3GPP TS 26.290: AMR-WB+ Codec & Transcoding in UMTS and LTE

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"3GPP TS 26.290 V9.0.0 (2010-01)" 是一份技术规范文档，详细介绍了数字蜂窝通信系统（Phase 2+）、通用移动通信系统（UMTS）、长期演进（LTE）中的音频编码处理功能，特别是扩展自适应多速率宽带（AMR-WB+）编解码器以及转码功能。此规范由3GPP发布，属于Release 9版本。 AMR-WB+（Adaptive Multi-Rate - Wideband Plus），是3GPP标准下的一种高级音频编码技术，用于提高语音和音频的质量，尤其是在有限带宽的无线通信环境中。它是AMR-WB（Adaptive Multi-Rate - Wideband）的增强版，提供更宽的频率响应范围，从而支持更高质量的音频传输。AMR-WB+编码器的设计目标是在保持较低比特率的同时，提供与CD音质相近的音频体验。该技术规范详细描述了AMR-WB+编解码器的工作原理和处理流程，包括编码、解码以及不同编码格式之间的转码操作。编码过程中，AMR-WB+采用自适应比特率调整策略，根据网络条件动态调整编码速率，以确保最佳的语音质量和带宽效率。解码器则负责将经过编码的信号还原为可听音频，同时兼容不同网络环境下的编码格式。转码函数部分则涉及如何在AMR-WB+与其他音频编码格式之间进行转换，例如从AMR-WB转换到AMR-WB+，或者从其他标准如GSM、UMTS或LTE的音频编码格式转换过来。这些转换功能对于保持跨平台和设备间的兼容性至关重要。文档还包含了关键词GSM、LTE和UMTS，表明此规范不仅适用于UMTS和LTE网络，还与传统的GSM网络相融合，确保在各种通信技术中的一致性和互操作性。 3GPP（Third Generation Partnership Project）是推动全球移动通信标准发展的组织，其发布的TS 26.290标准为行业提供了统一的技术指南，有助于设备制造商和网络运营商实现AMR-WB+技术的标准化实施。这份技术规范文档是移动通信领域的重要参考资料，对于理解并实现AMR-WB+编码器及其在UMTS、LTE网络中的应用具有很高的价值。它详细阐述了音频处理和转码的各个方面，有助于开发者和工程师优化通信系统的性能和用户体验。

ETSI

ETSI TS 126 290 V9.0.0 (2010

01)

3GPP TS 26.290 version 9.0

.0 Release 9

4.3.8 Bit allocation

The bit allocation for the different parameters in the low-frequency band coding (Core) (0-Fs/4 kHz band) is shown in

Tables 3, 4, 5, and 6. Note that there are two mode bits sent in each 256-sample packet. These mode bits are not shown

in the bit allocation tables. The bit allocations for the stereo part is shown in Tables 7, 8, and 9. Note that there are also

two additional mode bits for the VLF stereo encoder, which are not shown in the bit allocation. The bit allocation for

the stereo HF part is by definition that of the bandwidth extension, as presented in Tables 7,8 and 9.

Tables 2 and 3 show the total bits per 256-sample packet, including mode bits.

Table 3: Bit allocations for ACELP core rates including BWE (per frame)

Parameter

Number of bits

Mode bits 2

ISF Parameters 46

Mean Energy 2

Pitch Lag 30

Pitch Filter 4 × 1

Fixed-codebook

Indices

4 × 20 4 × 28 4 × 36 4 × 44 4 × 52 4 × 64 4 × 72 4 × 88

Codebook Gains 4 × 7

HF ISF Parameters 9

HF gain 7

Total in bits 208 240 272 304 336 384 416 480

Table 4: Bit allocations for 256-sample TCX window (Core)

Parameter

Number of bits

Mode bits 2

ISF Parameters 46

Noise factor 3

Global Gain 7

Algebraic VQ 134 166 198 230 262 310 342 406

HF ISF Parameters 9

HF gain 7

Total in bits 208 240 272 304 336 384 416 480

Table 5: Bit allocations for 512-sample TCX window (Core)

Parameter

Number of bits

Mode bits 2+2

ISF Parameters 46

Noise factor 3

Global Gain 7

Gain redundancy 6

Algebraic VQ 318 382 446 510 574 670 734 862

HF ISF Parameters 9

HF gain 7

HF Gain correction 8 × 2

Total in bits 416 480 544 608 672 768 832 960

ETSI

ETSI TS 126 290 V9.0.0 (2010

01)

3GPP TS 26.290 version 9.0.0 Release 9

5 Functional description of the encoder

In this clause, the different functions of the encoder extension modes represented in Figure 1 are described. Input

signals are understood as internal, i.e. sampled at the internal sampling frequency Fs.

