[vorbis] channel coupling in rc2

Gian-Carlo Pascutto gcp at sjeng.org
Mon Sep 3 06:46:23 PDT 2001

On Mon, 3 Sep 2001, Greg Wooledge wrote:

> Gian-Carlo Pascutto (gcp at sjeng.org) wrote:
> > What I've been wondering about is why you would want to pick the
> > channel coupling mode yourself.
> The same reason we want to pick the lowpass cutoff frequency ourselves
> (hint, hint).

Same question, why?

If you think the current lowpass is too low for your hearing then the
solution is more likely a higher *bitrate* than a higher lowpass.

My fear with allowing people to select channel coupling is exactly
the same. For some reason there seems to be a crazy idea with a lot
of people that mp3 joint stereo causes stereo image degradation.
Imagine the horror if they would find out that Vorbis's advanced
stereo modes are somewhat similar to intensity stereo. The WP expert
audiophiles even heard stereo degradation in uncoupled Vorbis files.
This is _all_ about psychology. (Guess that is why Vorbis has
'lossless stereo' instead of 'almost M/S stereo' :)

Some of the FAQ's of mp3 newsgroups even specifically _disrecommend_
joint stereo 'becauses it is the prime cause of bad sounding mp3s'.
Arghl. No wonder if you recommend BladeEnc...

Back to vorbis. Currently the stereo modes are not a black-and-white
issue. Several stereo modes are used at once, depending on the
frequencies that are being encoded and the actual audio content.
The encoder intelligently switches between them.

If you are going to let the user switch between channel coupling
modes, you can't depend on the user to understand these criteria
(hell, even I am not sure about them). So what will be left is
most likely a switch like

-mn   standard stereo coupling for selected bitrate
-ml   lossless channel coupling
-ms   seperate stereo (no channel coupling)

If you present this to the user, I am a whole lot will
blindly select lossless coupling or seperate stereo. And end up
with ogg's that sound like crap compared to what they would
have gotten with the standard coupling.

Now, as I understand the current encoder, disabling the coupling
will automatically give you higher bitrate oggs to compensate.
That would be good, because it prevents users from shooting themselves
in the foot with this. But it will cause problems because they
end up with bigger files than they expected.

But this brings us back to the beginning. If you want to fiddle
with this, you want to select a higher *bitrate* instead of
fiddling with the encoding setup.

Perhaps there is a misunderstanding here about what the bitrate
switches do.

Selecting a higher bitrate IS effectively picking an encoder
setup with a higher lowpasss, less aggressive coupling and

The only _good_ use of selecting a coupling mode that I can
imagine so far is lossless coupling (or no coupling) for (e.g.)
Dolby Surround encoded audio. So I'd propose a switch like

-md  Use this for Dolby Surround encoded audio

That way, you solve this case and prevent others from doing
things they don't really want to be doing.

Now, perhaps God(*) has some good ideas about how to let the
user pick their channel coupling modes _and_ prevent them
from shooting themselves in the foot at the same time. If
so, I'd love to hear about them.

(*) Monty


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