[linux-dvb] AW: Re: AC3 works with Pro7 and PW (was ...towards AC3sound)

[Andreas Vitting <Andreas@huji.de>]

The header is wrong !! See at DIGITAL AUDIO COMPRESSION (AC-3) ATSC
STANDARD. The frames has a exact framesize, starting with a sync sequence.
Ac3dec works because of his "resync" capability. He does ist for every frame
again (much work for nothing). Other ac3 decoders only search the first
sync, check the parameters(framesize, rate and so on) and then playing and
playing and ....  ;-)
Ok, i think at the first step you can cut the header away (but the better
place would be the driver)

Andreas

-----Ursprüngliche Nachricht-----
Von: kls@cadsoft.de [mailto:kls@cadsoft.de]Im Auftrag von Klaus
Schmidinger
Gesendet: Dienstag, 26. Juni 2001 09:29
An: linux-dvb@linuxtv.org
Betreff: [linux-dvb] Re: AC3 works with Pro7 and PW (was ...towards
AC3sound)


Stefan.Hagendorn@lindy.cc wrote:
>
> Hi all,
>
> after applying the patch to strip the header, it is now possible to replay
> ac3data from Pro7 and from Premiere World. Strangely the header strip is
> not necessary at Pro7 .. here it works with and without header ... on PW
> the header must be striped for ac3dec.

Well, then I guess generally stripping the header in VDR is the way to go.
I'll change it that way.

> I'm not an expert regarding mpeg2 .. but striping the PTS should be no
> problem at all because the external programm cannot sync with the video.
> Maybe vdr should check the PTS before striping it to get the most possible
> sync before the stream leaves VDR ?!?!
>
> Something like:
>
> -analyze pts
> -sync via pts to video
> -strip header
> -send to external pipe

The data that comes from the driver should already be in sync, so I don't
know if/how VDR could improve that.

>>From what I've seen so far it looks as if the Dolby Digital data arrives
somewhere in the area of 1/25 seconds before the video and "normal" audio
data. However, the test reports here on the list so far have shown that
the Dolby Digital sound is somewhat "behind" the video. Maybe some of you
can look further into this and observe whether there is always a constant
offset (in which direction?) or whether the offset grows/shrinks over time.

Klaus
--
_______________________________________________________________

Klaus Schmidinger                       Phone: +49-8635-6989-10
CadSoft Computer GmbH                   Fax:   +49-8635-6989-40
Hofmark 2                               Email:   kls@cadsoft.de
D-84568 Pleiskirchen, Germany           URL:     www.cadsoft.de
_______________________________________________________________


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