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

[Jeff Clemens]

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> 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.
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
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>
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