An idea would be like for WMA 9 lower bitrates (32-42-48Kbps) to use a
'crystallizer'; which is basically a harmonics booster focussed at
transposing sharp tones some octaves higher.
Eg:
A file has been recorded @ 20khz computer (or 10khz real life) to preserve
space.
While playing back the file sounds a bit mushy, almost as if someone was
speaking through a cardboard wall. The higher frequencies are missing, and you
can easily perceive that the file is a low bitrate file.
By harmonically transposing frequencies from 6 to 10Khz with one octave, and
overlaying the original file with that stream, one can create the perception
that an additional stream of 12-20Khz has been encoded. This effect is extremely
usefull since altered (or artificial) frequencies below 100Hz and above 6Khz are
enormously hard to hear!
Especially in speech!
In music this would translate to that the cymbals of all songs or drums sound
about the same, and one can clearly pick the original recording from the
compressed one.
But for voice this is hardly noticable since very few articulated tones actually
go over 10Khz.
The letter 'S' and 'C' are two of them.
By this a much higher quality is perceivable (on low bitrates), the software
only needs to be included in the decoder, and technically could still work with
older compressed Speex files.
I just wanted to throw in this idea, since it works quite nice on WMA; and for
now I'm encoding speech files with 48kbps (speech with background music) or
32kbps (speech as being preaching, bookreading or lectures).
I wish to see that number going down though. If Speex performs better, has
smaller files, and can compete with 32kbps WMA9 encoded files, I might become
interested in testing out the product, and perhaps switching over!
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