Feb 3 19:14:19 CET 2006

On Fri, 2006-02-03 at 15:53 +0100, Luca Abeni wrote:
> > 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).

Yes, I know :-) bt848/bt878. The bt878 also offers audio but it seems
not many people get it working.

> 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
> bug?

Completely default. If you have a cinergy card with a broken crystal,
you'll know very soon :-(

> >  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 ;-)

Hmmm, I doubt that ffmpeg can work around the "smartness" of the
saa7134, which, in this case, is counterproductive.

> 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... ;-)

Hmmm, ffmpeg alsa support for ffmpeg, that would be interesting. I don't
know if alsa supports timestamps, but there will undoubtedly be more
something like timing info.
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