[mythtv-users] spdif with simultaneous analog out (alsa)?

Misty P mistyp at thekorn.net
Fri May 2 16:38:48 UTC 2008


Richard Woelk wrote:
>> It doesn't quite work for me, though...  Things with PCM audio wind
>> up going out the analog, and it's *trying* to go out through the
>> SPDIF but something is amiss.  (The SPDIF light keeps coming on and
>> going out on the receiver.) AC3 does work exactly as you describe.
>>
>> Unfortunately, since I'm all ATSC OTA, 80% or better of my content
>> has AC3 on it.  (Or worse, is AC3 except for the commercials...
>> yuck!)
>>
>    If you uncheck the passthrough boxes in the mythfrontend setup
> screens, then Mythtv won't pass AC3 to the SPDIF, the stream will be
> decoded  internally and output as 2 channel PCM.

Yeah, thought about doing that, but the cure is almost worse than the 
disease at that point.  I may just have to be content with simply cranking 
the speakers in the other room for a while.

> I'm not sure why the PCM isn't working correctly, it sounds like its
> trying.

Yeah I'm a bit mistified about that myself.  Probably something to do with 
rate conversion, though with having AC3 going exclusively out spdif and 
apparently not being able to be snooped upon, I have to admit I'm not much 
interested in finding the cause.

>> on Line 164, you have to change device 2 to device 1 (judging from
>> your .asoundrc that you posted)

Thanks, caught that myself.  :)  (I read and made sure I mostly understood 
the whole file before letting it loose!)

>> you can also try this.
>> at line 54, you can configure what is default. Maybe try changing
>> that to digital-hw instead of dmix-analog
>> It will prevent other sounds from interfering. You can also use
>> these 4 device names directly in mythtv

Tried that as well, thinking that maybe the mixing was causing some sort of 
unacceptable to spdif overhead.  No dice, unfortunately.

> It is impossible, as far as I've seen, in windows, to output AC3 over
> SPDIF and decode to analog simultaneously.

You may be right; it's been a while since I was dinking about with it. 
Seems like it should be possible to clone the incoming AC3 stream, send one 
clone out spdif, then decode it for output on the analog jacks.

(Then again, this wouldn't be necessary if the stupid preamp I use would do 
it for me, but nooooooOOooo, it doesn't want to, either!)




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