[mythtv-users] Simultaneous AC3/DTS passthrough & stereo analog output

[Bob]

Ryan Steffes wrote:
> On 10/22/07, Bob wrote:
>   
>> Bob wrote:
>>     
>>> I'm using an Audigy2 in one PC and an SB Live value in another (both
>>> Debian etch) and I output to a 5.1 speaker system over S/PDIF coax and
>>> to my headphones (via the TV) over analogue stereo.
>>>
>>> I have AC3 passthrough working when I call mplayer with -afm hwac3, (I
>>> think DTS will work but can't find a test clip) and I can soft decode
>>> and downmix the AC3 audio with -afm liba52, so that's all great, except
>>> that with -afm liba52 rear left and right are mixed and I don't get
>>> anything out of the LFE track.
>>>
>>> What I can't do is both at once which would make life easer for SWMBO.
>>>
>>> My google search (below) has reviled I'm not alone in wanting this
>>> (including a number of threads here) but hints that it may not be
>>> possible, however, my search skills are week so I thought I'd ask.
>>> "ac3 passthrough" "analog (output | stereo | downmix)" (simultaneous |
>>> "both at" | "same time")
>>>
>>> Has anyone got any suggestions on how to get this working?
>>>
>>> I can install more than one sound card, so if it can be done but only on
>>> a second card I'm still interested in the solution
>>>
>>> Thanks for any help.
>>>
>>> PS: Though the PCs in question aren't MythTV frontends at the moment,
>>> that is their destiny and since this community seems to be one of the
>>> most knowledgeable with regard to linux sound issues I thought I'd ask here.
>>>
>>> PPS: I asked the same question on the debian-user list 10 days ago but
>>> with no luck so sorry to those who, like me, are subscribed to both.
>>>
>>>       
>> Someone on debian-user suggested I look in to PulseAudio but it appears
>> that PulseAudio cannot do AC3 pass through. it may be possible to get it
>> to work by having mplayer output 6 channel PCM, splitting that to 2
>> streams, downmixing one stream to stereo and outputting it to the analog
>> output and using a real time software AC3 encoder to re-encode the
>> second stream then outputting that to S/PDIF.
>>
>> There would be quite a quality/performance penalty though, particularly
>> at low volume.
>>
>> Oh well, guess it's not possible, it's funny, usually a product
>> implemented in software gives you more flexibility than a dedicated
>> hardware version but my cheep consumer DVD player automatically does this.
>>
>> Thank you for your time.
>>     
>
> Depending on your hardware, the easiest way to accomplish this may be
> to go digital to your receiver, and let it split the signal to your
> television.  What sort of receiver do you have?
>   

On my full featured A/V receiver in the living room it's not a problem, 
I can just plug into the headphone socket on the front, this does a 
better job downmixing than liba52 anyway as rear left and right remain 
discrete and I get sound out of the LFE track.

The more basic system in the spare room is a generic AC3 DTS system sold 
for cheap 6 ~ 7 years ago, but it's more than adequate for a bedroom or 
office and I'm reluctant to replace it just because it doesn't have a 
headphone port or cleaver downmixing, it's one of these.
http://www.dvd.reviewer.co.uk/reviews/hardware.asp?Index=3806

I see in other posts it's possible for Myth 0.20 and above to split the 
audio and achieve the desired result, I'm surprised it's not an alsa 
thing though as that way it would only need implementing once.

I think I'll sign up to their mailing list and ask over there. (Yay 
another 500 emails a day)