[mythtv-users] Does SPDIF Input generally work?

Nick Rout nick at rout.co.nz
Thu Aug 30 00:43:34 UTC 2007

On Thu, August 30, 2007 6:06 am, Ken Scales wrote:
> Mitch Gore wrote:
>> On 8/29/07, *Bearcat M. Sandor* <HomeTheater at feline-soul.com
>> <mailto:HomeTheater at feline-soul.com>> wrote:
>>     Folks,
>>     I've been looking all over the wiki for this one and i can't find
>> it.
>>     Provided that i get a sound card that is well supported, will
>>     MythTV record
>>     tv audio  using the spdif input?   Can i expect any problems with
>>     this? It
>>     appears to be just fine, but i'm about to make a purchase of an
>>     m-audio usb
>>     soundcard and it only has spdif in.
>>     Thank you,
>>     Bearcat M. Sandor
>>     _______________________________________________
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>> i would assume it would work but i have to ask what are you recording
>> that has digital sound and the sound isn't built into the steam as in
>> HDTV recorded via firewire or QAM or antenna.
>> Mitchell
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> I'm not the OP, but one example: my ExpressVu PVR5100 ( = Echostar
> PVR501) records the AC3 soundtrack when transmitted, and plays back with
> s-video and spdif (as well as analogue sound of course).  My older 4700
> receiver also has spdif output.  I'd like to try capturing the spdif and
> mplex-ing it with my video recordings, like Bearcat is proposing, so
> that I could put them onto my much larger Myth storage without losing
> the DD surround. (Aside: yes, I'm familiar with "PVRexplorer", but I'm
> not running Mac or Windows right now.)
> I expect some sync issues, but as long as both "tracks" are stable
> timewise (granted, a big hope), adjusting for a time-delay between them
> can be done with available tools (probably manually at first).
> So, to echo Bearcat's request, is there a recommended way of setting up
> and capturing spdif audio? (Even something as crude as "cat < /dev/dsp2
>  > filename.ac3" -- to start exploring the possibilities of doing this.)
> It could be an interesting project to work on over the winter months...
> Ken.

Fundamentally this doesn't seem any different to using an analogue
framegrabber that presents its audio as line-in or similar. the first step
would be to use alsamixer to set the capture device to be the spdif

This assumes of course that you have a sound card that *has* an spdif
input (as opposed to output).

If you want to play around with some manual muxing of an ac3 soundtrack
into a video stream I recommend avidemux. I regularly convert my stereo
captures to 5.1 ac3 with stereo2surround.sh [1] and mux them back into the
video. Of course I don't get a real 3D soundtrack (trains don't thunder
diagonally through my lounge, you just get noise out of all the speakers)..
Anyway avidemux will happily mux an ac3 soundtrack into an avi.


Nick Rout

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