[theora] Fixed Quantizer - Fixed Quality

Marco Al marco at simplex.nl
Tue Mar 25 17:52:53 PST 2003

From: "Dan Miller" <dan at on2.com>

> everything you say is basically true.  However, what you are not
> accounting for is that it is the job of the codec to define what "Q=8"
> means.

I think the general assumption was that you meant quantizer by Q, not
quality. Christoph most certainly means quantizer with Q.

> I suspect that audio codecs, particularly Vorbis, do this intrinsically,
> because their internal psycho-accoustic models tend to be rather complex.
> In the video world, for reasons that elude me, this is not the case.  I
> know of no codec that incorporates any useful psycho-visual model into its
> encoder (though there are encoding apps that sit on top of codecs that
> claim to do this).

I think audio is also easier because our hearing is mostly frequency
sensitive, and our sight more structure sensitive. To oversimplify ... our
hearing perceives the max error, which makes quantization for constant
quality much easier, but our sight perceives an error which is a more
complex function of the errors at the seperate frequencies.

A coding mode which puts a hard limit on a MB's MSE shouldnt be too slow or
hard to code BTW. Would be an easy point to start for constant quality


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