MFCC特征提取(C语言版本)
音频分析中，MFCC参数是经典参数之一。之前对于它的计算流程和原理，大体上是比较清楚的，所以仿真的时候，都是直接调用matlab的voicebox工具或者开发的时候直接调用第三方库。最近想整理一个纯C语言版本的MFCC函数，发现第三方开源的一部分是C++的，有些纯C的开源代码是针对语音固定了某些参数，不太灵活。干脆自己动手写一下，发现matlab写习惯了，都弱化了写C的思维，磕磕碰碰弄了2天，初版总算是完成了。
计算的大体流程：预加重->分帧->加窗->FFT->能量->Mel滤波器组滤波->log->dct解卷积->倒谱提升->差分。
在编码的过程中碰到了一些小问题，跟大家分享一下。
问题1：预加重也就高通滤波，它是针对整段语音的。而如果先进行分帧，然后按帧预加重，这种做法比较符合实时流，一帧一帧的实时处理，感觉应该也是可以的。但是严格来说，分帧预加重和对整段语音预加重还是有区别的，例如两种做法的每帧第一样本点不一致。
此处针对整段语音做预加重，然后一帧一帧的输出特征。
问题2：Mel尺度转换，两个公式都可以。
问题3：能量
幅度谱和用幅度谱的平方都可以。
问题4：相关公式
DCT变换，有好几个形式，见wiki。
http://en.wikipedia.org/wiki/Discrete_cosine_transform。
此处用到：
Mel滤波器组： 三角滤波器
倒谱提升窗：
w=1+0.5*K*sin(pi*m/K); 1<=m<=K
差分计算： 也有不同的形式，但是基本上是一致的，只是系数倍数不同而已。
差分计算比较麻烦，涉及到前后好几帧，也有对此处简化的处理。
，有的此处分母是开方。
FFT变换：调用了第三方库FFTW，也可以直接使用轻量级的源码FFT计算方法。
头文件：
#ifndef MFCC_H
#define MFCC_H
#include <stdio.h>
typedef struct AudioInfo
{
int sample_rate;
int frame_len;
int frame_shift;
float* data_in;
int data_len;
AudioInfo():sample_rate(0),frame_len(0),frame_shift(0),data_in(NULL),data_len(0){};//构造函数注释掉，变成纯C版本
};
typedef struct MelBankInfo
{
float **filter;
int nfilters;
int nfft;
int low;
int high;
MelBankInfo():filter(NULL),nfilters(0),nfft(0),low(0),high(0){};
};
typedef struct DctInfo
{
float **coeff;
int dctlen;
DctInfo():coeff(NULL),dctlen(0){};
};
typedef enum MFCC_TYPE
{
MFCC_STD,
MFCC_DIFF_1,
MFCC_DIFF_2,
};
typedef struct MfccInfo
{
MelBankInfo melbank;
DctInfo dct;
int nframes;
int out_nframes;//可输出的特征数
float *frame_data;
float *data_in;
float *window;
float *lift_window;
int frame_len;
int frame_shift;
float *pre1;
float *pre2;
float *cur;
float *next1;
float *next2;
float *diff_pre1;
float *diff_pre2;
float *diff_cur;
float *diff_next1;
float *diff_next2;
MFCC_TYPE m_type;
MfccInfo():nframes(0),out_nframes(0),window(NULL),lift_window(NULL),cur(NULL),frame_len(0),pre1(NULL),pre2(NULL),next1(NULL),next2(NULL),m_type(MFCC_STD),frame_data(NULL),frame_shift(0){};
};
MfccInfo* MfccInit(AudioInfo audioctx, int nfft, int low, int high, int nfilters, int ndcts, MFCC_TYPE type);
int Mfcc_Frame_std(MfccInfo *p, int iframe, float *out, int len);
int Mfcc_Frame_diff1(MfccInfo *p, int iframe, float *out, int len);
int Mfcc_Frame_diff2(MfccInfo *p, int iframe, float *out, int len);
void MfccDestroy(MfccInfo *data);
#endif
源文件：
#include "mfcc.h"
#include "Spl.h"
#include "fftw3.h"
#include <malloc.h>
#include <math.h>
#include <string.h>
#include <assert.h>
#define PI 3.1415926
#define EPS 0.0000001
#pragma comment(lib, "libfftw3f-3.lib")
using namespace std;
void PreEmphasise(const float *data, int len, float *out, float preF)//预加重
{
for(int i = len - 1; i <= 1; i--)
{
out[i] = data[i] - preF * data[i-1];
}
out[0] = data[0];
}
float HzToMel(float f)
{
return 1127*log(1.0 + f/700);
}
float MelToHz(float data)
{
return 700 * (exp(data/1127) - 1);
}
int HzToN(float f, int fs, int nfft)
{
return f/fs *nfft+1;
}
void MelBank( int fs, int nfft, int low, int high, int nfilters, float** coeff )//三角滤波器组。
{
float fre_bin = (float)fs / nfft;
float low_mel = HzToMel(low);
