[mythtv] Internal DVD Player multichannel support

matt mead m-mythtv at goof.com
Mon May 1 18:53:09 UTC 2006

I'm willing to lend a hand on this patch as well.  I'm not quite in the same 
boat as Ed, in that I have an actual receiver, but it is 11 years old, and 
originally you were to purchase an external AC3 decoder for it.  I had the 
external decoder, but it crapped out a few years back.  Because the receiver is 
quite good and there is no other reason to replace it, I've just purchased DVD 
players with built-in AC3/DTS decoders since the loss of the external AC3 
decoder.  Because the receiver has a DB25 input for carrying pre-decoded audio 
to the 5.1 channels, I'd like to wire up an adapter for the sound card output.

I have not taken a look at the patch yet, though I've been aware of its 
existence and intended to look at it once I get a few other things about my 
myth system stable.  Does it currently provide for both AC3/DTS decoding to 6 
analog outputs as well as decoding DPL/DPLII to 6 analog outputs?  Having 
mythtv (and whatever ends up playing DVDs when done through myth) properly 
output to all 6 channels given source material in DPL/DPLII and AC3/DTS would 
make life a lot easier - I could just leave the receiver in "external decoder" 
mode and be done with it.  Maybe a keystroke in the different playback modes to 
switch between surround and "bypass" (stereo (front L+R), but with intelligence 
to steer bass to the LFE channel) would be a good thing to do as well.

Let me know if there's a way I can help out on this.

Thanks for the effort so far on the patch.


Ed W wrote:
> Hi
>> this is what it does.
> Aha, wasn't obvious that it did that just by eyeballing the patch - I 
> could just see the AC3 stuff and misunderstood - sorry
> So what's the big barrier to getting some of this committed?  Most of it 
> looked quite reasonable just eyeballing the patch?
>> you can do this too. I was just supporting what people wanted. works fine.
>> AC3 reencoding is in audiobase only. however I needed to pass some info 
>> between the layers.
>> this is part of my rethink but it is necessary in some way. Ideally the 
>> audio layer should control the decode parameters instead of being pushed 
>>from decoder to audio layer. its a bit nasty.
>> no point reencoding if its already AC3 and no timestretch is being used. 
>> just wastes CPU. and it does use a fair bit.
> I guess the issue is probably that the decoder is setup before 
> initialising the audio?  If we initialised the audio first then we could 
> request a certain number of channels, have it fail and tell us how many 
> it's really got and then fall back to using that?  This wouldn't catch 
> the case of someone with a multichannel card and only two speakers 
> hooked up, but it would help for the two channel card or spdif setup?
> Seems really promising anyway!  What can I do to help some or all of 
> this get committed?!
> Ed W
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matt mead


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