From: shu@maestro.bellcore.com (Stephen Shu)
Subject: Re: Sound quality of DMX, DCC, Supersound
Organization: Bellcore (Red Bank, NJ)
Date: Tue, 29 Sep 92 17:55:46 GMT

>I was under the impression that Digital Music Express was NOT compressed
>and that it is in fact capable of providing a digital output suitable
>for d/a convertors in common use.
> 
>Is dmx compressed?  If so, is it a lossless compression or a sub-band
>scheme similar to those used by dcc or minidisc?  Inquiring minds want
>to know and all that.  A reference to ageneral description that goes a
>little deeper than, "oh, wow! isnt this great?" would be appreciated.
> 
>Mike.Coyne@utxvm.utexas.edu
>Coyne@utxvm.bitnet

Let me first state that I cannot divulge propriety information.
All information described here is publically available in some form
or another.

Broadly speaking, the DMX cable music service utilizes the compression
schemes and various hardware developed by a company called FSLI, 531
West Golf Road, Arlington Heights, IL 60005.  The firm works 
in the areas of audio compression, audio enhancement, digital audio 
applications, video compression, and video applications primarily.
At any rate, inventors at that firm have publically available patents
on the compression algorithms utilized.

As described in the patent, the compression scheme (known
at FSLI as "Supersound") is unlike the psychoacoustic compression 
scheme utilized in DCC.  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.  (By the
way, I did not invent Supersound, for those who snicker)

Facts:
	1) DCC is a lossy compression scheme.  This means that when
	one takes a CD and utilizes the DCC compression algorithms,
	some distortion occurs.  Lossless compression schemes, on
	the other hand, have exactly that ... no loss during
	compression and decompression.
	
	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
	exactly the same as that comes off the CD at the central office.
	This is largely because the compression scheme is so modest.
	Supersound utilizes a very non-aggressive approach to data
	reduction.  For lossy compression schemes, this generally means 
	that any distortion will be extremely low.

	3) All lossy compression schemes have artifacts.  Psychoacoustics
	is not magic, it is simply an aggressive compression technique.

	4) DCC utilizes the frequency domain to compress.  Thus,
	DCC is able to exploit characteristics of the frequency
	domain to get a higher rate of lossy compression.  DCC is
	thus able to reduce the music data by as much as 70%-80%.

	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.). 

	5) Supersound utilizes the time domain.  Hence, artifacts
	in the time domain spread themselves out in the frequency
	domain.  However, since the data reduction is so minimal, the
	generated artifacts can be easily "shaped" into inaudible regions
	of the spectrum. 

	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.

FSLI has developed a new audio compression scheme known as RQAT ("r-cat"),
which stands for Recording Quality Audio Transmission.  The audio philosophy
behind the technique is to achieve higher compression rates with the
same or better quality than Supersound (as Supersound has evidently
been accepted by the general public and audiophiles).  That is, in my 
opinion, the philosophy of RQAT and Supersound is audio quality first,
data reduction second.  On the other hand, DCC is of the philosophy 
to reduce the data as much as possible first, sound quality second 
or close second.

Again, let me state that one should not judge DCC before one hears
it for themselves.


Steve Shu