[mythtv] Internal DVD Player multichannel support

Ed W lists at wildgooses.com
Mon May 1 09:44:10 UTC 2006

>I assume you are talking about my patch #1104.

I have reviewed this patch and it's not actually quite what I was 
talking about.  I guess I had forgotten that many people use a surround 
sound amp for multichannel.

What I am after is N channel decoding in Myth, with output to N channels 
of my soundcard.  I don't have a surround amp, I just have a big pile 
(8) of stereo amplifiers which are wired up to the speakers and the PC 
(nothing else).  In this way I was able to build an extremely high end 
system for less than the price of a decent surround amp.

Some of the bits of the patch look very interesting.  Can I suggest that 
the best way to proceed is to split the patch out and get some bits into 
SVN now and then it will be easier to work on the complicated parts.  
For example the multichannel timestretch seems like an easy one to break 
out and submit (also submit upstream?).  I think if this went in then at 
least it would be easier to develop the other multichannel aspects 
without having to lug around a big chunk of patch to that file?

I was a bit nervous about all the changes in avformatdecoder - isn't 
this derived from upstream lavc?  Are we not reproducing stuff that is 
already written elsewhere (eg the conversion from 8 to 16 to 32 bit 
samples, etc?)

>what I wanted to do was incorporate some way of the audio layer handling any 
>number of source channels without being explicitly configured for them. 
>output channels (analog) I thought just limit to 2 and 6 which alsa can 

Well, I was wondering a bit about where the AC3 re-encoding was being 
introduced actually.  I wonder if it might make more sense to 
re-introduce this into the audiobase class instead of partly in 
avformatdecoder?  The fact that it looks like a two channel stereo 
digital output (for example) is something that we probably want to hide?

It would be a total bodge, but note also that there is a Jack to AC3 
decoder which could be used to patch in temporary AC3 output into Myth, 
and also the code may be worth examining for any clues on implementing 
this stuff.

However, it would seem to be one possible option to spit out 6 channel 
audio to the audio library and then have some plugin *there* either 
downmix it to stereo, 2.1, 4.1, 5.1, dolby, etc.

Any chance you could break out the timestretch bits and try to get them 
submitted and I will have a look at working on the core audio routines?


Ed W

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