From: jj@alice.att.com (James D. Johnston)
Newsgroups: comp.compression
Subject: Re: Too much lossy encoding?
Keywords: PASC, audio
Date: 10 Apr 92 19:00:03 GMT
Reply-To: jj@alice.UUCP (James D. Johnston)
Organization: Signal Processing Research Department, AT&T Bell Labs

In article <1992Apr8.160300.5981@tc.fluke.COM> kurt@tc.fluke.COM (Kurt Guntheroth) writes:
>Is it just me or is anyone else out there concerned that we are using lossy
>encoding "too much" as a substitute for denser storage and better transmission
>methods?
It's not just you.  A lot of people who don't understand what
"Perceptual Coding" means are concerned, and somewhat rightly
so, because some folks are proposing the use of perceptual coders
in places where they don't belong. (i.e. the users don't understand
what they do, either)

Perceptual coders have one basic hypothesis, namely that the
decompressed signal will be presented to the viewer/listener
with minimal processing.  If you violate this hypothesis (the
multi-encoding results from the CCIR testing are a good
example), all quality guarantees are off.

>Now proposals like PASC and audio MPEG are coming along
>that are lossy; they filter out sounds "we can't hear anyway".  We are moving
>down from being "about as good as a phonograph" to "about as good as FM radio"
>to achieve playback on media that are still undergoing rapid development.
I find your comments somewhat provoking. It is possible to use
PASC, PAC, ASPEC, OCF, PXFM, MPEG-Audio, and such incorrectly,
and thereby degrade a signal, but in general, with an enlightened
user, this won't happen.  Certainly, MPEG-Audio Layer III at
192 kb/s/channel with Psychoacoustic Model II is MUCH MUCH MUCH
better than "FM radio".  If you are claiming otherwise, you are
making a statement that is factually wrong.  In fact, a statement
that is shown wrong at 96kb/s for Layer III-Psych II, by the
MPEG-Audio tests.  Please do not make such assertions, unless
you have information not available to those of us who oversaw the
subjective tests and wrote the draft standard.

Certainly, the wrong combination of Layer and bitrate is very bad
news, but we didn't regard our mission as preventing user misuse.

>off that we accepted so much lossy encoding that we can in fact detect?
>I hope everybody implementing this technology will think about this.  You
>are going to have to live with your decisions.  

15 years ago, when I started to work in the compression field,
the claim was that new transmission media would make compression
unnecessary.  As time goes by, the required rate for transmission media
continues to rise even faster than the fantastic increase in the
ability of the technology to cope.  People have been predicting the
demise of compression technologies since the 1940's.  I'm not
holding my breath.

As to use of lossy coding, as long as one is careful how one uses a coder,
and learns what it's for in the first place, one should not be
"sorry".  Of course, somebody will get it wrong, but why should
we blame that on the technology.  That's like blaming the
inventor of the chopstick for a stabbing with a sharpened chopstick.
-- 
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