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From: adrian@eagle.UUCP (A.Freed)
Newsgroups: net.audio
Subject: Re: Vinyl vs. CD recordings- a reasonably informed view...
Message-ID: <1197@eagle.UUCP>
Date: Mon, 24-Sep-84 00:35:45 EDT
Article-I.D.: eagle.1197
Posted: Mon Sep 24 00:35:45 1984
Date-Received: Wed, 26-Sep-84 05:16:54 EDT
References: <3050@watcgl.UUCP> <61@unc.UUCP> <829@opus.UUCP>
Organization: AT&T Bell Laboratories, Summit, NJ
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This is a brief attempt at some sensible discussion on Vinyl and CD recordings,
from someone involved professionally in related things for some time:

Places where problems can be introduced:
	A/D conversion (filtering and sampling)
		unless you are computer synthesising your sound directly,
(and few of us are) some conversion process has to be performed. Designing
and measuring high quality A/D conversion systems for audio is not understood
by many engineers yet. There is no perfect anti-aliasing filter! You can
never do better than the original source. Alot of CD sources are analogue
masters, and even worse copies of copies of masters.

	Interpolation on bad CD data
		as far as I can tell, unless you go to a lot of trouble
to damage the surface of a CD, most CD machines won't ever have to invent
sound for you, to make up for uncorrectable erroneous data.

	D/A conversion and filtering on output
		This problem is also new to most engineers. The most important
way in which CD players differ as far as the sound is concerned is the
characteristics of their anti-aliasing filters. This business of one
or two DACS is a red herring. Fixed phase shifts are irrelevent. Frequency
dependent shifts are the problem. There is also the problem of the non-linearity
of the output of a DAC. The output can never be a step function (infinite
energy would be required). Careful correction has to be done to make sure
any distortion components are minimised or moved to a filterable frequency.
There are all sorts of other conventional noise and distortion problems
from the rest of the analogue chain to the output.

From what I have read, few people are measuring any of these new performance
parameters. Manufacturers are not publishing specifications of them
and people really can hear the difference! One of the reasons are that
they are not widely understood. Another is that they are not easy to measure.Even when they are published, there is not enough psychoacoustics
research for us to make deep conclusions. Listening to your favourite
music on machines that other people have recommended and that will be
well serviced, is still the best way of choosing HIFI gear. 
People are confusing the near perfect medium, with the far from perfect
reproduction process. (a problem that reproducing organisms have to!).

The following things excite me about CD:

	The possibility of storing data about the music on the disk
for manipulation by home computer.

	Being able to make perfect copies of what are not far from
masters (when the recording industry is totally digital). There are
lots of legal problems with this, but I am fed up with scratched disks
and cassette tape.

The Future

It may be a depressing thought but there will probably be several
CD formats. I would like a smaller one for a walkman (Sony have a prototype
CD walkman). I would also like a larger capacity format for encyclopaedic
collections. There will also be (some day) an integrated format for
video/sound/data.

Why don't we just post revues of CD's we have enjoyed and problems
with CD machines? The new CD magazines are not giving good revues of
enough disks, nor will they flame manufacturers who advertise in their
rags!