Speex编解码器手册

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"Speex 手册，涵盖了 Speex 编解码器的特性、标准命令行工具、编程API及算法原理。适用于理解和使用 Speex 的开发者或研究人员。" Speex 是一种开源的音频编解码器，专为语音通信优化，尤其在互联网VoIP应用中表现出色。手册详细介绍了Speex的各种功能和基本术语，旨在帮助用户更好地理解和使用该技术。 1. **介绍 Speex** - **获取帮助**：手册提供了获取关于Speex相关帮助的途径。 - **文档说明**：文档不仅阐述了Speex的工作原理，还包含了编译、移植以及优化方面的信息。 2. **编解码器描述** - **概念**：这部分讲解了与 Speex 相关的基本概念，包括编码、解码等。 - **Codec**：Speex 编解码器负责将语音信号转换为数字数据并进行压缩。 - **预处理器**：预处理器处理输入的语音信号，为编码做准备。 - **自适应抖动缓冲器**：用于管理网络传输中的延迟和不稳定性。 - **回声消除器**：Acoustic Echo Canceller (AEC) 减少回声和噪声，提高通话质量。 - **重采样器**：调整音频信号的采样率以适应不同的系统需求。 3. **编译和移植** - **平台支持**：Speex 支持多种平台，包括但不限于 Windows、Linux 和 MacOS。 - **移植与优化**：提供指导如何在不同平台上编译 Speex，以及如何针对特定CPU和内存进行性能优化。 4. **命令行编码/解码工具** - **speexenc**：用于将音频文件编码成 Speex 格式。 - **speexdec**：用于将 Speex 文件解码回原始音频格式。 5. **使用 Speex Codec API (libspeex)** - **编码**：详细介绍了如何使用 API 进行语音信号的编码过程。 - **解码**：解码部分解释了如何将已编码的 Speex 数据还原为原始音频信号。 - **Codec 选项**：speex_*_ctl 函数允许用户控制编码器和解码器的参数。 - **模式查询**：获取有关 Speex 编解码器的不同工作模式的信息。 - **打包和解包**：这部分涉及如何处理编码后的数据包，确保它们在网络传输中的正确性。 6. **算法原理** - **CELP 算法概述**：CELP（码本激励线性预测）是 Speex 基于的压缩技术，用于高效编码语音信号。 - **Speex 特有的算法**：章节9和10深入讨论了 Speex 在 CELP 基础上实现的独特优化和改进。手册适合希望深入了解 Speex 编解码器工作原理、开发基于 Speex 的应用或者研究语音处理的读者。通过学习，读者可以掌握 Speex 的使用方法，以及如何利用其特性进行音频处理和通信应用的开发。

3 Compiling and Porting

• TI C6xxx

• TriMedia (experimental)

Operating systems on top of which Speex is known to work include (it probably works on many others):

• Linux

•

Clinux

• MacOS X

• BSD

• Other UNIX/POSIX variants

• Symbian

The source code directory include additional information for compiling on certain architectures or operating systems in

README.xxx ﬁles.

3.2 Porting and Optimising

Here are a few things to consider when porting or optimising Speex for a new platform or an existing one.

3.2.1 CPU optimisation

The single that will affect the CPU usage of Speex the most is whether it is compiled for ﬂoating point or ﬁxed-point. If your

CPU/DSP does not have a ﬂoating-point unit FPU, then compiling as ﬁxed-point will be orders of magnitudes faster. If there

is an FPU present, then it is important to test which version is faster. On the x86 architecture, ﬂoating-point is generally

faster, but not always. To compile Speex as ﬁxed-point, you need to pass –ﬁxed-point to the conﬁgure script or deﬁne the

FIXED_POINT macro for the compiler. As of 1.2beta3, it is now possible to disable the ﬂoating-point compatibility API,

which means that your code can link without a ﬂoat emulation library. To do that conﬁgure with –disable-ﬂoat-api or deﬁne

the DISABLE_FLOAT_API macro. Until the VBR feature is ported to ﬁxed-point, you will also need to conﬁgure with

–disable-vbr or deﬁne DISABLE_VBR.

Other important things to check on some DSP architectures are:

• Make sure the cache is set to write-back mode

• If the chip has SRAM instead of cache, make sure as much code and data are in SRAM, rather than in RAM

If you are going to be writing assembly, then the following functions are usually the ﬁrst ones you should consider optimising:

• filter_mem16()

• iir_mem16()

• vq_nbest()

• pitch_xcorr()

• interp_pitch()

The ﬁltering functions filter_mem16() and iir_mem16() are implemented in the direct form II transposed (DF2T).

However, for architectures based on multiply-accumulate (MAC), DF2T requires frequent reload of the accumulator, which

can make the code very slow. For these architectures (e.g. Blackﬁn and Coldﬁre), a better approach is to implement those

functions as direct form I (DF1), which is easier to express in terms of MAC. When doing that however, it is important to

make sure that the DF1 implementation still behaves like the original DF2T behaviour when it comes to ﬁlter values.

This is necessary because the ﬁlter is time-varrying and must compute exactly the same value (not counting machine rounding)

on any encoder or decoder.

3 Compiling and Porting

3.2.2 Memory optimisation

Memory optimisation is mainly something that should be considered for small embedded platforms. For PCs, Speex is already

so tiny that it’s just not worth doing any of the things suggested here. There are several ways to reduce the memory usage of

Speex, both in terms of code size and data size. For optimising code size, the trick is to ﬁrst remove features you do not need.

Some examples of things that can easily be disabled if you don’t need them are:

• Wideband support (–disable-wideband)

• Support for stereo (removing stereo.c)

• VBR support (–disable-vbr or DISABLE_VBR)

• Static codebooks that are not needed for the bit-rates you are using (*_table.c ﬁles)

Speex also has several methods for allocating temporary arrays. When using a compiler that supports C99 properly (as of 2007,

Microsoft compilers don’t, but gcc does), it is best to deﬁne VAR_ARRAYS. That makes use of the variable-size array feature

of C99. The next best is to deﬁne USE_ALLOCA so that Speex can use alloca() to allocate the temporary arrays. Note that on

many systems, alloca() is buggy so it may not work. If none of VAR_ARRAYS and USE_ALLOCA are deﬁned, then Speex

falls back to allocating a large “scratch space” and doing its own internal allocation. The main disadvantage of this solution

is that it is wasteful. It needs to allocate enough stack for the worst case scenario (worst bit-rate, highest complexity setting,

...) and by default, the memory isn’t shared between multiple encoder/decoder states. Still, if the “manual” allocation is the

only option left, there are a few things that can be improved. By overriding the speex_alloc_scratch() call in os_support.h, it

is possible to always return the same memory area for all states

. In addition to that, by redeﬁning the NB_ENC_STACK and

NB_DEC_STACK (or similar for wideband), it is possible to only allocate memory for a scenario that is known in advange.

In this case, it is important to measure the amount of memory required for the speciﬁc sampling rate, bit-rate and complexity

level being used.

In this case, one must be careful with threads

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