在一般的VOIP软件或视频会议系统中，假设我们只有A和B两个人在通话，首先，A的声音传给B，B然后用喇叭放出来，而这时B的MIC呢则会采集到喇叭放出来的声音，然后传回给A，如果这个传输的过程中时延足够大，A就会听到一个和自己刚才说过的话一样的声音，这就是回声，声学回声消除器的作用就是在B端对B采集到的声音进行处理，把采集到声音包含的A的声音去掉再传给A，这样，A就不会听到自己说过的话了。
声学回声消除的原理我就不说了，这在网上有很多文档，网上缺少的是实现，所以，我在这把一个开源的声学回声消除器介绍一下，希望对有些有人用，如果有人知道怎么把这消除器用的基于实时流的VOIP软件中，希望能一起分享一下。
这个声学回声消除器是一个著名的音频编解码器speex中的一部分，1.1.9版本后的回声消除器才起作用，以前版本的都不行，我用的也是这个版本，测试表明，用同一个模拟文件，它有效果比INTEL IPP库4.1版中的声学回声消除器的还要好。
先说编译。首先，从
www.speex.org上下载speex1.1.9的源代码，解压，打开speex/win32/libspeex中的libspeex.dsw，这个工作区里有两个工程，一个是libspeex，另一个是libspeex_dynamic。然后，将libspeex中的mdf.c文件添加到工程libspeex中，编译即可。
以下是我根据文档封装的一个类，里面有一个测试程序：
//file name: speexEC.h
#ifndef SPEEX_EC_H
#define SPEEX_EC_H
#include <stdio.h>
#include <stdlib.h>
#include "speex/speex_echo.h"
#include "speex/speex_preprocess.h"
class CSpeexEC
{
public:
CSpeexEC();
~CSpeexEC();
void Init(int frame_size=160, int filter_length=1280, int sampling_rate=8000);
void DoAEC(short *mic, short *ref, short *out);
protected:
void Reset();
private:
bool m_bHasInit;
SpeexEchoState* m_pState;
SpeexPreprocessState* m_pPreprocessorState;
int m_nFrameSize;
int m_nFilterLen;
int m_nSampleRate;
float* m_pfNoise;
};
#endif
//fine name:speexEC.cpp
#include "SpeexEC.h"
CSpeexEC::CSpeexEC()
{
m_bHasInit = false;
m_pState = NULL;
m_pPreprocessorState = NULL;
m_nFrameSize = 160;
m_nFilterLen = 160*8;
m_nSampleRate = 8000;
m_pfNoise = NULL;
}
CSpeexEC::~CSpeexEC()
{
Reset();
}
void CSpeexEC::Init(int frame_size, int filter_length, int sampling_rate)
{
Reset();
if (frame_size<=0 || filter_length<=0 || sampling_rate<=0)
{
m_nFrameSize =160;
m_nFilterLen = 160*8;
m_nSampleRate = 8000;
}
else
{
m_nFrameSize =frame_size;
m_nFilterLen = filter_length;
m_nSampleRate = sampling_rate;
}
m_pState = speex_echo_state_init(m_nFrameSize, m_nFilterLen);
m_pPreprocessorState = speex_preprocess_state_init(m_nFrameSize, m_nSampleRate);
m_pfNoise = new float[m_nFrameSize+1];
m_bHasInit = true;
}
void CSpeexEC::Reset()
{
if (m_pState != NULL)
{
speex_echo_state_destroy(m_pState);
m_pState = NULL;
}
if (m_pPreprocessorState != NULL)
{
speex_preprocess_state_destroy(m_pPreprocessorState);
m_pPreprocessorState = NULL;
}
if (m_pfNoise != NULL)
{
delete []m_pfNoise;
m_pfNoise = NULL;
}
m_bHasInit = false;
}
void CSpeexEC:DoAEC(short* mic, short* ref, short* out)
{
if (!m_bHasInit)
return;
speex_echo_cancel(m_pState, mic, ref, out, m_pfNoise);
speex_preprocess(m_pPreprocessorState, (__int16 *)out, m_pfNoise);
}
测试程序：
#define NN 160
void main()
{
FILE* ref_fd, *mic_fd, *out_fd;
short ref[NN], mic[NN], out[NN];
ref_fd = fopen ("ref.pcm", "rb"); //打开参考文件，即要消除的声音
mic_fd = fopen ("mic.pcm", "rb");//打开mic采集到的声音文件，包含回声在里面
out_fd = fopen ("echo.pcm", "wb");//消除了回声以后的文件
CSpeexEC ec;
ec.Init();
while (fread(mic, 1, NN*2, mic_fd))
{
fread(ref, 1, NN*2, ref_fd);
ec.DoAEC(mic, ref, out);
fwrite(out, 1, NN*2, out_fd);
}
fclose(ref_fd);
fclose(mic_fd);
fclose(out_fd);
}
以上的程序是用文件来模拟回声和MIC，但在实时流中是大不一样的，在一般的VOIP软件中，接收对方的声音并传到声卡中播放是在一个线程中进行的，而采集本地的声音并传送到对方又是在另一个线程中进行的，而声学回声消除器在对采集到的声音进行回声消除的同时，还需要播放线程中的数据作为参考，而要同步这两个线程中的数据是非常困难的，因为稍稍有些不同步，声学回声消除器中的自适应滤波器就会发散，不但消除不了回声，还会破坏原始采集到的声音，使被破坏的声音难以分辨。我做过好多尝试，始终无法用软件来实现对这两个线程中的数据进行同步，导致实现失败，希望有经验的网友们一起分享一下这方面的经验。
示例代码：
Sample code
This section shows sample code for encoding and decoding speech using the Speex API. The commands can be used to encode and decode a file by calling:
% sampleenc in_file.sw | sampledec out_file.sw
where both files are raw (no header) files encoded at 16 bits per sample (in the machine natural endianness).
