lowpass option (Was: RE: [vorbis] channel coupling in rc2)

[Ross Levis]

I would very much like a lowpass option because for FM radio broadcasting I
don't want to encode frequencies above 15khz.  I'm waiting for this option
before switching to ogg from mp3(lame).

Ross.

> -----Original Message-----
> From: owner-vorbis at xiph.org [mailto:owner-vorbis at xiph.org]On Behalf Of
> Gian-Carlo Pascutto
> Sent: Tuesday, 4 September 2001 01:46
> To: vorbis at xiph.org
> Subject: Re: [vorbis] channel coupling in rc2
> 
> 
> 
> 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
> masking. INTERNALLY VORBIS DOES _NOT_ USE BITRATES (yet).
> 
> 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
> 
> -- 
> GCP
> 
> 
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