国际标准ISO/IEC13818-3：音频编码技术详解

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"ISO/IEC 13818-3是国际标准化组织（ISO）和国际电工委员会（IEC）联合技术委员会1/分技术委员会29/工作组11制定的一项标准，属于信息技术——运动图像及其伴音的通用编码的一部分，特别是音频部分。这个标准扩展了ISO/IEC 11172-3音频编码，适用于更低的采样频率，并且涵盖了低比特率多声道音频的编码方法。" ISO/IEC 13818-3，通常称为MPEG-2 Audio，是多媒体编码标准MPEG-2的一部分，用于高效编码运动图像和其相关的音频信号。这个标准定义了音频数据如何被压缩、编码和解码，以便在有限的带宽下传输或存储高质量的音频内容。标准的范围包括： 1. **扩展ISO/IEC 11172-3音频编码到更低的采样频率**：ISO/IEC 11172-3，也被称为MPEG-1 Audio Layer III（即MP3），主要设计用于44.1kHz的采样率。ISO/IEC 13818-3扩展了这一标准，使其适应更低的采样率，如32kHz、24kHz和16kHz，这在某些应用和环境中是必要的，例如数字电视广播（DVB）或卫星通信。 2. **低比特率多声道音频编码**：标准还关注在低比特率下的多声道音频编码，这对于创建环绕声体验尤其重要。它提供了一种高效的方式，将多个音频通道的数据压缩成更小的比特流，同时保持足够的音频质量，适合广播和家庭娱乐系统。技术元素包括： - **定义**：标准定义了与编码和解码过程相关的术语和概念。 - **符号和缩写**：提供了用于描述比特流语法的符号和缩写的清单。 - **比特流语法描述方法**：详细规定了音频比特流的结构和表示方式，这对于解码器正确解析和恢复音频信号至关重要。 - **扩展和低比特率编码要求**：明确了对扩展ISO/IEC 11172-3标准和低比特率多声道编码的具体技术要求。此外，标准还包含了一些附录，如： - **附录A：图表**：提供了有助于理解标准的图形表示。 - **附录B：表格**：包含了相关参数和配置的列表。 - **附录C：编码过程**：详细描述了音频信号的编码流程。 - **附录D：心理声学模型**：讨论了用于优化压缩效率的心理声学理论。 - **附录E：专利持有者列表**：列出了涉及到标准实施的专利所有者。 ISO/IEC 13818-3标准对于数字音频广播、卫星电视、DVD等领域的应用起到了关键作用，使得高质量的音频能够在有限的带宽下得以传输，极大地推动了数字媒体的发展。

©ISO/IEC ISO/IEC 13818-3:1994(E)

2.1.91 layer [video and systems]: One of the levels in the data hierarchy of the video and system

specifications defined in ISO/IEC 11172-1 and ISO/IEC 11172-2.

2.1.92 low frequency enhancement channel [audio]: A limited bandwidth channel for low frequency

audio effects in a multichannel system.

2.1.93 luminance (component) [video]: A matrix, block or single pel representing a monochrome

representation of the signal and related to the primary colours in the manner defined in CCIR Rec 601. The

symbol used for luminance is Y.

2.1.94 macroblock [video]: The four 8 by 8 blocks of luminance data and the two corresponding 8 by 8

blocks of chrominance data coming from a 16 by 16 section of the luminance component of the picture.

Macroblock is sometimes used to refer to the pel data and sometimes to the coded representation of the pel

values and other data elements defined in the macroblock layer of the syntax defined in ISO/IEC 11172-2.

The usage is clear from the context.

2.1.95 mapping [audio]: Conversion of an audio signal from time to frequency domain by subband

filtering and/or by MDCT.

2.1.96 masking [audio]: A property of the human auditory system by which an audio signal cannot be

perceived in the presence of another audio signal .

2.1.97 masking threshold [audio]: A function in frequency and time below which an audio signal cannot

be perceived by the human auditory system.

2.1.98 MDCT [audio]: Modified Discrete Cosine Transform which corresponds to the Time Domain

Aliasing Cancellation Filter Bank.

2.1.99 motion compensation [video]: The use of motion vectors to improve the efficiency of the

prediction of pel values. The prediction uses motion vectors to provide offsets into the past and/or future

reference pictures containing previously decoded pel values that are used to form the prediction error

signal.

2.1.100 motion estimation [video]: The process of estimating motion vectors during the encoding process.

2.1.101 motion vector [video]: A two-dimensional vector used for motion compensation that provides an

offset from the coordinate position in the current picture to the coordinates in a reference picture.

2.1.102 MS stereo [audio]: A method of exploiting stereo irrelevance or redundancy in stereophonic audio

programmes based on coding the sum and difference signal instead of the left and right channels.

