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

[Bob]

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.