ISO_IEC_11172-2MPEG1标准

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TITLE PAGE PROVIDED BY ISOCD 11172-2

CODING OF MOVING PICTURES AND ASSOCIATED AUDIO --

FOR DIGITAL STORAGE MEDIA AT UP TO ABOUT 1.5 Mbit/s --

Part 2 Video

CONTENTS

FOREWORD 4

INTRODUCTION - PART 2: VIDEO 5

I.1 Purpose 5

I.1.1 Coding Parameters 5

I.2 Overview of the Algorithm 5

I.2.1 Temporal Processing 6

I.2.2 Motion Representation - Macroblocks 6

I.2.3 Spatial Redundancy Reduction 6

I.3 Encoding 7

I.4 Decoding 8

I.5 Structure of the Coded bitstream 9

I.6 Features Supported by the Algorithm 9

I.6.1 Random Access 10

I.6.2 Fast Search 10

I.6.3 Reverse Playback 10

I.6.4 Error Robustness 10

I.6.5 Editing 10

1 GENERAL NORMATIVE ELEMENTS 11

1.1 Scope 11

1.2 References 11

2 TECHNICAL NORMATIVE ELEMENTS 12

2.1 Definitions 12

2.2 Symbols and Abbreviations 19

2.2.1 Arithmetic Operators 19

2.2.2 Logical Operators 19

2.2.3 Relational Operators 20

2.2.4 Bitwise Operators 20

2.2.5 Assignment 20

2.2.6 Mnemonics 20

2.2.7 Constants 21

2.3 Method of Describing Bit Stream Syntax 21

2.4 Requirements 23

2.4.1 Coding Structure and Parameters 23

2.4.2 Specification of the Coded Video Bitstream Syntax 26

2.4.3 Semantics for the Video Bitstream Syntax 33

2.4.4 The Video Decoding Process 41

2-ANNEX A (normative) 8 by 8 Inverse Discrete Cosine Transform A-1

2-ANNEX B (normative) Variable Length Code Tables B-1

2-B.1 Macroblock Addressing B-1

2-B.2 Macroblock Type B-2

2-B.3 Macroblock Pattern B-3

2-B.4 Motion Vectors B-4

2-B.5 DCT Coefficients B-5

2-ANNEX C (normative) Video Buffering Verifier C-1

2-C.1 Video Buffering Verifier C-1

2-ANNEX E (informative) Bibliography E-1

FOREWORD

This standard is a committee draft that was submitted for approval to ISO-IEC/JTC1 SC29 on 23 November 1991. It was

prepared by SC29/WG11 also known as MPEG (Moving Pictures Expert Group). MPEG was formed in 1988 to

establish a standard for the coded representation of moving pictures and associated audio stored on digital storage media.

This standard is published in four Parts. Part 1 - systems - specifies the system coding layer of the standard. It defines a

multiplexed structure for combining audio and video data and means of representing the timing information needed to

replay synchronized sequences in real-time. Part 2 - video - specifies the coded representation of video data and the

decoding process required to reconstruct pictures. Part 3 - audio - specifies the coded representation of audio data. Part

4 - conformance testing - is still in preparation. It will specify the procedures for determining the characteristics of coded

bit streams and for testing compliance with the requirements stated in Parts 1, 2 and 3.

In Part 1 of this standard all annexes are informative and contain no normative requirements.

In Part 2 of this standard 2-Annex A, 2-Annex B and 2-Annex C contain normative requirements and are an integral part

of this standard. 2-Annex D and 2-Annex E are informative and contain no normative requirements.

In Part 3 of this standard 3-Annex A and 3-Annex B contain normative requirements and are an integral part of this

standard. All other annexes are informative and contain no normative requirements.

INTRODUCTION - PART 2: VIDEO

I.1 Purpose

This Part of this standard was developed in response to the growing need for a common format for representing

compressed video on various digital storage media such as CDs, DATs, Winchester disks and optical drives. The

standard specifies a coded representation that can be used for compressing video sequences to bitrates around 1.5 Mbit/s.

The use of this standard means that motion video can be manipulated as a form of computer data and can be transmitted

and received over existing and future networks. The coded representation can be used with with both 625-line and 525-

line television and provides flexibility for use with workstation and personal computer displays.

The standard was developed to operate principally from storage media offering a continual transfer rate of about 1.5

Mbit/s. Nevertheless it can be used more widely than this because the approach taken is generic.

I.1.1 Coding Parameters

The intention in developing this standard has been to define a source coding algorithm with a large degree of flexibility

that can be used in many different applications. To achieve this goal, a number of the parameters defining the

characteristics of coded bitstreams and decoders are contained in the bitstream itself. This allows for example, the

algorithm to be used for pictures with a variety of sizes and aspect ratios and on channels or devices operating at a wide

range of bitrates.

