G729音频编解码技术详解

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"G729音频标准是一个国际电信联盟(ITU)制定的音频编码标准，主要用于高效压缩语音信号，特别是在有限带宽的通信系统中。这个标准在1996年由ITU-T（电信标准化部门）发布，旨在全球范围内推动通信技术的标准化。G.729标准使用了一个称为“码激励线性预测”(Code Excited Linear Prediction, CELP)的压缩算法，以8 kbps的数据速率提供接近CD音质的语音通信，这对于当时的语音压缩技术是一个重大突破。 G729音频编解码标准的主要特点是： 1. **高效压缩**：在8 kbps的数据速率下，G.729能够将语音信号压缩到原来的1/8左右，极大地节省了传输带宽，适合于有限带宽的网络环境，如早期的ISDN和移动通信网络。 2. **高质量语音**：尽管压缩率高，但G.729通过复杂的编码技术，如CELP算法，仍能保持相对清晰、自然的语音质量，接近未压缩的语音水平。 3. **快速编码和解码**：G729的设计考虑了实时通信的需求，其编码和解码过程相对较快，适合电话、VoIP等实时通信应用。 4. **低延迟**：由于实时通信的要求，G.729具有较低的编码和传输延迟，确保通话双方几乎无感知的延迟体验。 5. **广泛采用**：G729被广泛应用于多种通信系统，包括VoIP服务、会议系统、移动通信网络（如2G和3G）等。 G.729标准的开发和批准遵循了ITU-T的流程，由相关的研究小组（在本例中是1993-1996年的ITU-T Study Group 15）进行研究并提出建议，然后通过世界电信标准化大会（WTSC）的决议进行审批。此标准的实施和应用有助于在全球范围内统一通信标准，提高不同系统之间的互操作性。学习G729音频编解码，不仅需要理解基本的音频编码原理，还应掌握CELP算法的细节，以及如何在实际通信系统中实现和优化G729编码器和解码器。这份资料可能对理解G729的工作机制及其在实际应用中的优势非常有帮助，尤其对于那些希望在音频处理和通信领域深化知识的人来说。"

Recommendation G.729 (03/96) 3

T1518650-95/D02

Input

speech

Pre-

processing

Fixed

codebook

Synthesis

filter

Pitch

analysis

LPC info

Perceptual

weighting

Gain

quantization

Parameter

encoding

Transmitted

bitstream

LP analysis

quantization

interpolation

Adaptive

codebook

Fixed CB

LPC info

FIGURE 2/G.729

Encoding principle of the CS-ACELP encoder

LPC info

FIGURE 2/G.729...[D02] = 16 CM

2.2 Decoder

The decoder principle is shown in Figure 3. First, the parameter’s indices are extracted from the received bitstream.

These indices are decoded to obtain the coder parameters corresponding to a 10 ms speech frame. These parameters are

the LSP coefficients, the two fractional pitch delays, the two fixed-codebook vectors, and the two sets of adaptive and

fixed-codebook gains. The LSP coefficients are interpolated and converted to LP filter coefficients for each subframe.

Then, for each 5 ms subframe the following steps are done:

• the excitation is constructed by adding the adaptive and fixed-codebook vectors scaled by their respective

gains;

• the speech is reconstructed by filtering the excitation through the LP synthesis filter;

• the reconstructed speech signal is passed through a post-processing stage, which includes an adaptive

postfilter based on the long-term and short-term synthesis filters, followed by a high-pass filter and

scaling operation.

4 Recommendation G.729 (03/96)

T1518660-95/d03

Fixed

codebook

Short-term

filter

Post-

processing

Adaptive

codebook

FIGURE 3/G.729

Principle of the CS-ACELP decoder

FIGURE 3/G.729...[D03]

7 CM

2.3 Delay

This coder encodes speech and other audio signals with 10 ms frames. In addition, there is a look-ahead of 5 ms,

resulting in a total algorithmic delay of 15 ms. All additional delays in a practical implementation of this coder are due

to:

•

processing time needed for encoding and decoding operations;

•

transmission time on the communication link;

•

multiplexing delay when combining audio data with other data.

2.4 Speech coder description

The description of the speech coding algorithm of this Recommendation is made in terms of bit-exact, fixed-point

mathematical operations. The ANSI C code indicated in clause 5, which constitutes an integral part of this

Recommendation, reflects this bit-exact, fixed-point descriptive approach. The mathematical descriptions of the encoder

(clause 3), and decoder (clause 4), can be implemented in several other fashions, possibly leading to a codec

implementation not complying with this Recommendation. Therefore, the algorithm description of the ANSI C code of

clause 5 shall take precedence over the mathematical descriptions of clauses 3 and 4 whenever discrepancies are found.

A non-exhaustive set of test signals, which can be used with ANSI C code, are available from the ITU.

2.5 Notational conventions

Throughout this Recommendation, it is tried to maintain the following notational conventions:

• Codebooks are denoted by caligraphic characters (e.g.

• Time signals are denoted by their symbol and a sample index between parenthesis [e.g. s(n)]. The symbol

n is used as sample index.

• Superscript indices between parenthesis (e.g. g

(m)

are used to indicate time-dependency of variables. The

variable m refers, depending on the context, to either a frame or subframe index, and the variable n to a

sample index.

• Recursion indices are identified by a superscript between square brackets (e.g. E

[k]

• Subscripts indices identify a particular element in a coefficient array.

• The symbol ^ identifies a quantized version of a parameter (e.g. g

• Parameter ranges are given between square brackets, and include the boundaries (e.g. [0.6, 0.9]).

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