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An Introduction to Digital Video

An Introduction to Digital

Video

Second Edition

John Watkinson

Focal Press

OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

Focal Press An imprint of Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801– A division of Reed Educational and Professional Publishing Ltd

A member of the Reed Elsevier plc group

First published 1994 Reprinted 1995, 1998, 1999 Second edition 2001

© John Watkinson 2001

All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 0LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers

British Library Cataloguing in Publication Data Watkinson, John An introduction to digital video

1. Digital video I. Title 621.3'

Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress

ISBN 0 240 51637 0

For information on all Focal Press publications visit our website at www.focalpress.com

Composition by Genesis Typesetting, Rochester, Kent Printed and bound in Great Britain

• Chapter 1 Introducing digital video Preface xi
• 1.1 Video as data
• 1.2 What is a video signal?
• 1.3 Why binary?
• 1.4 Colour
• 1.5 Why digital?
• 1.6 Some digital video processes outlined
• 1.7 Time compression and expansion
• 1.8 Error correction and concealment
• 1.9 Product codes
• 1.10 Shuffling
• 1.11 Channel coding
• 1.12 Video compression and MPEG
• 1.13 Disk-based recording
• 1.14 Rotary-head digital recorders
• 1.15 DVD and DVHS
• 1.16 Digital television broadcasting
• 1.17 Networks
  • References
• Chapter 2 Video principles
• 2.1 The eye
• 2.2 Gamma
• 2.3 Scanning
• 2.4 Synchronizing
• 2.5 Bandwidth and definition
• 2.6 Aperture effect
• 2.7 Colour
• 2.8 Colour displays
• 2.9 Colour difference signals
• 2.10 Motion portrayal and dynamic resolution
• 2.11 Progressive or interlaced scan?
• 2.12 Binary codes vi Contents
• 2.13 Introduction to digital logic
• 2.14 The computer
• 2.15 The processor
• 2.16 Timebase correction
• 2.17 Multiplexing
• 2.18 Statistical multiplexing
• 2.19 Filters and transforms
• 2.20 FIR filters
• 2.21 Sampling-rate conversion
• 2.22 Transforms and duality
• 2.23 The Fourier transform
• 2.24 The discrete cosine transform (DCT)
• 2.25 Modulo- n arithmetic
• 2.26 The Galois field
• 2.27 The phase-locked loop
  • References
• Chapter 3 Conversion
• 3.1 Introduction to conversion
• 3.2 Sampling and aliasing
• 3.3 Reconstruction
• 3.4 Aperture effect
• 3.5 Two-dimensional sampling
• 3.6 Choice of sampling rate
• 3.7 Jitter
• 3.8 Quantizing
• 3.9 Introduction to dither
• 3.10 Requantizing and digital dither
• 3.11 Basic digital-to-analog conversion
• 3.12 Basic analog-to-digital conversion
• 3.13 Oversampling
• 3.14 Gamma in the digital domain
• 3.15 Colour in the digital domain
  • References
• Chapter 4 Digital video processing
• 4.1 A simple digital vision mixer
• 4.2 Keying
• 4.3 Digital video effects
• 4.4 Graphics
• 4.5 Applications of motion compensation
• 4.6 Motion-compensated standards conversion
• 4.7 De-interlacing
• 4.8 Noise reduction
  • References
• Chapter 5 Video compression and MPEG Contents vii
• 5.1 Introduction to compression
• 5.2 What is MPEG?
• 5.3 Spatial and temporal redundancy in MPEG
• 5.4 I and P coding
• 5.5 Bidirectional coding
• 5.6 Spatial compression
• 5.7 A bidirectional coder
• 5.8 Handling interlaced pictures
• 5.9 An MPEG-2 coder
• 5.10 The elementary stream
• 5.11 An MPEG-2 decoder
• 5.12 Coding artifacts
• 5.13 Processing MPEG-2 and concatenation
  • References
• Chapter 6 Digital coding principles
• 6.1 Introduction
• 6.2 Types of transmission channel
• 6.3 Transmission lines
• 6.4 Types of recording medium
• 6.5 Magnetic recording
• 6.6 Azimuth recording and rotary heads
• 6.7 Optical and magneto-optical disks
• 6.8 Equalization and data separation
• 6.9 Slicing and jitter rejection
• 6.10 Channel coding
• 6.11 Simple codes
• 6.12 Group codes
• 6.13 Randomizing and encryption
• 6.14 Partial response
• 6.15 Synchronizing
• 6.16 Basic error correction
• 6.17 Concealment by interpolation
• 6.18 Parity
• 6.19 Block and convolutional codes
• 6.20 Cyclic codes
• 6.21 Introduction to the Reed–Solomon codes
• 6.22 Correction by erasure
• 6.23 Interleaving
• 6.24 Product codes - polynomials Appendix 6.1 Calculation of Reed–Solomon generator
  • References
• Chapter 7 Disks in digital video viii Contents
• 7.1 Types of disk
• 7.2 Magnetic disks
• 7.3 Accessing the blocks
• 7.4 Servo-surface disks
• 7.5 Winchester technology
• 7.6 The disk controller
• 7.7 Defect handling
• 7.8 RAID arrays
• 7.9 Disk servers
• 7.10 Optical disk principles
• 7.11 Focus and tracking systems
• 7.12 Structure of a DVD player
• 7.13 Non-linear video editing
• 7.14 The structure of a workstation
• 7.15 Locating the edit point
• 7.16 Editing with disk drives
• Chapter 8 Introduction to the digital VTR
• 8.1 History of DVTRs
• 8.2 The rotary-head tape transport
• 8.3 Digital video cassettes
• 8.4 DVTR block diagram
• 8.5 Operating modes of a DVTR
• 8.6 Confidence replay
• 8.7 Colour framing
• 8.8 Timecode
• 8.9 Picture in shuttle
• 8.10 Digital Betacam
• 8.11 DVC and DVCPRO
• 8.12 The D-9 format
• 8.13 Digital audio in VTRs
• 8.14 AES/EBU compatibility
  • References
• Chapter 9 Digital communication
• 9.1 Introduction
• 9.2 Production-related interfaces
• 9.3 SDI
• 9.4 SDTI
• 9.5 ASI
• 9.6 AES/EBU
• 9.7 Telephone-based systems
• 9.8 Digital television broadcasting
• 9.9 MPEG packets and time stamps
• 9.10 Program clock reference
• 9.11 Program Specific Information (PSI) Contents ix
• 9.12 Transport stream multiplexing
• 9.13 Broadcast modulation techniques
• 9.14 OFDM
• 9.15 Error correction in digital television broadcasting
• 9.16 DVB
• 9.17 ATSC
• 9.18 Networks
• 9.19 FireWire
• 9.20 Broadband networks and ATM
  • References
• Glossary
• Index

