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_1_ THORN EMI
.. Home Electronics Development Centre
ISBN 978-0-333-44904-2 ISBN 978-1-349-09765-4 (eBook)
DOI10.1007/978-1-349-09765-4
Copyright © 1986 THORN EMI Any person who does any
Home Electronics Ltd unauthorised act in relation to this
publication may be liable to criminal
All rights reserved. No reproduction, prosecution and civil claims for
copy or transmission of this publication damages.
may be made without permission.
Published by:
No paragraph ofthis publication may THORN EMI Home Electronics Ltd
be reproduced, copied or transmitted Development Centre
save with written persmission or in Sigma, Windmill Hill,
accordance with the provision of the Swindon, Wilts SN5 9YY
Copyright Act 1956 (as amended).
Please quoter eference PT1165with
any enquiry regarding this book.
0186/1296
Contents
2. Disc structure
3. PCM (Pulse Code Modulation) Sampling, Quantization, Coding
9. Aliasing
13. Bit modulation methods NRZ (Non Return to Zero), NRZ-I, PE (Phase Encode)
14. EFM (Eight to Fourteen Modulation)
17. Frame format Frame sync, Sub-coding,CIRC parity
19. CIRC prinicples
26. Sub coding Sub code sync pattern, user word, control, address, data, CRC
Introduction
Compact Audio Disc is the latest development in high performance audio
reproduction systems. It offers, at a reasonable cost, a technical
specification unattainable with conventional tape or turntable equipment. It
has a dynamic range and signal to noise ratio of -98dB. and no measurable
wow or flutter.
To achieve this high performance the audio signals are converted from
analogue to digital using PCM (Pulse Code Modulation) techniques. Once
converted, additional digital information is added to allow the compact disc
player to automatically correct for a wide range of errors. This means that
the final audio reproduction is less susceptible to deterioration when the
compact disc receives scratches and other damage during use.
The use of lasers for both the mastering of compact discs, and reading the
stored information into the compact disc player means the digital
information can be compacted to provide one hour of stereo reproduction
on a single sided 4.5" disc. A facility called sub-coding, enables the compact
disc player to display play time and track numbers allowing features such as
auto repeat and track sequencing to be used. Spare capacity in sub-coding is
provided for future expansion of the CD system.
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--- --- --- --- __j Figure 1
A simple block diagram of a compact To help relate the information given in
disc encoder is shown in figure 1. In this the block diagram, dotted boxes indicate
book each block has been dealt with on the parts of the encoder covered by a
a discrete basis to enable an easy specific section of the book.
understanding of the principles
involved. However in practice the latter
half of the encoder would be one or
several microcomputers.
1
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Figure2 Figure4
Loser beam
Disc Structure
The pit width is 0.41Lm and the spacing The recovered signal corresponds
between adjacent tracks is 1.2j.J..m. This precisely with the original PCM audio
During the disc mastering process a gives a pitch of 1.6j.J..m. signal used during mastering.
series of 'pits' are pressed into the
surface of a transparent plastic disc. The The pitted surface of the disc is coated A coating of clear plastic is applied to
pit pattern corresponds to the PCM with a reflective material. During protect the pits and reflective coating
(Pulse Code Modulation) audio signal. playback the disc is scanned from from mechnical damage, and the disc
underneath by a laser beam (see figure label is attached to the upper surface.
The depth of each pit is 0.11 ILm. To give 4) which is reflected back to photo-diode
an idea of the depth involved, the paper sensors.
on which this text is printed is approxi
mately 451Lm thick (approximately 400 The pits appear as 'bumps' which
times thicker). modulate the scanning laser beam and
produce a modulated output from the
The pits form a spiral track that starts sensors.
near the centre of the disc and spirals
outward. Each disc is capable of storing a
maximum of 75 minutes of information.
2
I
I
I
FigureS
PCM (Pulse Code
Because the process of quantization
Modulation) (converting an analogue sampled value
to a digital representation) takes time, it
follows that between each successive
PCM !Pulse Code Modulation) is the sample, the value of the analogue
process of converting an analogue signal waveform will appear to be constant.
to a digital representation. This is commonly known as sample and
hold.
PCM consists of three functions:
sampling, quantization and coding.
Sampling
In order to convert an analogue
waveform to ad igital representation, the
instantaneous value of the waveform is
measured at regular intervals. This
prores~ is known as sampling. (figure 58).
3
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H1ghe$l lrequentt
for g1v.n sample rate
Figure?
When considering the final The highest frequency that can be
reproduction of the converted analogue reproduced by PCM is one half of the
signal, it is apparent that the higher the frequency of sampling.
sampling rate (the less time between
each successive sample) the more In compact audio disc the highest audio
accurate will be the result (figure 6). frequency which will be reproduced is
20kHz, therefore the minimum sample
By the same reasoning for a constant is 40kHz
sampling rate the lower frequencies will
be reproduced more accurately than
higher frequencies (figure 7).
4
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11
II
10
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Figure 8
Quantization A quantum is defined ast he smallest
Quantization is the process of dividing change in the analogue waveform that
the amplitude of the analogue waveform will be reproduced by quantization ('a'
into a number of discreet values. in figure 8 is an example).
5