sampleenc takes a raw 16 bits/sample file, encodes it and outputs a Speex stream to stdout. Note that the packing used is NOT compatible with that of speexenc/speexdec.
#include <speex/speex.h>
#include <stdio.h>
/**//*The frame size in hardcoded for this sample code but it doesn't have to be*/
#define FRAME_SIZE 160
int main(int argc, char **argv)
{
char *inFile;
FILE *fin;
short in[FRAME_SIZE];
float input[FRAME_SIZE];
char cbits[200];
int nbBytes;
/**//*Holds the state of the encoder*/
void *state;
/**//*Holds bits so they can be read and written to by the Speex routines*/
SpeexBits bits;
int i, tmp;
/**//*Create a new encoder state in narrowband mode*/
state = speex_encoder_init(&speex_nb_mode);
/**//*Set the quality to 8 (15 kbps)*/
tmp=8;
speex_encoder_ctl(state, SPEEX_SET_QUALITY, &tmp);
inFile = argv[1];
fin = fopen(inFile, "r");
/**//*Initialization of the structure that holds the bits*/
speex_bits_init(&bits);
while (1)
{
/**//*Read a 16 bits/sample audio frame*/
fread(in, sizeof(short), FRAME_SIZE, fin);
if (feof(fin))
break;
/**//*Copy the 16 bits values to float so Speex can work on them*/
for (i=0;i<FRAME_SIZE;i++)
input[i]=in[i];
/**//*Flush all the bits in the struct so we can encode a new frame*/
speex_bits_reset(&bits);
/**//*Encode the frame*/
speex_encode(state, input, &bits);
/**//*Copy the bits to an array of char that can be written*/
nbBytes = speex_bits_write(&bits, cbits, 200);
/**//*Write the size of the frame first. This is what sampledec expects but
it's likely to be different in your own application*/
fwrite(&nbBytes, sizeof(int), 1, stdout);
/**//*Write the compressed data*/
fwrite(cbits, 1, nbBytes, stdout);
}
/**//*Destroy the encoder state*/
speex_encoder_destroy(state);
/**//*Destroy the bit-packing struct*/
speex_bits_destroy(&bits);
fclose(fin);
return 0;
}
sampledec reads a Speex stream from stdin, decodes it and outputs it to a raw 16 bits/sample file. Note that the packing used is NOT compatible with that of speexenc/speexdec.
#include <speex/speex.h>
#include <stdio.h>
/**//*The frame size in hardcoded for this sample code but it doesn't have to be*/
#define FRAME_SIZE 160
int main(int argc, char **argv)
{
char *outFile;
FILE *fout;
/**//*Holds the audio that will be written to file (16 bits per sample)*/
short out[FRAME_SIZE];
/**//*Speex handle samples as float, so we need an array of floats*/
float output[FRAME_SIZE];
char cbits[200];
int nbBytes;
/**//*Holds the state of the decoder*/
void *state;
/**//*Holds bits so they can be read and written to by the Speex routines*/
SpeexBits bits;
int i, tmp;
/**//*Create a new decoder state in narrowband mode*/
state = speex_decoder_init(&speex_nb_mode);
/**//*Set the perceptual enhancement on*/
tmp=1;
speex_decoder_ctl(state, SPEEX_SET_ENH, &tmp);
outFile = argv[1];
fout = fopen(outFile, "w");
/**//*Initialization of the structure that holds the bits*/
speex_bits_init(&bits);
while (1)
{
/**//*Read the size encoded by sampleenc, this part will likely be
different in your application*/
fread(&nbBytes, sizeof(int), 1, stdin);
fprintf (stderr, "nbBytes: %d/n", nbBytes);
if (feof(stdin))
break;
/**//*Read the "packet" encoded by sampleenc*/
fread(cbits, 1, nbBytes, stdin);
/**//*Copy the data into the bit-stream struct*/
speex_bits_read_from(&bits, cbits, nbBytes);
/**//*Decode the data*/
speex_decode(state, &bits, output);
/**//*Copy from float to short (16 bits) for output*/
for (i=0;i<FRAME_SIZE;i++)
out[i]=output[i];
/**//*Write the decoded audio to file*/
fwrite(out, sizeof(short), FRAME_SIZE, fout);
}
/**//*Destroy the decoder state*/
speex_decoder_destroy(state);
/**//*Destroy the bit-stream truct*/
speex_bits_destroy(&bits);
fclose(fout);
return 0;
}