[CELT-dev] On guessing theta

[Timothy B. Terriberry]

Benjamin M. Schwartz wrote:
> However, this orthogonality is still useful.  If m is encoded in N
> dimensions, then from this orthogonality s can be encoded in N-1
> dimensions.  That means that transmitting theta no longer represents an
> extra degree of freedom.

This was one of the first things I proposed to Jean-Marc. Well, more
accurately, I proposed to still encode s in N dimensions, but to use the
orthogonality constraint to take the place of the spectral folding used
to add a noise floor for mono. The reason is that it is very
computationally expensive to determine a basis for s that is orthogonal
to an arbitrary m (O(N^3)). We could have special cased N=3, but...

This approach turns out to harm quality. The quantization in m is too
large, especially for HF bands, for this constraint to actually be
useful, and it was doing more harm than good.