2.1.103 multichannel [audio]: A combination of audio channels used to create a spatial sound field.

2.1.104 multilingual [audio]: A presentation of dialogue in more than one language.

2.1.105 non-intra coding [video]: Coding of a macroblock or picture that uses information both from itself

and from macroblocks and pictures occurring at other times.

2.1.106 non-tonal component [audio]: A noise-like component of an audio signal.

2.1.107 Nyquist sampling: Sampling at or above twice the maximum bandwidth of a signal.

2.1.108 pack [system]: A pack consists of a pack header followed by one or more packets. It is a layer in

the system coding syntax described in ISO/IEC 11172-1.

2.1.109 packet data [system]: Contiguous bytes of data from an elementary stream present in a packet.

2.1.110 packet header [system]: The data structure used to convey information about the elementary

stream data contained in the packet data.

2.1.111 packet [system]: A packet consists of a header followed by a number of contiguous bytes from an

elementary data stream. It is a layer in the system coding syntax described in ISO/IEC 11172-1.

2.1.112 padding [audio]: A method to adjust the average length in time of an audio frame to the duration

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

©ISO/IEC ISO/IEC 13818-3:1994(E)

2.1.113 past reference picture [video]: The past reference picture is the reference picture that occurs at an

earlier time than the current picture in display order.

2.1.114 pel aspect ratio [video]: The ratio of the nominal vertical height of pel on the display to its

nominal horizontal width.

2.1.115 pel [video]: Picture element.

2.1.116 picture period [video]: The reciprocal of the picture rate.

2.1.117 picture rate [video]: The nominal rate at which pictures should be output from the decoding

process.

2.1.118 picture [video]: Source, coded or reconstructed image data. A source or reconstructed picture

consists of three rectangular matrices of 8-bit numbers representing the luminance and two chrominance

signals. The Picture layer is one of the layers in the coding syntax defined in this CD. Note that the term

"picture" is always used in this CD in preference to the terms field or frame.

2.1.119 polyphase filterbank [audio]: A set of equal bandwidth filters with special phase

interrelationships, allowing for an efficient implementation of the filterbank.

2.1.120 prediction [audio]: The use of a predictor to provide an estimate of the subband sample in one

channel from the subband samples in other channels.

2.1.121 prediction [video]: The use of a predictor to provide an estimate of the pel value or data element

currently being decoded.

2.1.122 predictive-coded picture; P-picture [video]: A picture that is coded using motion compensated

prediction from the past reference picture.

2.1.123 prediction error [video]: The difference between the actual value of a pel or data element and its

predictor.

2.1.124 predictor [video]: A linear combination of previously decoded pel values or data elements.

2.1.125 presentation channel [audio]: audio channels at the output of the decoder corresponding to the

loudspeaker positions left, centre, right, left surround and right surround.

2.1.126 presentation time-stamp; PTS [system]: A field that may be present in a packet header that

indicates the time that a presentation unit is presented in the system target decoder.

2.1.127 presentation unit; PU [system]: A decoded audio access unit or a decoded picture.

2.1.128 psychoacoustic model [audio]: A mathematical model of the masking behaviour of the human

auditory system.

2.1.129 quantisation matrix [video]: A set of sixty-four 8-bit values used by the dequantiser.

2.1.130 quantised DCT coefficients [video]: DCT coefficients before dequantisation. A variable length

coded representation of quantised DCT coefficients is stored as part of the compressed video bit stream.

2.1.131 quantiser scalefactor [video]: A data element represented in the bit stream and used by the

decoding process to scale the dequantisation.

2.1.132 random access: The process of beginning to read and decode the coded bit stream at an arbitrary

point.

2.1.133 reference picture [video]: Reference pictures are the nearest adjacent I- or P-pictures to the

current picture in display order.

2.1.134 reorder buffer [video]: A buffer in the system target decoder for storage of a reconstructed

I-picture or a reconstructed P-picture.

2.1.135 requantisation [audio]: Decoding of coded subband samples in order to recover the original

quantised values.

©ISO/IEC ISO/IEC 13818-3:1994(E)

2.1.136 reserved: The term "reserved" when used in the clauses defining the coded bit stream indicates that

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

2.1.137 reverse playback [video]: The process of displaying the picture sequence in the reverse of display

order.

2.1.138 scalefactor band [audio]: A set of frequency lines in Layer III which are scaled by one

scalefactor.

2.1.139 scalefactor index [audio]: A numerical code for a scalefactor.

2.1.140 scalefactor [audio]: Factor by which a set of values is scaled before quantisation.

2.1.141 sequence header [video]: A block of data in the coded bit stream containing the coded

representation of a number of data elements.

2.1.142 side information: Information in the bit stream necessary for controlling the decoder.

