MPEG2 AAC编码标准详解：ISO/IEC 13818-7 (2014)与MPEG4-AAC的关系

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ISO/IEC 13818-7:2004(E)，即《信息技术——通用移动图像和关联音频信息——第7部分：高级音频编码(AAC)》是MPEG-2 AAC标准的第三个版本，于2004年10月15日发布。这个标准旨在定义和规范高级音频编码技术在数字媒体中的应用，尤其是在视频流传输中，如电视广播和互联网多媒体内容。AAC（Advanced Audio Coding）是一种高效、高质量的音频压缩算法，它能够提供接近CD音质的音频，同时保持较小的文件大小，这对于存储和网络传输非常关键。 AAC技术最初源于ISO/IEC 13818-7，该标准主要关注文件格式层面，规定了AAC编码后的数据包（ADTS，Advanced Deflate Transform Syntax）结构，这种数据包常用于在ISO Base Media File Format（MPEG-2 Transport Stream）中封装音频数据。因此，如果你在研究MPEG4_AAC（ISO/IEC 14496-3）时遇到某些概念的起源或细节问题，13818-7文档会提供必要的背景信息和上下文。 ISO/IEC 13818-7定义了`.AAC`作为文件扩展名，这与`.MP4`格式中的音频部分不同，后者通常包含AAC编码但以`.MP4`为整体容器。Apple的`.M4A`文件格式虽然也使用AAC，但它是在`.MP4`文件中，且可能是为了兼容性或版权策略而设计的。在查看ISO/IEC 13818-7的PDF版本时，需要注意其中可能包含了嵌入的字体许可证问题。Adobe的政策规定，除非获取字体的适当授权并安装在使用编辑功能的计算机上，否则用户只能打印或查看PDF，而不能对其进行编辑，以避免侵犯Adobe的字体使用许可。下载和使用该文件时，接收方需承担确保遵守此类版权和许可协议的责任。总结来说，ISO/IEC 13818-7是音频编码技术的核心文档，对于理解MPEG-2 AAC以及其在多媒体文件格式中的应用至关重要。无论是音频开发者、系统集成者还是标准遵从者，理解和掌握这一标准都是确保高质量音频流传输和存储的关键。同时，版权和许可证问题也是在使用这类标准文档时需要关注的重要法律事项。

ISO/IEC 13818-7:2004(E)

3.21

CRC

the Cyclic Redundancy Check to verify the correctness of data.

3.22

critical band

this unit of bandwidth represents the standard unit of bandwidth expressed in human

auditory terms, corresponding to a fixed length on the human cochlea. It is approximately

equal to 100 Hz at low frequencies and 1/3 octave at higher frequencies, above

approximately 700 Hz.

3.23

data element

an item of data as represented before encoding and after decoding.

3.24

decoded stream

the decoded reconstruction of a compressed bitstream.

3.25

decoder

an embodiment of a decoding process.

3.26

decoding (process)

the process defined in this part of ISO/IEC 13818 that reads an input coded bitstream and

outputs decoded audio samples.

3.27

digital storage media; DSM

a digital storage or transmission device or system.

3.28

discrete cosine transform; DCT

either the forward discrete cosine transform or the inverse discrete cosine transform. The

DCT is an invertible, discrete orthogonal transformation.

3.29

downmix

a matrixing of n channels to obtain less than n channels.

3.30

editing

the process by which one or more coded bitstreams are manipulated to produce a new

coded bitstream. Conforming edited bitstreams must meet the requirements defined in this

part of ISO/IEC 13818.

3.31

encoder

an embodiment of an encoding process.

3.32

encoding (process)

a process, not specified in ISO/IEC 13818, that reads a stream of input audio samples and

produces a valid coded bitstream as defined in this part of ISO/IEC 13818.

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ISO/IEC 13818-7:2004(E)

3.45

intensity stereo

a method of exploiting stereo irrelevance or redundancy in stereophonic audio programmes

based on retaining at high frequencies only the energy envelope of the right and left

channels.

3.46

joint stereo coding

any method that exploits stereophonic irrelevance or stereophonic redundancy.

3.47

joint stereo mode

a mode of the audio coding algorithm using joint stereo coding.

