Walter Knapp <> wrote:
>It's really a gross simplification that it removes 80 percent. It
does
>reduce the disk space required by that amount, but that's not the
same
>as removing 80 percent of the sound or the sound samples. It gets
to
>that disk space by analyzing the sound and writing instructions as
to
>ow it's decoder can synthesize a reproduction of the sound. It
comes
>very close to preserving 100% of the sound, some would say it
actually
>succeeds at that.
In fact, the compression is achieved by reducing the bit-depth in
some frequency bands. This is an adaptive process that is controlled
by the properties of the sound being recorded (and the properties of
the human auditory system). If the bandwidth of the incoming signal
limited (lets say below 8 kHz), the compression will have little
effect on the sound quality (even in more complex sounds). However,
if there are larger signal signal bandwidths (or rapid frequency
modulations over large frequency ranges), the compression will lead
to some degeneration. This would explain why low-frequency impulsive
frog calls are less affected than higher pitched bird or insect
sounds. The example posted by Jeremy is a nice illustration, how
rapid frequency attacks with larger bandwidths may lead to serious
distortion.
> > I had two or three ideas about the differences I'm hearing:
> >=A0 (a) the Sony is doing some un-specified post-processing to
handle cases
> > where it must generate a lot of 'filler' white noise, to insure
that it's
> > either masked by higher frequency noise, or as non-periodic as
possible;
>
> It needs to generate no filler. I've not seen any sonograms of any
ATRAC
> recording where deliberate filler was introduced. It does not do
this,
> it only works to reproduce what it got not invent new stuff.
I agree, that ATRAC does not add 'filler' noise in order to mask
artifacts. However, the decoder itself will introduce some kind of
white noise. This is a (unwanted and system-inherent) result of the
reduced bit-depths (also called quantization noise). This kind of
noise is clearly visible in Jeremy's recording at t=3D6,3 sec:
http://www.avisoft-saslab.com/compression/MDtest2MD.gif
Best regards,
Raimund Specht
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