2.1.143 skipped macroblock [video]: A macroblock for which no data are stored.

2.1.144 slice [video]: A series of macroblocks. It is one of the layers of the coding syntax defined in

ISO/IEC 11172-2.

2.1.145 slot [audio]: A slot is an elementary part in the bit stream. In Layer I a slot equals four bytes, in

Layers II and III one byte.

2.1.146 source stream: A single non-multiplexed stream of samples before compression coding.

2.1.147 spreading function [audio]: A function that describes the frequency spread of masking effects.

2.1.148 start codes [system and video]: 32-bit codes embedded in that coded bit stream that are unique.

They are used for several purposes including identifying some of the layers in the coding syntax.

2.1.149 STD input buffer [system]: A first-in first-out buffer at the input of the system target decoder for

storage of compressed data from elementary streams before decoding.

2.1.150 stereo-irrelevant [audio]: a portion of a stereophonic audio signal which does not contribute to

spatial perception.

2.1.151 stereo mode [audio]: Mode, where two audio channels which form a stereo pair (left and right) are

encoded within one bit stream. The coding process is the same as for the dual channel mode.

2.1.152 stuffing (bits); stuffing (bytes) : Code-words that may be inserted into the compressed bit stream

that are discarded in the decoding process. Their purpose is to increase the bitrate of the stream.

2.1.153 subband [audio]: Subdivision of the audio frequency band.

2.1.154 subband filterbank [audio]: A set of band filters covering the entire audio frequency range. In

this part of the CD, the subband filterbank is a polyphase filterbank.

2.1.155 subband samples [audio]: The subband filterbank within the audio encoder creates a filtered and

subsampled representation of the input audio stream. The filtered samples are called subband samples.

From 384 time-consecutive input audio samples, 12 time-consecutive subband samples are generated

within each of the 32 subbands.

2.1.156 surround channel [audio]: An audio presentation channel added to the front channels (L and R or

L, R, and C) to enhance the spatial perception.

2.1.157 syncword [audio]: A 12-bit code embedded in the audio bit stream that identifies the start of a

frame.

2.1.158 synthesis filterbank [audio]: Filterbank in the decoder that reconstructs a PCM audio signal from

subband samples.

2.1.159 system header [system]: The system header is a data structure defined in ISO/IEC 11172-1 that

carries information summarising the system characteristics of the ISO/IEC 11172 multiplexed stream.

2.1.160 system target decoder; STD [system]: A hypothetical reference model of a decoding process used

to describe the semantics of an ISO/IEC 11172 multiplexed bit stream.

2.1.161 time-stamp [system]: A term that indicates the time of an event.

2.1.162 triplet [audio]: A set of 3 consecutive subband samples from one subband. A triplet from each of

the 32 subbands forms a granule.

2.1.163 tonal component [audio]: A sinusoid-like component of an audio signal.

2.1.164 variable bitrate: Operation where the bitrate varies with time during the decoding of a compressed

bit stream.

2.1.165 variable length coding; VLC: A reversible procedure for coding that assigns shorter code-words

to frequent events and longer code-words to less frequent events.

2.1.166 video buffering verifier; VBV [video]: A hypothetical decoder that is conceptually connected to

the output of the encoder. Its purpose is to provide a constraint on the variability of the data rate that an

encoder or editing process may produce.

2.1.167 video sequence [video]: A series of one or more groups of pictures. It is one of the layers of the

coding syntax defined in ISO/IEC 11172-2.

2.1.168 zigzag scanning order [video]: A specific sequential ordering of the DCT coefficients from

(approximately) the lowest spatial frequency to the highest.

2.2 Symbols and abbreviations

The mathematical operators used to describe this Recommendation | International Standard are similar to

those used in the C programming language. However, integer division with truncation and rounding are

specifically defined. The bitwise operators are defined assuming twos-complement representation of

integers. Numbering and counting loops generally begin from zero.

2.2.1 Arithmetic operators

+ Addition.

− Subtraction (as a binary operator) or negation (as a unary operator).

++ Increment.

− − Decrement.

* Multiplication.

^ Power.

/ Integer division with truncation of the result toward zero. For example, 7/4 and −7/−4 are

truncated to 1 and −7/4 and 7/−4 are truncated to −1.

// Integer division with rounding to the nearest integer. Half-integer values are rounded away from

zero unless otherwise specified. For example 3//2 is rounded to 2, and −3//2 is rounded to −2.

DIV Integer division with truncation of the result towards −∞.

| | Absolute value. | x | = x when x > 0

| x | = 0 when x == 0

| x | = −x when x < 0

% Modulus operator. Defined only for positive numbers.

Sign( ) Sign. Sign(x) = 1 when x > 0

Sign(x) = 0 when x == 0

Sign(x) = −1 when x < 0

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