[linux-dvb] Re: video pts differences on dvb-t in the uk

[Måns Rullgård <mru@mru.ath.cx>]

Ragnar Sundblad <ragge@nada.kth.se> writes:

> --On den 10 oktober 2004 17:08 +0100 Torgeir Veimo <torgeir@pobox.com>
> wrote:
>
>> But surely the difference should be detectable in some way?
>
> Sure, just compare the PTS fields? I probably don't
> understand your question.

There is no requirement for the PTS values in different streams to
ever be equal.  The only requirement is that a PTS is present in all
streams at least once every 0.7 seconds.

>> I think the difference is less on some channels (TMF, some news
>> channel). I delay video constantly by 160ms to compensate for this,
>> but automating it would be preferable.
>
> If you are talking about a complete audio + video + other-media
> decoder, I think the answer is that you will have to establish
> a decoder clock, and buffer and sync the incoming streams by
> looking at their PTSes so that they all get played at the
> right times.
>
> Note that the PTS of a MPEG 2 video frame can be in the
> future, since some of them are sent in advande to be ably to
> be used for forward referencing frames that are sent later
> but presented earlier.

This can be detected from the presence of a DTS field.  The standard
requires that a DTS, if present, is different from the PTS, and
requires the presence of a DTS if the decoding time differs from the
presentation time.

-- 
Måns Rullgård
mru@mru.ath.cx