Because of the large range of the characteristics of bitstreams that can be represented by this standard, a sub-set of these

coding parameters known as the "Constrained Parameters bitstream" has been defined. The aim in defining the

constrained parameters is to offer guidance about a widely useful range of parameters. Conforming to this set of

constraints in not a requirement of this standard. A flag in the bitstream indicates whether or not it is a Constrained

Parameters bitstream.

Summary of the Constrained Parameters:

Horizontal picture size Less than or equal to 768 pels

Vertical picture size Less than or equal to 576 lines

Picture area Less than or equal to 396 macroblocks

Pixel rate Less than or equal to 396x25 macroblocks per second

Picture rate Less than or equal to 30 Hz

Motion vector range Less than +/- 64 pels (using half-pel vectors)

[Vector scale code <= 4)

Input buffer size (in VBV model) Less than or equal to 327 680 bits

Bitrate Less than or equal to 1 856 000 bits/second (constant bitrate)

I.2 Overview of the Algorithm

The coded representation defined in this standard achieves a high compression ratio while preserving good picture

quality. The algorithm is not lossless as the exact pel values are not preserved during coding. The choice of the

techniques is based on the need to balance a high picture quality and compression ratio with the requirement to make

random access to the coded bitstream. Obtaining good picture quality at the bitrates of interest demands very high

compression, which is not achievable with intraframe coding alone. The need for random access, however, is best

satisfied with pure intraframe coding. This requires a careful balance between intra- and interframe coding and between

recursive and non-recursive temporal redundancy reduction.

A number of techniques are used to achieve high compression. The first, which is almost independent from this standard,

is to select an appropriate spatial resolution for the signal. The algorithm then uses block-based motion compensation to

reduce the temporal redundancy. Motion compensation is used both for causal prediction of the current picture from a

previous picture, and for non-causal, interpolative prediction from past and future pictures. Motion vectors are defined

for each 16-pel by 16-line region of the image. The difference signal, the prediction error, is further compressed using

the discrete cosine transform (DCT) to remove spatial correlation before it is quantized in an irreversible process that

discards the less important information. Finally, the motion vectors are combined with the residual DCT information,

and transmitted using variable length codes.

I.2.1 Temporal Processing

Because of the conflicting requirements of random access and highly efficient compression, three main picture types are

defined. Intra pictures (I-Pictures) are coded without reference to other pictures. They provide access points to the

coded sequence where decoding can begin, but are coded with only moderate compression. Predicted pictures (P-

Pictures) are coded more efficiently using motion compensated prediction from a past intra or predicted picture and are

generally used as a reference for further prediction. Bidirectionally-predicted pictures (B-Pictures) provide the highest

degree of compression but require both past and future reference pictures for motion compensation. Bidirectionally-

predicted pictures are never used as references for prediction. The organisation of the three picture types in a sequence is

very flexible. The choice is left to the encoder and will depend on the requirements of the application. Figure I.1

illustrates the relationship between the three different picture types.

1 2 3 4 5 6 7 8 9

I B B B P B B B P

Fo rw a rd Pre d ic tion

Bid irec tion a l P red ic tio n

Figure I.1 Example of temporal picture structure, M=3

The fourth picture type defined in the standard, the DC-picture, is provided to allow a simple, but limited quality, fast-

forward mode.

I.2.2 Motion Representation - Macroblocks

The choice of 16 by 16 macroblocks for the motion-compensation unit is a result of the trade-off between the coding gain

provided by using motion information and the overhead needed to store it. Each macroblock can be one of a number of

different types. For example, intra-coded, forwards- prediction-coded, backwards-prediction coded, and bidirectionally-

prediction-coded macroblocks are permitted in bidirectionally-predicted pictures. Depending on the type of the

macroblock, motion vector information and other side information is stored with the compressed prediction error signal

in each macroblock. The motion vectors are encoded differentially with respect to the last transmitted motion vector,

using variable length codes. The maximum length of the vectors that may be represented can be programmed, on a

picture-by-picture basis, so that the most demanding applications can be met without compromising the performance of

the system in more normal situations.

It is the responsibility of the encoder to calculate appropriate motion vectors. The standard does not specify how this

should be done.

I.2.3 Spatial Redundancy Reduction

Both original pictures and prediction error signals have high spatial redundancy. This standard uses a block-based DCT

method with visually weighted quantization and run-length coding. After motion compensated prediction or

interpolation, the residual picture is split into 8 by 8 blocks. These are transformed into the DCT domain where they are

weighted before being quantized. After quantization many of the coefficients are zero in value and so two-dimensional

run-length and variable length coding is used to encode the remaining coefficients efficiently.

I.3 Encoding

The standard does not specify an encoding process. It specifies the syntax and semantics of the bitstream and the signal

processing in the decoder. As a result, many options are left open to encoders to trade-off cost and speed against picture

quality and coding efficiency. This clause is a brief description of the functions that need to be performed by an encoder.

Figure I.2 shows the main functional blocks.

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