From: monta@image.mit.edu (Peter Monta)
Subject: Psychoacoustic source coders
Organization: MIT Advanced Television Research Program
Date: Tue, 29 Sep 1992 22:17:58 GMT

In article <1992Sep29.175546.9983@porthos.cc.bellcore.com> shu@maestro.bellcore.com (Stephen Shu) writes:

> ... Supersound performs compression in the time domain rather than
> in the frequency domain.  This is not to say that Supersound does
> not utilize some psychoacoustics.   However unlike DCC, Supersound
> utilizes extremely moderate psychoacoustic principles.  As a result,
> in my professional opinion, Supersound is one of the highest
> CD-quality audio compression schemes comercially in the market,
> if not the highest quality.

> ...
>	   2) Supersound is also a lossy compression scheme. However,
>	   speaking in layman's terms, Supersound does not try to
>	   throw away as much data as possible.  Supersound reduces
>	   the data (and hence your music in some sense) by only roughly
>	   20%.  In fact, in many cases, the reconstructed signal is

Any coder can achieve high quality at high rates.  If I interpret
the 20% number correctly, then it runs at about 14 bits per sample.
Even trivial block-companding schemes are transparent at this rate.

>	   Not a fact, but my opinion: 
>	   At this point, compression schemes are like comparing
>	   apples and oranges, although some qualitative statements
>	   can be made (In the field there are also quantitative
>	   measures).  Because DCC utilizes the frequency domain and
>	   because DCC pushes psychoacoustics to the limit, some artifacts
>	   will be audible.  Furthermore, "errors" in the frequency
>	   domain spread themselves out in the time domain (ask any
>	   electrical engineer).  These artifacts tend to smear themselves 
>	   out and become audible in passages like triangle hits.  In short,
>	   I contest that people who utilize psychoacoustics for CD-quality
>	   audio must inevitably find way to "patch" their algorithms
>	   so that these extreme artifacts are less audible (using special
>	   windowing techniques, etc.). 

You are referring to preecho, which can indeed be addressed by a
time-varying analysis/synthesis window, although there are other
techniques.  These are not "patches"; they're an essential part of
the design.

The situation with artifacts is substantially the same as with PCM:
wind the rate down far enough, and you'll hear stuff.  This is not
a fault of psychoacoustics, it's a fact of life.  Current coders,
including the one I'm working on, perform very well at their target
rates, just as PCM performs very well at 16 bits.

>	   My opinion:
>	   Psychoacoustics is bad from an audiophile's perspective.  One
>	   should not have to "hide" such large artifacts.  The Supersound
>	   approach is much better ... don't have such large artifacts
>	   in the first place.

"Psychacoustics is bad from an audiophile's perspective"?
Psychoacoustics *is* the audiophile's perspective, as well as that of
all the	rest of us poor humans.  These new coders address the issue
of how to send high-quality audio at low rates.  They use models
that are more closely matched to a human listener; they give high
quality at rates on the order of 2.0--2.5 bits/sample.

No one argues that if you've capacity to burn, a simple coder may
be the right thing.  Certainly CD is a good design.  Coders at rates
lower than 16 bits/sample were around in 1980, but they would have
been expensive, and they weren't needed:  the medium could support
a high data rate, and so a simple PCM scheme was selected.  There is
nothing magic about PCM---it can have high quality, it's easy to
do arithmetic on, etc. but it's a bandwidth hog.

Peter Monta   monta@image.mit.edu
MIT Advanced Television Research Program
Newsgroups: rec.audio