[mythtv-users] Not getting Dolby Prologic out of s/pdif

[Warren]

I guess it depends on the providers.  I capture Star Trek: TNG on my
replaytv (2200 kbps video 192k mpeg-2 audio) and the pro-logic survives
just fine for playback.  I am new to myth, have played with WinPVR 250,
350 and 150 cards and the encoding they do is better than that of the
replaytv so there is no reason from a hardware standpoint that the dolby
pro-logic encoding should be lost.

W

Jeff Clemens wrote:
> Prologic (the original) is basic stereo sound with the rear channel
> encoded in both left and right out of phase with each other, and the
> center channel encoded in both left and right in phase.  The prologic
> decoder in your reciever then sends L+R to the center channel, and L-R
> to the surround channel. It also has some more advanced logic to try
> to remove true L and R channel information from the front and rear
> speakers, but its not inherently encoded.  In any case, its a far from
> perfect encoding scheme, but does allow the TV tuner card to capture
> the encoding without having to do anything special (all it does is
> record the L and R signals). 
>  
> That said, the redundant information necessary to allow prologic to
> work may not make it through the compression process, its hard to
> say.  Generally, the human ear/brain system does not process phase
> very well, so phase information may be dropped in the compression as
> irrelevent (depending on the compression level and type used by your
> tuner card).  Analog TV may or may not be encoded in prologic (that
> depends on the broadcaster).  If it is, it would still not be
> comparable in audio quality to Dolby Digital or DTS, which both use
> independently recorded and compressed channels for each of the 6
> channels of audio.  If you are comparing analog TV audio with a DVD or
> something, there's no comparison.  However, even with poorly encoded,
> compressed and decompressed prologic, you should get some improvement
> over straight stereo.  Try switching back and forth between stereo and
> prologic modes on your reciever while watching analog TV and see if
> you notice a difference. 
>  
> I've noticed with my local broadcasters, and some national cable
> channels, that the stereo image seems to be pretty narrow.  In other
> words, almost all signal is recorded in both the left and right
> channels, in phase.  Thus, when I have stereo on, its like running the
> same signal through both speakers.  When I have Pro-Logic on, almost
> the whole signal comes through the center channel, and only background
> noises and some music come through the L, R  and surround channels. 
> This is just due to basically un-encoded, or poorly encoded Pro-Logic
> on the transmission end, and there's nothing you can really do about it. 
>  
> heh,
> Probably way more information than you really wanted,
>  
> Jeff
>
>  
> On 3/15/06, *Bryan Bennetts* <bbennetts at blueyonder.co.uk
> <mailto:bbennetts at blueyonder.co.uk>> wrote:
>
>     Hi all,
>
>     I know there's been loads of posts on s/pdif & 5.1 topics, but
>     I've been
>     bashing my head against this for quite a while and still am stuck :-(
>
>     I'm using the s/pdif output on my VIA 82xx sound card with success
>     when
>     playing DVDs in myth-dvd, all my speakers produce sound and
>     everything is
>     hunkydory.
>
>     However, when watching live tv I'm only getting the front L & R
>     speaker
>     producing (not very good) sound.  I was expecting all the speakers
>     to emit
>     sound in a dolby prologic stylee, or am I mad?
>
>     I have AC3 pass through set in both my Xine config and mythtv
>     settings.
>
>     Here's the relevant (?) snippets from my .asoundrc :
>
>     pcm.!default {
>     type plug
>     ## Uncomment the following to use mixed analog by default
>     #  slave.pcm "dmix-analog"
>     ## Uncomment the following to use unmixed digital by default
>     slave.pcm "digital-hw"
>     ## Uncomment the following to use mixed digital by default
>     #  slave.pcm "dmix-digital"
>     }
>
>     pcm.digital-hw {
>     type hw
>     card 0
>     device 1
>     }
>
>     # Control device (mixer, etc.)
>     ctl.digital-hw {
>     type hw
>     card 0
>     }
>
>     Any guidance or pointers to relevant threads/docs would be greatly
>     appreciated.
>
>     Cheers, Bryan.
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