[vorbis] frequency cutoff?

[newslists]

> When I tested -q4 after the RC3 release I was getting very little energy
> above 17.5Khz.  It may well be a maximum of 20.5 but it must have a very
> long lowpass gradient/width (I'm not sure of the scientific term).

Actually, its probably more due to the absolute threshold of hearing (ATH),
which basically translates into that we cannot distinguish high frequencies
if there are low frequencies present at levels that mask according to the
ATH curve. Vorbis takes this into account when encoding.  Here's a graph of
a typical curve: http://ccrma-www.stanford.edu/~bosse/proj/node11.html  Note
the x-axis is in log scale so the high frequency end is really bunched up,
but as you can see high frequencies need to be much louder to be heard than
low frequencies...

Later,
Alan

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