Speex 编码器/解码器手册 v1.2简介与API指南

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Speex 是一个专为语音编码设计的高质量、低带宽音频编解码器，主要用于实时通信和存储空间有限的应用。这份名为 "The Speex Codec Manual" 版本 1.2 Beta 3 的文档由 Jean-Marc Valin 编写，日期为 2007 年 12 月 8 日，受 GNU Free Documentation License 版权保护。该手册详细介绍了 Speex 编码器的工作原理、架构和关键组件，以及如何在不同平台上编译和优化代码。主要内容包括： 1. **Introduction to Speex**： - 获取帮助：提供了入门指南，可能包括官方文档、社区支持和资源链接。 - 关于文档：文档的目的、目标读者和预期用途，可能强调了其作为开发者的实用性和技术深度。 2. **Codec Description**： - **Concepts**：涵盖了 Speex 编码的核心理念和技术，如高效编码算法、语音处理技术和帧结构设计。 - **Codec**：深入剖析了编码器和解码器的具体实现，可能涉及语音信号的数字化、编码步骤和解码还原过程。 - **Preprocessor**：介绍了任何预处理步骤，如信号处理、噪声抑制或失真补偿。 - **Adaptive Jitter Buffer**：讲解了如何通过自适应缓冲来处理网络延迟和不稳定传输，确保语音连续性。 - **Acoustic Echo Canceller (AEC)**：重点阐述了消除回声的技术，提高通话质量。 - **Resampler**：讨论了采样率调整的机制，确保编码后的音频与原始信号同步。 3. **Compiling and Porting**： - **Platforms**：涵盖了支持 Speex 的多种操作系统和硬件平台，如 Windows、Linux 和嵌入式系统。 - **Porting and Optimising**： - **CPU optimisation**：提供了针对不同处理器架构的性能调优建议，包括指令集优化、多线程和缓存利用等。 - **Memory optimisation**：探讨了如何最小化内存消耗，例如数据结构的设计和内存管理策略。 4. **Command-line encoder/decoder**： - **speexenc**：介绍了一个命令行工具，用于编码音频文件，可能包含参数和使用示例。 - **speexdec**：对应地，它提供了解码器工具，用户可以使用这些工具对编码后的 Speex 数据进行解码恢复原始音频。 5. **Using the Speex Codec API (libspeex)**： - **Encoding**：详述了如何通过 libspeex 库进行编程式编码，包括接口函数和示例代码。 - **Decoding**：同样，这部分解释了如何通过 API 进行解码操作，确保正确性和灵活性。 - **Codec Options (speex_*_ctl)**：列出了可配置的编码参数，开发者可以根据应用需求调整。 - **Mode queries**：介绍如何查询当前编码模式和性能特性。 - **Packing and Integration**：指导如何将 Speex API 集成到其他项目中的步骤和注意事项。这份手册是 Speex 编码器开发人员和使用者的宝贵参考资料，涵盖了从技术原理到实际应用的全面内容。通过阅读和理解这些信息，开发者可以更好地利用 Speex 提供的高质量语音编码解决方案。

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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