[mythtv] Re: Software AC3 decoding when watching TV

[Juha Kuikka]

> > (3) Tuned to HDTV channel. No go. AC3 has the same startcode as MPEG2,
> > thus myth(or ffmpeg?) gives it the codec_id = CODEC_ID_MPEG2. Hence all
> > along Myth is trying to use the MPEG2 decoder to decode AC3 streams
> > (that's why I am hearing rubbish when I use a AC3 APID).
> 
> Depends. AC3 should be in privete_stream_1 PES packets in TS stream.
> At least in DVB and American ATSC.  This is where ffmpeg searches for
> it, see mpegps_read_packet and mpegps_read_pes_header. What do you
> mean with "startcode"?

I'll have to correct myself right away.

At least in ATSC content AC3 frame begins just after PES headers, in
DVD there seems to be a four byte header in front of the AC3 frame.
Ffmpeg seems to be expecting this header and not AC3 frame straight
away so it does not recognise these. I don't have any dvd
documentation so I cannot confirm this.  Just some conversations on
the 'net. :)

I do not have any DVB AC3 content, if you have any I could check it
out and make a patch for ffmpeg.

AC3 frame begins with (0x0b 0x77) so we I'd guess we have to modify
mpegps_read_packet and mpegps_read_pes_header to take this into
account.

 - Juha

-- 
Madness takes it's toll. Please have exact change.