float high_mel = HzToMel(high);
float mel_bw = (high_mel - low_mel)/(nfilters + 1);
int valid_nfft = nfft/2 + 1;
for(int j = 1; j <= nfilters; j++)
{
float mel_cent = j * mel_bw + low_mel;
float mel_left = mel_cent - mel_bw;
float mel_right = mel_cent + mel_bw;
float freq_cent = MelToHz(mel_cent);
float freq_left = MelToHz(mel_left);
float freq_bw_left = freq_cent - freq_left;
float freq_right = MelToHz(mel_right);
float freq_bw_right = freq_right - freq_cent;
for(int i = 1; i <= valid_nfft; i++)
{
float freq = (i-1) * fre_bin ;
if( freq > freq_left && freq < freq_right )
{
if( freq <= freq_cent)
{
coeff[j-1][i-1] = (freq - freq_left) / freq_bw_left;
}
else
{
coeff[j-1][i-1] = (freq_right - freq) / freq_bw_right;
}
}
}
}
}
void DctCoeff( int m, int n, float** coeff )//标准DCT变换。
{
for( int i = 1; i <= m; i++)
{
for(int j = 0; j < n; j++)
{
coeff[i-1][j] = cos( (2*j + 1) * i *PI / (2 * n));
}
}
}
void lift_window(float* p, int m)//倒谱提升窗归一化。
{
float max_value = 0.0f;
for(int i = 1; i <= m; i++)
{
p[i-1] = 1+ 0.5 * m * sin( PI * i/m );
if( p[i-1] > max_value)
{
max_value = p[i-1];
}
}
for(int i = 1; i <= m; i++)
{
p[i-1] /= max_value;
}
}
float Product(float *data1, float* data2, int len)
{
float result = 0.0;
for(int i = 0; i < len; i++)
{
result += data1[i] * data2[i];
}
return result;
}
float** MallocMatrix(int m, int n)
{
float **in = (float**)malloc(m * sizeof(float*));
float* data = (float*)malloc( m*n*sizeof(float));
memset( data, 0, sizeof(float)*m*n );
for(int i = 1; i <= m; i++)
{
in[i-1] = &data[(i-1)*n];
}
return in;
}
void FreeMatrix(float **in)
{
float *data = *in;
if(data != NULL)
{
free(data);
}
if(in != NULL)
{
free(in);
}
}
int Mfcc_Frame_diff1_temp(MfccInfo *p, int iframe, float *out, int len);
//初始化，预加重，获取滤波器组系数，DCT系数，倒谱提升窗系数等。
MfccInfo* MfccInit(AudioInfo audioctx, int nfft, int low, int high, int nfilters, int ndcts, MFCC_TYPE type)
{
MfccInfo* p = (MfccInfo*)malloc(sizeof(MfccInfo));
p->melbank.nfft = nfft;
p->melbank.low = low;
p->melbank.high = high;
p->melbank.nfilters = nfilters;
p->dct.dctlen = ndcts;
p->pre1 = NULL;
p->pre2 = NULL;
p->cur = NULL;
p->next1 = NULL;
p->next2 = NULL;
p->m_type = type;
p->data_in = audioctx.data_in;//整段语音的数据流
p->frame_shift = audioctx.frame_shift;
int valid_nfft = nfft/2 + 1;
p->melbank.filter = MallocMatrix( nfilters, valid_nfft);
MelBank( audioctx.sample_rate, nfft, low, high, nfilters, p->melbank.filter);//Mel滤波器系数
p->dct.coeff = MallocMatrix( ndcts, nfilters);
DctCoeff( ndcts, nfilters, p->dct.coeff );//DCT系数
float preF = 0.9375;
//整段语音高通滤波，预加重
PreEmphasise( audioctx.data_in, audioctx.data_len, audioctx.data_in, preF);
int nframes = (audioctx.data_len - audioctx.frame_len)/audioctx.frame_shift + 1;
p->nframes = nframes;
p->out_nframes = nframes;
p->frame_len = audioctx.frame_len;
p->window = (float*) malloc( audioctx.frame_len * sizeof(float));
hamming( p->window, audioctx.frame_len);//加窗
p->lift_window = (float*)malloc( ndcts * sizeof(float));
lift_window(p->lift_window, ndcts);//倒谱提升窗
int buffer_len = audioctx.frame_len > nfft ? audioctx.frame_len:nfft;