5.1 Input signal pre-processing

Input signals are pre-processed in order to bring them to the internal sampling frequency of the encoder Fs kHz. The

signal is upsampled by a factor K (related to the desired internal sampling frequency), filtered by a a low pass filter and

then downsampled by a factor 180. This operation is efficiently implemented by a polyphase filter implementation.

Output signal Fs kHz

1/180



180



Input signal 180Fs /K kHz

The resulting signals are further decomposed into two equal critically sampled bands as shown in the following figure:

Input signal Fs kHz

2

)(nx

2048 samples

1024 samples

At an internal sampling rate of Fs kHz, the lower band signals are obtained by first low-pass filtering to Fs/4 kHz

critically downsampling the low-pass filtered signal to Fs/2 kHz. The higher band signals are obtained by band-pass

filtering the input signals to frequencies above Fs/4 kHz, and critically downsampling the high-pass filtered signal to

Fs/2kHz sampling frequency.

5.1.1 High Pass Filtering

The lower band signals are high pass filtered. The high-pass filter serves as a precaution against undesired low

frequency components. A high pass filter is used, and it is given by

aa1

bbb

)(

−−

+−

where the filter parameters are dependent on the internal sampling rate.

5.1.2 Stereo Signal Downmixing/Bandsplitting

When the input audio signal is stereo, the lower band mono signal is obtained by downmixing the left and right

channels according to the following

()

)()(5.0)( nxnxnx

RLLLML

where

)(nx

, resp. )(nx

, is the lower band signal from the left, resp. right, channels. The lower band mono signal

is supplied to the core low band encoder for TCX/ACELP encoding.

ETSI

ETSI TS 126 290 V9.0.0 (2010

01)

3GPP TS 26.290 version 9.0.0 Release 9

For stereo encoding, the obtained downmixed mono signal )(nx

and the right channel signal )(nx

are further

split into two bands: a critically sampled low frequency band and a residual high frequency band according to the

following diagram



5/32



32/5

delay

)(nx

MLo

)(nx

RLo

)(nx

MMid

)(nx

RMid

The critically sampled low band output signals, )(nx

MLo

and )(nx

RLo

are fed to the stereo low band encoder, while

the signals

)(nx

MMid

and )(nx

RMid

to the stereo mid band encoder.

5.2 Principle of the hybrid ACELP/TCX core encoding

The encoding algorithm at the core of the AMR-WB+ codec is based on a hybrid ACELP/TCX model. For every block

of input signal, the encoder decides (either in open-loop or closed-loop) which encoding model (ACELP or TCX) is

best. The ACELP model is a time-domain, predictive encoder, best suited for speech and transient signals. The AMR-

WB encoder is used in ACELP modes. Alternatively, the TCX model is a transform-based encoder, and is more

appropriate for typical music samples. Frame lengths of variable sizes are possible in TCX mode, as will be explained

in Section 5.2.1.

In Sections 5.2.1 to 5.2.4, the general principles of the hybrid ACELP/TCX core encoder will be presented. Then

Section 5.3 and its subsections will give the details of the ACELP and TCX encoding modes.

5.2.1 Timing chart of the ACELP and TCX modes

The ACELP/TCX core encoder takes a mono signal as input, at a sampling frequency of Fs/2 kHz. This signal is

processed in super-frames of 1024 samples in duration. Within each 1024-sample super-frame, several encoding modes

are possible, depending on the signal structure. These modes are: 256-sample ACELP, 256-sample TCX, 512-sample

TCX and 1024-sample TCX. These encoding modes will be described further, but first we look at the different possible

mode combinations, described by a timing chart.

Figure 4 shows the timing chart of all possible modes within an 1024-sample superframe. As the figure shows, each

256-sample frame within a super-frame can be into one of four possible modes, which we call ACELP, TCX256,

TCX512 and TCX1024. When in ACELP mode, the corresponding 256-sample frame is encoded with AMR-WB. In

TCX256 mode, the frame is encoded using TCX with a 256-sample support, plus 32 samples of look-ahead used for

overlap-and add since TCX is a transform coding approach. The TCX512 mode means that two consecutive 256-sample

frames are grouped to be encoded as a single 512-sample block, using TCX with a 512-sample support plus 64 samples

look-ahead. Note that the TCX512 mode is only allowed by grouping either the first two 256-sample frames of the

super-frame, or the last two 256-sample frames. Finally, the TCX1024 mode indicates that all 256-sample frames within

the super-frame are grouped together to be encoded in a single block using TCX with an 1024-sample support plus 128

samples look-ahead.

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