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

Steven Adeff adeffs.mythtv at gmail.com
Wed Mar 15 15:55:13 UTC 2006


On 3/15/06, Jeff Clemens <teedub303 at gmail.com> 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> 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.

Pro-Logic should make it through the encode process untouched if you
give the audio enough bitrate, My PVR-150 creates audio with a bitrate
of 384kbit which is defnitely enough.

--
Steve


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