Speex手册----Speex编/解码API的使用(libspeex)

he libspeex library contains all the functions for encoding and decoding speech with the Speex codec. When linking on a UNIX system, one must add -lspeex -lm to the compiler command line. One important thing to know is that libspeex calls are reentrant, but not thread-safe. That means that it is fine to use calls from many threads, but calls using the same state from multiple threads must be protected by mutexes. Examples of code can also be found in Appendix A and the complete API documentation is included in the Documentation section of the Speex website (http://www.speex.org/).

Speex编解码器的libspeex包囊括了所有的语音编码和解码函数。在Linux系统中连接时，必须在编译器命令行中加入-lspeex –lm。需要知道的是，虽然libspeex的函数调用是可重入的，但不是线程安全的，所以在多线程调用时，如果使用共享资源需要进行互斥保护。附录A中有代码实例，在Speex站点(http://www.speex.org/ )的文档部分能下到完整的API文档。

 

 

5.1 编码

In order to encode speech using Speex, one first needs to:
#include <speex/speex.h>
Then in the code, a Speex bit-packing struct must be declared, along with a Speex encoder state:
SpeexBits bits;

void *enc_state;
The two are initialized by:
speex_bits_init(&bits);
enc_state = speex_encoder_init(&speex_nb_mode);
For wideband coding, speex_nb_mode will be replaced by speex_wb_mode. In most cases, you will need to know the frame size used at the sampling rate you are using. You can get that value in the frame_size variable (expressed in samples, not
bytes) with:
speex_encoder_ctl(enc_state,SPEEX_GET_FRAME_SIZE,&frame_size);
In practice, frame_size will correspond to 20 ms when using 8, 16, or 32 kHz sampling rate. There are many parameters that can be set for the Speex encoder, but the most useful one is the quality parameter that controls the quality vs bit-rate tradeoff.
This is set by:
speex_encoder_ctl(enc_state,SPEEX_SET_QUALITY,&quality);
where quality is an integer value ranging from 0 to 10 (inclusively). The mapping between quality and bit-rate is described in Fig. 9.2 for narrowband.
Once the initialization is done, for every input frame:
speex_bits_reset(&bits);
speex_encode_int(enc_state, input_frame, &bits);
nbBytes = speex_bits_write(&bits, byte_ptr, MAX_NB_BYTES);
where input_frame is a (short *) pointing to the beginning of a speech frame, byte_ptr is a (char *) where the encoded frame will be written,MAX_NB_BYTES is the maximumnumber of bytes that can be written to byte_ptr without causing an overflow and nbBytes is the number of bytes actually written to byte_ptr (the encoded size in bytes). Before calling speex_bits_write, it is possible to find the number of bytes that need to be written by calling speex_bits_nbytes(&bits), which returns a number of bytes.
It is still possible to use the speex_encode() function, which takes a (float *) for the audio. However, this would make an eventual port to an FPU-less platform (like ARM) more complicated. Internally, speex_encode() and speex_encode_int() are processed in the same way. Whether the encoder uses the fixed-point version is only decided by the compile-time flags, not at the API level.
After you’re done with the encoding, free all resources with:
speex_bits_destroy(&bits);
speex_encoder_destroy(enc_state);
That’s about it for the encoder.

使用Speex进行语音编码，首先要：

#include < speex/speex.h >

在代码中，需要声明Speex比特包结构体，同时设置Speex编码器状态：

SpeexBits bits;

void * enc_state;

初始化两变量：

speex_bits_init( &bits );

enc_state = speex_encoder_init( &speex_nb_mode );

用speex_wb_mode代替为speex_nb_mode，即可转换为宽带编码。很多时候，你在使用采样率的需要知道帧的大小，可以通过变量frame_size（用样本中的单位表示，不以字节为单位）获得，调用下面函数：

speex_encoder_ctl( enc_state, SPEEX_GET_FRAME_SIZE, &frame_size );

实践表明，在采用8、16或32kHz采样率的时候，frame_size大约对应于20ms。Speex编码器还有很多参数可以设置，其中最有用的一个是质量参数，控制着比特率（bit-rate）交换的质量，通过下面函数设置：
speex_encoder_ctl( enc_state, SPEEX_SET_QUALITY, &quality );

quality是一个0～10（包含10&