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

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 bug?

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... ;-)

            Luca

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