Mailing List Archive

On Tuesday 11 February 2003 02:00 pm, Cliff Draper wrote:
> Hi,
>
> After spending sometime configuring hardware (I had to replace my
> Fortismo III with a SB Live, and replace an old ATI video card with an
> old nVidia card, and get rid of the generated mysql password), I'm now
> at the point where I can start trying to get the best results out of
> my system.
>
> Some broadcasts are sent in Dolby ProLogic encoded in 2 channels and I
> want to make sure that when LAME is used to do compression, it doesn't
> "compress out" the Dolby ProLogic information. My understanding is
> that this means LAME should be run with full stereo (sometimes simply
> known as "stereo" I believe) instead of "joint stereo". Does MythTV
> use "joint stereo"? And if so, how can I switch it to use full
> stereo?

It always compresses in full stereo mode, not joint stereo.

Isaac

----- Original Message -----
From: "Isaac Richards" <ijr@po.cwru.edu>
To: "Discussion about mythtv" <mythtv-users@snowman.net>
Sent: Wednesday, February 12, 2003 4:10 AM
Subject: Re: [mythtv-users] MythTV & Dolby ProLogic question


> On Tuesday 11 February 2003 02:00 pm, Cliff Draper wrote:
> > Hi,
> >
> > After spending sometime configuring hardware (I had to replace my
> > Fortismo III with a SB Live, and replace an old ATI video card with an
> > old nVidia card, and get rid of the generated mysql password), I'm now
> > at the point where I can start trying to get the best results out of
> > my system.
> >
> > Some broadcasts are sent in Dolby ProLogic encoded in 2 channels and I
> > want to make sure that when LAME is used to do compression, it doesn't
> > "compress out" the Dolby ProLogic information. My understanding is
> > that this means LAME should be run with full stereo (sometimes simply
> > known as "stereo" I believe) instead of "joint stereo". Does MythTV
> > use "joint stereo"? And if so, how can I switch it to use full
> > stereo?
>
> It always compresses in full stereo mode, not joint stereo.
>
> Isaac

Why would you want full stereo mode? It's just an urbanlegend (kind of,
since it used to be true) that full stereo is superior to joint-stereo. For
those not familiar with join-stereo, it compares the difference between the
two channels and thus mannages to have a higher quality/filesize ratio.
Encoders used to be kind of bad at doing this, and so it got a bad rep.
However, nowadays lame does a terrific job.

Rasmus

ps. this is what I've read up on over the internet, so I might be wrong. If
so, please do correct me :) .ds

From: "Rasmus Ekman" <rasmus.ekman@kryddsill.net>
Date: Wed, 12 Feb 2003 04:51:00 +0900
> those not familiar with join-stereo, it compares the difference between the
> two channels and thus mannages to have a higher quality/filesize ratio.
> Encoders used to be kind of bad at doing this, and so it got a bad rep.
> However, nowadays lame does a terrific job.

Joint stereo is great for 2 channel sound sources, as the compressed
stream is based off of both channels, compressing out little
differences. And it is exactly those little differences where Dolby
ProLogic stuffs it's extra info.

http://www.digvid.info/tmpgenc/beginners.php:
"Joint-stereo: This makes low frequency bands mono, and high frequency
bands stereo. The human ear is more sensitive to the direction of high
frequency sounds. Note that this setting destroys Dolby Prologic
information, so if you source contains Dolby Prologic information
choose Stereo mode instead."

From http://lame.sourceforge.net/USAGE:

"jstereo means the encoder can use (on a frame by frame bases) either
regular stereo (just encode left and right channels independently) or
mid/side stereo. In mid/side stereo, the mid (L+R) and side (L-R)
channels are encoded, and more bits are allocated to the mid channel
than the side channel. This will effectively increase the bandwidth
if the signal does not have too much stereo separation.

Mid/side stereo is basically a trick to increase bandwidth. At 128 kbps,
it is clearly worth while. At higher bitrates it is less useful."

Cliff Draper Sun Microsystems, Forte Tools
My opinions may or may not reflect those of my employer.
---------------------------- food for thought ---------------------------
If you can lead it to water and force it to drink, it isn't a horse.