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1

Chapter 1

Introducing digital video

1.1 Video as data

The most exciting aspects of digital video are the tremendous possibilities which were not available with analog technology. Error correction, compression, motion estimation and interpolation are difficult or impossible in the analog domain, but are straightforward in the digital domain. Once video is in the digital domain, it becomes data, and only differs from generic data in that it needs to be reproduced with a certain timebase. The worlds of digital video, digital audio, communication and computation are closely related, and that is where the real potential lies. The time when television was a specialist subject which could evolve in isolation from other disciplines has gone. Video has now become a branch of information technology (IT); a fact which is reflected in the approach of this book. Systems and techniques developed in other industries for other purposes can be used to store, process and transmit video. IT equipment is available at low cost because the volume of production is far greater than that of professional video equipment. Disk drives and memories developed for computers can be put to use in video products. Communications networks developed to handle data can happily carry digital video and accompanying audio over indefinite distances without quality loss. Techniques such as ADSL allow compressed digital video to travel over a conventional telephone line to the consumer. As the power of processors increases, it becomes possible to perform under software control processes which previously required dedicated hardware. This causes a dramatic reduction in hardware cost. Inevitably the very nature of video equipment and the ways in which it is used is changing along with the manufacturers who supply it. The computer industry is competing with traditional manufacturers, using the economics of mass production. Tape is a linear medium and it is necessary to wait for the tape to wind to a desired part of the recording. In contrast, the head of a hard disk drive can access any stored data in milliseconds. This is known in computers as direct access and in television as non-linear access. As a result the non-linear editing workstation based on hard drives has eclipsed the use of videotape for editing. Digital TV Broadcasting uses coding techniques to eliminate the interference, fading and multipath reception problems of analog broadcasting. At the same time, more efficient use is made of available bandwidth.

