[vorbis-dev] New LSP code committed

[Ian]

Monty wrote:
> 
> So, it turns out (and another implementation actually explicitly mentions it)
> that LSP->LPC computation using the FIR algorithm is very sensitive to noise
> (iterative algorithm) and really really requires doubles [we're not kidding].
> This was complicating things for folks pursuing fixed point implementations,
> and also was a potential source for bugs if FP optimizations got out of hand.
> This may be the cause of the Win32 encoder 'blorps' (which also seem to affect
> PhatNoize).
> 
> (Note that the LSP problem, when it exists identically in the encoder and decoder will seem to almost disappear).
> 
> In any case, the new LSP code is committed and should head off all of the above possible problems.  As a useful side effect, LPC IIR filters and the iFFT are now completely eliminated from the decode side and decode with current streams is about 10% faster.

If I remember correctly, Tony Million (of Sonique/nad) used complex
numbers (that could only be computed in small chunks) to exact even more
precision than doubles for this reason in several versions of his nad
MP3 decoder.  (At the time, the CPU overhead was horrible, but now might
be acceptable.)

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