From: peter@memex.co.uk (Peter Ilieve)
Newsgroups: uk.misc
Subject: Re: Record shops (LPs)
Date: 16 Dec 92 14:15:12 GMT
Distribution: uk
Organization: Memex Information Systems Ltd, East Kilbrde, Scotland

In article <JRG.92Dec14162857@owl.doc.ic.ac.uk>
jrg@doc.ic.ac.uk (James Grinter) writes:
>
>We're told, that the latest players are always getting better and
>reading and playing the data from the CD more accurately. Does this
>give the possibility that the data on the disc, when it was recorded,
>could be incorrect too? Or are they saying that the recording
>equipment has always been perfect, but we've only just fixed the
>players?

This is a very good point. It depends on what you mean by `incorrect'.
The advantage of a digital system like CD is that the player can reconstruct
the exact sequence of bits that was recorded on the disk, and any
subsequent digital copying system can also do so, unto the nth generation.

In order to make this possible, two compromises must be made at the
recording stage. The first is the sampling, which places an upper limit
on the input bandwidth. In theory, sampling alone introduces no loss
of quality. Given an input signal with no components above half the
sampling rate the input signal can be reconstructed exactly from analogue
samples of the signal. The CD sampling rate of 44.1...KHz gives enough
leeway to cope with the `normal' 20KHz hi-fi bandwidth.

The other compromise is quantization, the turning of the analogue samples
into digits. This restricts the infinitely precise analogue samples to
one of a finite number of levels, introducing an error into each sample
of some fraction of a LSB. (I think that for a uniformly distributed signal
this is 1/sqrt(12) * LSB, but my memory is very hazy on this.)
The CD uses 16 bit accuracy, which explains the figures of 96dB bandied
about --- 20 * log(1/65536).

So, by introducing a specified, known, amount of degradation you gain
the ability to maintain this quality level everywhere downstream.

Now of course the recording equipment will not be perfect. Filters will
not be infinitely steep and will have dubious phase characteristics.
A-D converters will not be completely linear, etc., etc.
So even before you get your digits something will be lost.

There are lots of other places to lose things as well. Before the A-D stage
the recording engineer can lose things by not using the available dynamic
range. If he keeps the gain knob turned down in anticipation of something
loud then not all of the 64K levels are being used, but the quantization
noise is still as big. If old analogue tape is used all its noise is
faithfully reproduced.

Even in the digital domain you still need to be careful. Any processing
has the possibility of loss. Even something simple like adding two
signals together requires an extra bit to be used in the output of the
adder if overflow is to be avoided or the bottom bit is not to be
discarded, doubling the noise.
Things like a 48KHz to 44.1KHz interpolator for converting from a studio
digital recorder to CD format will also introduce some extra noise.

The upshot of all this is that you just have to take the bits on the CD
on trust, you don't really know how they got there.
There is certainly as much scope for improving the recording side of things
as the playback side in my opinion.


		Peter Ilieve		peter@memex.co.uk
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