p->frame_data = (float*) malloc( buffer_len * sizeof(float));
switch(type)
{
case MFCC_DIFF_1:
{
p->out_nframes = nframes - 4;
p->pre1 = (float*)malloc( ndcts*sizeof(float));
p->pre2 = (float*)malloc( ndcts*sizeof(float));
p->cur = (float*)malloc( ndcts*sizeof(float));
p->next1 = (float*)malloc( ndcts*sizeof(float));
p->next2 = (float*)malloc( ndcts*sizeof(float));
Mfcc_Frame_std(p, 1, p->pre1, ndcts);
Mfcc_Frame_std(p, 2, p->pre2, ndcts);
Mfcc_Frame_std(p, 3, p->cur, ndcts);
Mfcc_Frame_std(p, 4, p->next1, ndcts); //一阶差分需要相邻两帧数据
break;
}
case MFCC_DIFF_2:
{
p->out_nframes = nframes - 8;
p->pre1 = (float*)malloc( ndcts*sizeof(float));
p->pre2 = (float*)malloc( ndcts*sizeof(float));
p->cur = (float*)malloc( ndcts*sizeof(float));
p->next1 = (float*)malloc( ndcts*sizeof(float));
p->next2 = (float*)malloc( ndcts*sizeof(float));
Mfcc_Frame_std(p, 1, p->pre1, ndcts);
Mfcc_Frame_std(p, 2, p->pre2, ndcts);
Mfcc_Frame_std(p, 3, p->cur, ndcts);
Mfcc_Frame_std(p, 4, p->next1, ndcts); //一阶差分需要相邻两帧数据
p->diff_pre1 = (float*)malloc( ndcts*sizeof(float));
p->diff_pre2 = (float*)malloc( ndcts*sizeof(float));
p->diff_cur = (float*)malloc( ndcts*sizeof(float));
p->diff_next1 = (float*)malloc( ndcts*sizeof(float));
p->diff_next2 = (float*)malloc( ndcts*sizeof(float));
Mfcc_Frame_diff1_temp(p,1,p->diff_pre1,ndcts);
Mfcc_Frame_diff1_temp(p,2,p->diff_pre2,ndcts);
Mfcc_Frame_diff1_temp(p,3,p->diff_cur,ndcts);
Mfcc_Frame_diff1_temp(p,4,p->diff_next1,ndcts);//二阶差分需要相邻一阶差分数据
}
}
return p;
}
int Mfcc_Frame_std(MfccInfo *p, int iframe, float *out, int len)//输出mfcc，任意帧输出
{
if(iframe > p->nframes)
{
return -1;
}
memcpy(p->frame_data, p->data_in + (iframe - 1) * p->frame_shift, sizeof(float) * p->frame_len);
apply_window( p->frame_data, p->window, p->frame_len);
int nfft = p->melbank.nfft;
int valid_nfft = nfft/2 + 1;
fftwf_plan r2cP;
fftwf_complex* temp = (fftwf_complex*)fftwf_malloc(sizeof( fftwf_complex ) * valid_nfft);
r2cP = fftwf_plan_dft_r2c_1d( p->frame_len, p->frame_data, temp, FFTW_ESTIMATE ); //完成FFT运算
fftwf_execute( r2cP );
for (int j = 0; j < valid_nfft; ++j)
{
p->frame_data[j] = pow( temp[j][0], 2 ) + pow( temp[j][1], 2 );//平方能量值，也可以用谱幅度值
}
fftwf_destroy_plan( r2cP );
for(int i = 1; i <= p->dct.dctlen; i++)
{
float temp = 0.0;
for(int j = 1; j <= p->melbank.nfilters; j++)
{
//DCT变换，解卷积
temp += p->dct.coeff[i-1][j-1] * log(Product(p->frame_data, p->melbank.filter[j-1], valid_nfft)+ EPS)/log(10.0);
}
out[i-1] = temp * p->lift_window[i-1];//倒谱提升
}
fftwf_free(temp);
return 0;
}
int Mfcc_Frame_diff1(MfccInfo *p, int iframe, float *out, int len)//标准一阶差分，输出 mfcc + 一阶差分。 逐帧输出
{
assert(p->nframes >= 5 && iframe <= p->nframes -4 && p->m_type == MFCC_DIFF_1);
int ret = Mfcc_Frame_std(p, iframe + 4, p->next2, len);
int dctlen = p->dct.dctlen;
memcpy( out, p->cur, sizeof(float)* dctlen);//mfcc
float factor = sqrt(10.0);
for(int i = 0; i < dctlen; i++)
{
out[i + dctlen] = (2 * p->next2[i] + p->next1[i] - 2*p->pre1[i] - p->pre2[i])/factor ;//一阶差分
}
float *temp = p->pre1;
