[vdr] Re: Lip-Synchronisation

[Henning Holtschneider <hh@holtschneider.com>]

--On Mittwoch, 12. Februar 2003 14:56 +0100 Wolfgang Goeller 
<wgoeller@heraklit.ch> wrote:

> Audio PTS: first packet 07:00:50.602, last packet 07:50:49.330
> Video PTS: start 1.GOP 07:00:51.418, end last GOP 07:50:49.898
>
> Might be that simply one time-stamp is not correct?
> Does audio-recording start "too early?"
>
> Does recording discard the first few video-frames without
> discarding the corresponding audio-frames?
As Klaus wrote already, VDR basically just saves the data coming from the 
DVB card to a file.

The audio and video data is being aligned (sync'ed) in the MPEG decoder 
using the PTS (Presentation TimeStamp) information which is sent with each 
audio and video packet. Thus, if VDR starts to record a stream at a given 
moment, the audio data for a video sequence might already have been 
broadcast or vice versa.

This is no problem for the MPEG decoder on the DVB card because it will 
drop the audio or video data which it has no counterpart for. You will only 
run into trouble if you demux or remux the stream with a program that does 
treat the PTS information correctly (e.g. for conversion to (S)VCD). VDR 
could re-write the PTS information while writing the data from the card to 
a file but this would require quite a lot of precious memory and CPU time.

In practice, audio and video will almost always be broadcast out of sync 
and the offset will vary over time.

hh


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