[CELT-dev] improve CELT

[Timothy B. Terriberry]

> There are still several ways to improve CELT, such as by incorporating better
 > psychoacoustic masking in the dynamic bit allocation. This is a difficult
 > problem both because there are few bits available for coding the 
allocation
 > and because the analysis window is short.
>
> it is possible to implement that and what are the correct approaches to do that.

Several people have tried, including Garf and Jean-Marc. No one has been 
able to demonstrate an improvement, even when you don't count the 
overhead required to signal the allocation, i.e., even when you flat-out 
cheat. This does not mean it can't be done, but it does mean we don't 
know how to do it.

We have considered adding a method of coding a boost to one or two bands 
(e.g., reusing the PVQ code), which is signaled with low probability. 
That would have little impact on the normal case where it is not used, 
but the lower the probability, the bigger the gain you'd have to get 
from your psychoacoustics before it would be useful. The other drawback 
is that if the feature is not used in practice, it will not be 
well-tested, and thus unlikely to work if you ever do figure out how to 
use it.

If anyone _can_ identify a method of doing dynamic allocation that makes 
a consistent improvement, that would be very useful. We are hoping to 
finalize the bitstream in the near future, though, so there isn't a lot 
of time left to try.