3.48

low frequency enhancement (LFE) channel

a limited bandwidth channel for low frequency audio effects in a multichannel system.

3.49

main audio channels

all channels represented by either single_channel_element()'s (see subclause 8.2.1) or

channel_pair_element()´s (see subclause 8.2.1)

3.50

Mapping

conversion of an audio signal from time to frequency domain by subband filtering and/or by

MDCT.

3.51

Masking

a property of the human auditory system by which an audio signal cannot be perceived in

the presence of another audio signal.

3.52

masking threshold

a function in frequency and time below which an audio signal cannot be perceived by the

human auditory system.

3.53

modified discrete cosine transform (MDCT)

a transform which has the property of time domain aliasing cancellation. An analytical

espression for the MDCT can be found in subclause C.3.1.2.

3.54

M/S stereo

a method of removing imaging artefacts as well as exploiting stereo irrelevance or

redundancy in stereophonic audio programmes based on coding the sum and difference

signal instead of the left and right channels.

3.55

Multichannel

a combination of audio channels used to create a spatial sound field.

3.56

Multilingual

a presentation of dialogue in more than one language.

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ISO/IEC 13818-7:2004(E)

3.57

non-tonal component

a noise-like component of an audio signal.

3.58

NCC

Number of Considered Channels. The number of channels represented by the elements

SCE, independently switched CCE and CPE, i.e. once the number of SCEs plus once the

number of independently switched CCEs plus twice the number of CPEs. With respect to

the naming conventions of the MPEG-AAC decoders and bitstreams, NCC=A+I. This

number is used to derive the required decoder input buffer size (see subclause 8.2.2).

3.59

Nyquist sampling

sampling at or above twice the maximum bandwidth of a signal.

3.60

Padding

a method to adjust the average length of an audio frame in time to the duration of the

corresponding PCM samples, by conditionally adding a slot to the audio frame.

3.61

Parameter

a variable within the syntax of this specification which may take one of a range of values. A

variable which can take one of only two values is a flag or indicator and not a parameter.

3.62

Parser

functional stage of a decoder which extracts from a coded bitstream a series of bits

representing coded elements.

3.63

polyphase filterbank

a set of equal bandwidth filters with special phase interrelationships, allowing for an efficient

implementation of the filterbank.

3.64

prediction error

the difference between the actual value of a sample or data element and its predictor.

3.65

Prediction

the use of a predictor to provide an estimate of the sample value or data element currently

being decoded.

3.66

predictor

a linear combination of previously decoded sample values or data elements.

3.67

presentation channel

an audio channel at the output of the decoder.

3.68

presentation unit

In the case of compressed audio a decoded audio access unit.

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ISO/IEC 13818-7:2004(E)

3.69

Program

a set of main audio channels, coupling_channel_element()'s (see subclause 8.2.1),

lfe_channel_element()'s (see subclause 8.2.1), and associated data streams intended to be

decoded and played back simultaneously. A program may be defined by default (see

subclause 8.5.2.1 and subclause 8.5.2.3) or specifically by a program_config_element()

(see subclauses 8.5.2.2). A given single_channel_element() (see subclause 8.2.1),

channel_pair_element() (see subclause 8.2.1), coupling_channel_element(),

lfe_channel_element() or data channel may accompany one or more programs in any given

bitstream.

3.70

psychoacoustic model

a mathematical model of the masking behaviour of the human auditory system.

3.71

random access

the process of beginning to read and decode the coded bitstream at an arbitrary point.

3.72

Reserved

the term "reserved" when used in the clauses defining the coded bitstream indicates that

the value may be used in the future for ISO/IEC defined extensions.

3.73

Sampling Frequency (Fs)

defines the rate in Hertz which is used to digitise an audio signal during the sampling

process.

3.74

Scalefactor

factor by which a set of values is scaled before quantization.

3.75

scalefactor band

a set of spectral coefficients which are scaled by one scalefactor.

3.76

scalefactor index

a numerical code for a scalefactor.

3.77

side information

information in the bitstream necessary for controlling the decoder.

3.78

spectral coefficients

discrete frequency domain data output from the analysis filterbank.

3.79

spreading function

a function that describes the frequency spread of masking effects.

3.80

stereo-irrelevant

a portion of a stereophonic audio signal which does not contribute to spatial perception.

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