[vorbis-dev] Impulses

[Mark Taylor]

> 
> > 
> > Do you guys really think window switching is so bad?  It clearly
> > works very well and is not just 'mp3' quailty, since it is used
> > in AAC which is pretty much the best encoder out there.
> 
> AAC, on paper, does not rely only on window switching.  Its 'noise shaping'
> (although not done in the time domain) is effectively a nifty time domain hack
> to better deal with very wide dynamic range variance across a window.
> 

I've also never seen a real AAC encoder, but I'm pretty sure 
they use a lot of window switching.  This paper
Bosi et al. "ISO/IEC MPEG-2 AAC", J. Audio Eng. Soc. 45 (1997) p 789-814,
discusses it quite extensively.  Here's a quote from the introduction
to the window switching section:

"
The adaption of the time-frequency resolution of the filter bank to
the charactersitics of the input singal is done by shifting
between transforms whose input lengths are either 2048 or 256 samples.
The 256 sample length for transient signal coding was selected as
the best compromise between frequency selectivity and preecho 
suppression at a data rate of around 64kbs per channel
"

Mark

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