[mythtv-users] How do you send TV-Out audio to the sound card?

[Michael T. Dean]

Kevin Barsby wrote:

>>Take the L/R audio out and feed it into a L/R female RCA to 1/8 male 
>>adapter.  Plug this into the input of your sound card and make sure mute 
>>is off.  HTH.
>>    
>>
>
>I found this yesterday while musing over the same problem, and it raises
>some other questions for me.
>
>Presumably the reason for this workaround is once the mpeg2 stream is fed 
>into the decoder, what comes out the other end goes straight out of the
>output leads and the audio can't be rerouted?
>  
>
... without a cable.  :)

>So, firstly, what about AC3 (dolby 5.1) sound on DVDs, by running this
>though the MPEG2 decoder on the 350 do you lose this feature of the DVD?
>  
>
Unless you have a DVD player application that was patched to send the 
(decrypted) DVD MPEG2 stream directly to the on-board decoder (and set 
all appropriate registers on the card to accept DVD input), all DVD 
decoding is performed by your CPU.  Therefore, your DVD player is also 
decoding the DVD's audio stream, so you have full access to any audio 
output supported by your DVD player/audio drivers.

The only patch I know of to allow sending the DVD stream directly to the 
on-board decoder is Chris Kennedy's mplayer-ivtvosd patch ( 
http://kmos.org/~ckennedy/ivtv/mplayer-ivtvosd.diff ), which only works 
with the PVR-350's TV out (because the decoder only outputs to the 
card's TV out--much like the decoder only outputs audio to the card's 
audio out).  To use the patch, you must start MPlayer and specify the 
audio and video output devices.  Therefore, a command line of (assuming 
all other MPlayer options and .asoundrc are properly specified and 
assuming you're using the latest MPlayer, version 1.0pre4):

mplayer -vo ivtvosd -ao alsa1x:TheNameIUsedInMyAsoundrcForMySPDIFOutput 
-ac hwac3

Would send the stream to the hardware decoder for video output and to 
the CPU to strip the AC3 signal for output (without decoding) by the 
ALSA drivers through your S/PDIF output.  You could also have MPlayer 
decode the AC3 stream in software and output 6 channels through three 
1/8" analog jacks on the sound card (or motherboard, if integrated sound).

Note that creating a proper .asoundrc is a whole subject in itself...

The mplayer-ivtvosd patch (obviously) only works with MPlayer.  Since 
MPlayer doesn't support DVD navigation (menus), it leaves something to 
be desired.  Xine supports DVD navigation, but--since the on-board 
decoder does not--your choices with xine would be to write a xine video 
driver to use hardware decoding and lose navigation support or use CPU 
decoding and have navigation support (which is probably why no one has 
written a xine video driver).

Note that you could write a program/script to initialize the card for 
decoding DVD streams and send the stream directly to the hardware 
decoder.  The decoder itself would output only 2-channel analog audio 
(possibly with Pro Logic encoding), but media players that can handle 
DVD's can handle DVD audio streams...

>Also are there sync issues by routing the audio back though the sound card?
>  
>
Haven't yet purchased my RCA to 1/8" Y-adapter, yet, so I can't say...  
(But I would venture a guess that if there were issues, no one would be 
using that solution.  I would also say that you could probably induce a 
problem with improper cabling, but it would probably take some really 
long wires--which would probably degrade/destroy the audio quality.)

>Is there a planned hack to the ivtv driver (OT here I know, but you might
>know) to syphon off the audio stream post mpeg2 decoding?
>  
>
Not necessary.  You want to siphon off the encoded audio stream for your 
media player.  The media player does the rest.  See above.

>I'm a little unsure of the inner workings here so any info on the problem
>would be appreciated.
>  
>
HTH.

Mike