- & 2-11172 DISx
INTRODUCTION - PART 2: VIDEO
Note: Readers interested in an overview of the MPEG Video layer should read this Introduction and then
proceed to the Informative Annex 2-D, before returning to the normative Clauses 1 and 2.
I.1 Purpose
This Part of this International 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 International Standard specifies a coded representation that can be used for compressing video sequences
to bitrates around 1.5 Mbit/s. The use of this International 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 both 625-line and 525-line television and provides flexibility for use with workstation
and personal computer displays.
The International Standard was developed to operate principally from storage media offering a continuous 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 International 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 International Standard, a
sub-set of these coding parameters known as the "Constrained Parameters" 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 is not a requirement of this International 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
Pel 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 to +63.5 pels (using half-pel vectors)
[backward_f_code and forward_f_code <= 4 (see Table 2-
D.7)]
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 International 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 a very
high compression ratio, 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.
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