[mythtv-users] Missing --passthrough option in mythtranscode

Jean-Yves Avenard jyavenard at gmail.com
Tue Jun 7 12:59:02 UTC 2011


On 7 June 2011 20:37, Paul Gardiner <lists at glidos.net> wrote:
> Not quite, but close I think. The audio passes through perfectly,
> but it now seems to be that only every Nth video frame is passed
> to the video pipe, where N is somewhere between 3 and 10. I don't
> mean it varies between 3 and 10. It is a constant factor. It's just
> that I don't know what the factor is. (I've just realised that I could
> find out by changing the frame rate I reencode at until I get the
> right speed - I can do that if you like). The effect I see is
> the audio is fine, and the video looks to be on fast forward. It's
> not that ffmpeg is messing up the mux, because I can do a video-only
> encode and I still see the same problem (and not if I remove --passthrough).

Probably because timestamp calculations is based on the size of the
decoded audio (in PCM). We know the audio rate; we now the sample and
frame size so to calculate the timestamps and length of the audio data
we simply divide the length of the decoded audio by the frame size and
framerate...

With raw audio, that calculation will obviously return a wrong result.

I guess, I will have to decode the audio completely as well as passing
the raw compressed audio ; in order to calculate the timestamps
properly.

Glad it's working that well already though ; it was completely untested ...

JY


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