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Path: utzoo!watmath!watdcsu!herbie
From: herbie@watdcsu.UUCP (Herb Chong, Computing Services)
Newsgroups: net.audio
Subject: Sheffield CDs and why they sound bad
Message-ID: <460@watdcsu.UUCP>
Date: Mon, 1-Oct-84 20:01:05 EDT
Article-I.D.: watdcsu.460
Posted: Mon Oct  1 20:01:05 1984
Date-Received: Wed, 3-Oct-84 07:44:09 EDT
Organization: U of Waterloo, Ontario
Lines: 30

Some people have been complaining about the sound of Sheffield Lab's CD
releases.  One of the problems with digital recording as attempted in
the manner that Sheffield does is that the digital recorder does not
have enough dynamic range.  We have people in our physics audio lab
designing and building a dynamic range compression device for digital
recorders because of that.  One theory has it that a digital recorder
with at least 120 dB SNR is required to provide adequate dynamic range
for live digital recording.  If you record to capture the peaks of
something like LAB-2 (I've Got the Music in Me by Thelma Houston), the
lowest levels will be about 40 dB below that.  This leaves you with 50
dB SNR, which is fine for analog systems, though not too listenable,
but not for digital systems.  This ratios is a voltage difference of
only about 300 times.  This means that the lowest range is quantized
with only about 8 bits (2^8 = 256).  What does 8 bit quantization sound
like?  I've heard demos at 10, 8, and 4 bits quantization.  Something
sounds wrong at 10 bits; at 8 bits, everything sounds harsh; at 4 bits,
the sound is barely recognizable.  A compressor can be used to place
the signal up where more bits are being used and quantization error is
not as noticeable.  This is a problem with all digital recording
systems handling a high dynamic range signal.  More bits are required,
or a signal compression system, to get around this.  Of course, 120 dB
SNR is hard to maintain in analog equipment, and a digital system using
linear encoding would require 20 bits to get that much SNR, and
finally, the number of bits generated would be 1.5 times as great.  Any
comments?

Herb Chong, BASc
Computer Consultant 
Department of Computing Services
University of Waterloo