[mythtv-users] Capturing 5.1 audio

[Michael T. Dean]

John DiLorenzo wrote:

> I'm planning my myth setup and I'm a little confused about how to 
> handle capturing and playing back surround sound.  Here's the plan so far:
>
> Backend/frontend with a single pvr-150 (for now) and a Geforce MX460.
>
> The backend will be connected to a Scientific Atlanta digital cable 
> box.  The only digital audio out is a digital coax.  I have a tuner 
> that will accept an optical input. 
>
> It's clear that I need a sound card with an optical out to output 
> audio to the receiver, but how do I get 5.1 captured in the first place?
>
> Do I:
>
> 1.  Get a soundcard with both coax and optical ports and somehow 
> configure coax for input and optical for output?
>
> 2.  If I rely on the pvr-150 for capturing the audio by using coax 
> cable from the digital tuner to the 150, will it capture in 5.1?  If 
> so, would I be sacrificing video quality by not using s-video?
>
> 3.  Or am I missing something else?

PVR-150 records NTSC or PAL.

NTSC/PAL don't do 5.1 channel audio--they support only 2-channel stereo 
sound.

Therefore, PVR-150 cannot record 5.1 channel audio.

However, Dolby Surround ( http://en.wikipedia.org/wiki/Dolby_Surround ) 
is a technology whereby multiple audio channels (4.0 channels) are 
encoded within a standard 2-channel (stereo) signal and decoded using a 
Dolby Pro Logic ( http://en.wikipedia.org/wiki/Dolby_Pro_Logic ) 
decoder.  Often, when a show is broadcast (on digital stations) in Dolby 
Digital=AC-3=5.1 ( http://en.wikipedia.org/wiki/Dolby_Digital ), the 
analog stations transmit Dolby Surround encoded in the stereo signal.

The PVR-150 records the stereo signal broadcast with the show, so--in 
theory--it would be possible for Hauppauge to support Dolby Surround.  
However, the PVR-x50's do not contain a Dolby Surround decoder--instead 
only a stereo decoder--so only two channels of audio are encoded in the 
stream.

However, most ALSA drivers will automatically replicate the left- and 
right-front channels to the left- and right-rear (and surround if you 
have a 7.1 channel setup) speakers since it has only a minor effect on 
the perceived positionality of the sounds.  If your driver doesn't, you 
can easily add a translation table to your .asoundrc to do it yourself.

If you really want 5.1 channel, though, you'll have to go to a digital 
video broadcast technology (like HDTV (US) or DVB).

Mike