Introducing digital video 3

Note that when the hard drive is used to time shift or record, it simply stores the MPEG bitstream. On playback the bitstream is decoded and the picture quality will be unimpaired. The generation loss due to using an analog VCR is eliminated. Ultimately digital technology will change the nature of television broadcasting out of recognition. Once the viewer has non-linear storage technology and electronic program guides, the traditional broadcaster’s transmitted schedule is irrelevant. Increasingly viewers will be able to choose what is watched and when, rather than the broadcaster deciding for them. The broadcasting of conventional commercials will cease to be effective when viewers have the technology to skip them. Anyone with a web site which can stream video can become a broadcaster.

1.2 What is a video signal?

When a two-dimensional image changes with time the basic information is three- dimensional. An analog electrical waveform is two-dimensional in that it carries a voltage changing with respect to time. In order to convey three-dimensional picture information down a two-dimensional channel it is necessary to resort to scanning. Instead of attempting to convey the brightness of all parts of a picture at once, scanning conveys the brightness of a single point which moves with time. The scanning process converts spatial resolution on the image into the temporal frequency domain. The higher the resolution of the image, the more lines are necessary to resolve the vertical detail. The line rate is increased along with the number of cycles of modulation which need to be carried in each line. If the frame rate remains constant, the bandwidth goes up as the square of the resolution. In an analog system, the video waveform is conveyed by some infinite variation of a continuous parameter such as the voltage on a wire or the strength or frequency of flux on a tape. In a recorder, distance along the medium is a further, continuous, analog of time. It does not matter at what point a recording is examined along its length, a value will be found for the recorded signal. That value can itself change with infinite resolution within the physical limits of the system. Those characteristics are the main weakness of analog signals. Within the allowable bandwidth, any waveform is valid. If the speed of the medium is not constant, one valid waveform is changed into another valid waveform; a timebase error cannot be detected in an analog system. In addition, a voltage error simply changes one valid voltage into another; noise cannot be detected in an analog system. Noise might be suspected, but how is one to know what proportion of the received signal is noise and what is the original? If the transfer function of a system is not linear, distortion results, but the distorted waveforms are still valid; an analog system cannot detect distortion. Again distortion might be suspected, but it is impossible to tell how much of the energy at a given frequency is due to the distortion and how much was actually present in the original signal. It is a characteristic of analog systems that degradations cannot be separated from the original signal, so nothing can be done about them. At the end of a system a signal carries the sum of all degradations introduced at each stage

4 An Introduction to Digital Video

through which it passed. This sets a limit to the number of stages through which a signal can be passed before it is useless. Alternatively, if many stages are envisaged, each piece of equipment must be far better than necessary so that the signal is still acceptable at the end. The equipment will naturally be more expensive. Digital video is simply an alternative means of carrying a moving image. Although there are a number of ways in which this can be done, there is one system, known as pulse code modulation (PCM) which is in virtually universal use.^1 Figure 1.2 shows how PCM works. Instead of being continuous, the time axis is represented in a discrete, or stepwise manner. The video waveform is not carried by continuous representation, but by measurement at regular intervals. This process is called sampling and the frequency with which samples are taken is called the sampling rate or sampling frequency F s.

