H.264/AVC：视频编码标准概述与技术特性

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H.264/AVC是视频编码领域的一项重要标准，由国际电信联盟电信标准部门VCEG（Video Coding Experts Group）和国际标准化组织/国际电工委员会MPEG（Moving Picture Experts Group）联合制定。该标准的主要目标在于提升视频压缩性能，提供一种对网络环境友好的视频表示方式，以适应多样化的应用场景，包括实时对话（如视频电话）和非实时应用（存储、广播或流媒体）。相比于现有的标准，H.264/AVC在率-失真效率上实现了显著的改进。本文概述了H.264/AVC的技术特性，包括其先进的编码算法、帧结构设计、熵编码方法等，这些都旨在提高编码效率和解码速度。文章还介绍了H.264/AVC的不同 profiles（例如Main Profile、High Profile和High 422 Profile），每个profile针对不同的画质要求和带宽限制进行了优化，以满足不同终端设备的需求。 H.264/AVC的成功之处在于它引入了诸如帧内预测、运动矢量预测、变量化、熵编码（如HEVC中的Context-adaptive binary arithmetic coding）以及多参考帧策略等创新技术。同时，它还支持高效的视频序列级并行处理，使得在硬件上实现高效的解码成为可能，这对于移动设备和云计算环境下的多媒体处理至关重要。此外，文章回顾了H.264/AVC的标准化历程，从最初的提案到最终的批准，涉及到了多个阶段的协商、测试和修订，展示了全球标准制定者和技术专家们的共同努力。与前一代标准MPEG-2、H.263和MPEG-4的关系也得到了提及，H.264/AVC是对这些标准的继承和发展，它不仅提供了更高的压缩效率，还为后续的AVC标准如H.265/HEVC奠定了基础。 H.264/AVC标准因其卓越的性能、广泛的适用性和对网络友好性的关注，在视频编码领域占据了主导地位，并且对于推动高清视频、互联网电视和移动通信的发展起到了关键作用。随着技术的进步，H.264/AVC将继续作为视频传输和存储的基础，为未来的视频通信和娱乐产业奠定基石。

WIEGAND et al.: OVERVIEW OF THE H.264/AVC VIDEO CODING STANDARD 4

Robustness to data errors/losses and flexibility for operation

over a variety of network environments is enabled by a

number of design aspects new to the H.264/AVC standard

including the following highlighted features.

• Parameter set structure: The parameter set

design provides for robust and efficient

conveyance header information. As the loss of a

few key bits of information (such as sequence

header or picture header information) could have a

severe negative impact on the decoding process

when using prior standards, this key information

was separated for handling in a more flexible and

specialized manner in the H.264/AVC design.

• NAL unit syntax structure: Each syntax structure

in H.264/AVC is placed into a logical data packet

called a NAL unit. Rather than forcing a specific

bitstream interface to the system as in prior video

coding standards, the NAL unit syntax structure

allows greater customization of the method of

carrying the video content in a manner appropriate

for each specific network.

• Flexible slice size: Unlike the rigid slice structure

found in MPEG-2 (which reduces coding

efficiency by increasing the quantity of header data

and decreasing the effectiveness of prediction),

slice sizes in H.264/AVC are highly flexible, as

was the case earlier in MPEG-1.

• Flexible macroblock ordering (FMO): A new

ability to partition the picture into regions called

slice groups has been developed, with each slice

becoming an independently-decodable subset of a

slice group. When used effectively, flexible

macroblock ordering can significantly enhance

robustness to data losses by managing the spatial

relationship between the regions that are coded in

each slice. (FMO can also be used for a variety of

other purposes as well.)

• Arbitrary slice ordering (ASO): Since each slice

of a coded picture can be (approximately) decoded

independently of the other slices of the picture, the

H.264/AVC design enables sending and receiving

the slices of the picture in any order relative to

each other. This capability, first found in an

optional part of H.263+, can improve end-to-end

delay in real-time applications, particularly when

used on networks having out-of-order delivery

behavior (e.g., internet protocol networks).

• Redundant pictures: In order to enhance

robustness to data loss, the H.264/AVC design

contains a new ability to allow an encoder to send

redundant representations of regions of pictures,

enabling a (typically somewhat degraded)

representation of regions of pictures for which the

primary representation has been lost during data

transmission.

• Data Partitioning: Since some coded information

for representation of each region (e.g., motion

vectors and other prediction information) is more

important or more valuable than other information

for purposes of representing the video content,

H.264/AVC allows the syntax of each slice to be

separated into up to three different partitions for

transmission, depending on a categorization of

syntax elements. This part of the design builds

further on a path taken in MPEG-4 Visual and in

an optional part of H.263++. Here the design is

simplified by having a single syntax with

partitioning of that same syntax controlled by a

specified categorization of syntax elements.

• SP/SI synchronization/switching pictures: The

H.264/AVC design includes a new feature

consisting of picture types that allow exact

synchronization of the decoding process of some

decoders with an ongoing video stream produced

by other decoders without penalizing all decoders

with the loss of efficiency resulting from sending

an I picture. This can enable switching a decoder

between representations of the video content that

used different data rates, recovery from data losses

or errors, as well as enabling trick modes such as

fast-forward, fast-reverse, etc.

In the following two sections, a more detailed description of

the key features is given.

III. N

ETWORK ABSTRACTION LAYER

The network abstraction layer (NAL) is designed in order to

provide "network friendliness" to enable simple and

effective customization of the use of the VCL for a broad

variety of systems.

The NAL facilitates the ability to map H.264/AVC VCL

data to transport layers such as

• RTP/IP for any kind of real-time wire-line and wireless

Internet services (conversational and streaming)

• File formats, e.g. ISO MP4 for storage and MMS

• H.32X for wireline and wireless conversational

services

• MPEG-2 systems for broadcasting services, etc.

The full degree of customization of the video content to fit

the needs of each particular application is outside the scope

of the H.264/AVC standardization effort, but the design of

the NAL anticipates a variety of such mappings. Some key

concepts of the NAL are NAL units, byte stream, and

packet format uses of NAL units, parameter sets, and access

units. A short description of these concepts is given below

whereas a more detailed description including error

resilience aspects is provided in [6] and [7].

A. NAL units

The coded video data is organized into NAL units, each of

which is effectively a packet that contains an integer

number of bytes. The first byte of each NAL unit is a

header byte that contains an indication of the type of data in

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