[mythtv] Re: Software AC3 decoding when watching TV

[htpc]

 >> Recording ATSC AC3 will be broken without doing it this way.. Juha 
said he would
 > I think the problem with recording AC3 is that in
 > CorrectStreamNumber() we mandle stream_id's of all audio pids to 0xC0.
 > This is wrong for AC3 audio and propably breaks it.
Ah, there's the code I was looking for.... I remember seeing something 
like that when I searhed for AC3 in gasommer..

 >> have a MUCH easier time doing subtitles as well so I am completely 
sold.. Sorry
 >Well yes, we could save on the trickery. :)
More trickery... From the posts I realised that 0xbd is a private stream 
(and not AC3 as I previously thought). But If I modified transform.c and 
treat 0xbd as an audio stream, the function find_next_start_code will 
return that. Myth then detects this AC3 audio stream correctly and loads 
the A52 decoder. I can even change to a channel using MPEG2 stream and 
Myth will automatically use the MPEG2 decoder. All provided that 
bit_rates, frequencies, etc computes to some non zero values in 
get_ainfo or get_vinfo. Otherwise, backend will crash in the function 
ps_pes() somewhere at line 1981 (all in transform.c):

                   if (pv->has_vi && pv->vi.bit_rate && !pv->muxr ) 
(something similar to that)

pv isn't null when it crashed, memory corruption perhaps (don't think 
get_ainfo is meant to read 0xbd packets)? Will find out more tonight, 
think I must have missed something when I forced 0xbd to an audio stream.


 >> if I shot from the hip earlier.. Do you have a quick patch that I 
could make to
 >> transform.c (since its your code) to make it dump an ATSC AC3 
stream? None of
 >> ATSC channels ive watched have sound and I am pretty sure tranform 
just isn't
 >> putting them in the ringbuffer since its looking for a differnet 
header than
 >> PrivateStream..
 > Please see above. I think that's the problem.
When tuning to MPEG (0xc0) audio, I find that there are some ipackets(? 
or something)  with a code of 0xbd as well.. Although there's no 
apparant breakage in sound, will this be a problem?

 >> The biggest issue right now is really time.. I want to get this code 
into
 >> myth-cvs as soon as I can and doing TS recording may not fit at 
first.. I have
 >> more than enough on my plate.. Its possible Jesper could do this, 
but I do not
 >> set his schedule.. I know both John and I have tons of work to do 
before this
 >> whole thing is ready..
 >I can help.
So can I. Though I think I am restricted to a tester role for now as I 
still have no idea yet how Myth works or  details about the DVB format.

OT: Turns out I am duped by the marketing hype as most programs in 
Australia Perth broadcasts their programs in AC3 stereo, not AC3 
surround sound..


Regards.