In analog video systems, the time axis is sampled into frames, and the vertical axis is sampled into lines. Digital video simply adds a third sampling process along the lines. Each sample still varies infinitely as the original waveform did. To complete the conversion to PCM, each sample is then represented to finite accuracy by a discrete number in a process known as quantizing. It is common to make the sampling rate a whole multiple of the line rate. Samples are then taken in the same place on every line. If this is done, a monochrome digital image becomes a rectangular array of points at which the brightness is stored as a number. The points are known as picture cells, pixels or pels. As shown in Figure 1.3, the array will generally be arranged with an even spacing between pixels, which are in rows and columns. Obviously the finer the pixel spacing, the greater the resolution of the picture will be, but the amount of data needed to store one picture will increase as the square of the resolution, and with it the costs. At the ADC (analog-to-digital convertor), every effort is made to rid the sampling clock of jitter, or time instability, so every sample is taken at an exactly even time step. Clearly, if there is any subsequent timebase error, the instants at which samples arrive will be changed and the effect can be detected. If samples arrive at some destination with an irregular timebase, the effect can be eliminated by storing the samples temporarily in a memory and reading them out using a stable, locally generated clock. This process is called timebase correction and all properly engineered digital video systems must use it.

Figure 1.2 When a signal is carried in numerical form, either parallel or serial, the mechanisms described in the text ensure that the only degradation is in the conversion process.

6 An Introduction to Digital Video

binary digit, abbreviated to bit. One bit is a datum and many bits are data. Logically, binary allows a system of thought in which statements can only be true or false. The great advantage of binary systems is that they are the most resistant to misinterpretation. In information terms they are robust. Figure 1.4(b) shows some binary terms and (c) some non-binary terms for comparison. In all real processes, the wanted information is disturbed by noise and distortion, but with only two possibilities to distinguish, binary systems have the greatest resistance to such effects. Figure 1.5(a) shows an ideal binary electrical signal is simply two different voltages: a high voltage representing a true logic state or a binary 1 and a low voltage representing a false logic state or a binary 0. The ideal waveform is also shown at (b) after it has passed through a real system. The waveform has been considerably altered, but the binary information can be recovered by comparing the voltage with a threshold which is set half-way between the ideal levels. In this way any received voltage which is above the threshold is considered a 1 and any voltage below is considered a 0. This process is called slicing, and can reject significant amounts of unwanted noise added to the signal. The signal will be carried in a channel with finite bandwidth, and this limits the slew rate of the signal; an ideally upright edge is made to slope. Noise added to a sloping signal (c) can change the time at which the slicer judges that the level passed through the threshold. This effect is also eliminated when the output of the slicer is reclocked. Figure 1.5(d) shows that however many stages the binary signal passes through, the information is unchanged except for a delay.

Figure 1.4 Binary digits (a) can only have two values. At (b) are shown some everyday binary terms, whereas (c) shows some terms which cannot be expressed by a binary digit.

What is binary?

(a) Mathematically: The simplest numbering scheme possible, there are only two symbols: 1 and 0

Logically: A system of thought in which there are only two states: True and False

(b) Binary information is not subject to misinterpretation Black White In Out Guilty Innocent

(c) Variables or non-binary terms: Somewhat Undecided Probably Not proven Grey Under par

Introducing digital video 7

Of course, an excessive noise could cause a problem. If it had sufficient level and the correct sense or polarity, noise could cause the signal to cross the threshold and the output of the slicer would then be incorrect. However, as binary has only two symbols, if it is known that the symbol is incorrect, it need only be set to the other state and a perfect correction has been achieved. Error correction really is as trivial as that, although determining which bit needs to be changed is somewhat harder. Figure 1.6 shows that binary information can be represented by a wide range of real phenomena. All that is needed is the ability to exist in two states. A switch can be open or closed and so represent a single bit. This switch may control the voltage in a wire which allows the bit to be transmitted. In an optical system, light may be transmitted or obstructed. In a mechanical system, the presence or absence of some feature can denote the state of a bit. The presence or absence of

Figure 1.5 An ideal binary signal (a) has two levels. After transmission it may look like (b), but after slicing the two levels can be recovered. Noise on a sliced signal can result in jitter (c), but reclocking combined with slicing makes the final signal identical to the original as shown in (d).