p->pre1 = p->pre2;
p->pre2 = p->cur;
p->cur = p->next1;
p->next1 = p->next2;
p->next2 = temp;
return ret;
}
int Mfcc_Frame_diff1_temp(MfccInfo *p, int iframe, float *out, int len)//输出一阶差分
{
int ret = Mfcc_Frame_std(p, iframe + 4, p->next2, len);
int dctlen = p->dct.dctlen;
float factor = sqrt(10.0);
for(int i = 0; i < dctlen; i++)
{
out[i] = (2 * p->next2[i] + p->next1[i] - 2*p->pre1[i] - p->pre2[i])/factor ;//一阶差分
}
float *temp = p->pre1;
p->pre1 = p->pre2;
p->pre2 = p->cur;
p->cur = p->next1;
p->next1 = p->next2;
p->next2 = temp;
return ret;
}
int Mfcc_Frame_diff2(MfccInfo *p, int iframe, float *out, int len)//输出mfcc+1+2
{
assert(p->nframes >= 9 && iframe <= p->nframes -8 && p->m_type == MFCC_DIFF_2);
int ret = Mfcc_Frame_diff1_temp(p, iframe + 8, p->diff_next2, len);
int dctlen = p->dct.dctlen;
memcpy( out, p->next2, sizeof(float)* dctlen);//mfcc
memcpy( out + dctlen, p->diff_cur, sizeof(float)* dctlen);//一阶差分
float factor = sqrt(10.0);
for(int i = 0; i < dctlen; i++)
{
out[i + 2*dctlen] = (2 * p->diff_next2[i] + p->diff_next1[i] - 2*p->diff_pre1[i] - p->diff_pre2[i])/factor ;//二阶差分
}
float *temp = p->diff_pre1;
p->diff_pre1 = p->diff_pre2;
p->diff_pre2 = p->diff_cur;
p->diff_cur = p->diff_next1;
p->diff_next1 = p->diff_next2;
p->diff_next2 = temp;
return ret;
}
void MfccDestroy(MfccInfo *data)
{
FreeMatrix(data->melbank.filter);
FreeMatrix(data->dct.coeff);
if(data->window)
{
free(data->window);
data->window = NULL;
}
if(data->lift_window)
{
free(data->lift_window);
data->lift_window = NULL;
}
if(data->pre1)
{
free(data->pre1);
data->pre1 = NULL;
}
if(data->pre2)
{
free(data->pre2);
data->pre2 = NULL;
}
if(data->cur)
{
free(data->cur);
data->cur = NULL;
}
if(data->next1)
{
free(data->next1);
data->next1 = NULL;
}
if(data->next2)
{
free(data->next2);
data->next2 = NULL;
}
if(data->frame_data)
{
free(data->frame_data);
data->frame_data = NULL;
}
if(data->diff_pre1)
{
free(data->pre1);
data->pre1 = NULL;
}
if(data->diff_pre2)
{
free(data->pre2);
data->pre2 = NULL;
}
if(data->diff_cur)
{
free(data->cur);
data->cur = NULL;
}
if(data->diff_next1)
{
free(data->next1);
data->next1 = NULL;
}
if(data->diff_next2)
{
free(data->next2);
data->next2 = NULL;
}
}
其它文件：
#include "Spl.h"
#include <math.h>
#define TWOPI 6.283185307179586
void hanning( float *win, int N)
{
int half = 0;
if ( N % 2 == 0 )
{
half = N / 2;
for (int i = 1; i <= half; ++i)
{
win[i - 1] = 0.5 - 0.5*cos(TWOPI*i / (N + 1.0));
}
int index = half + 1;
for (int i = half; i >= 1; i--)
{
win[index - 1] = win[i - 1];
index++;
}
}
else
{
half = (N + 1) / 2;
for (int i = 1; i <= half; ++i)
{
win[i - 1] = 0.5 - 0.5*cos(TWOPI*i / (N + 1.0));
}
int index = half + 1;
for (int i = half-1; i >= 1; i--)
{
win[index - 1] = win[i - 1];
index++;
}
}
}
void hamming( float *win, int N)
{
int half = 0;
if ( N % 2 == 0 )
{
half = N / 2;
for (int i = 1; i <= half; ++i)
{
win[i - 1] = 0.54 - 0.46*cos(TWOPI*i / (N + 1.0));
}
int index = half + 1;
for (int i = half; i >= 1; i--)
{
win[index - 1] = win[i - 1];
index++;
}
}
else
{
half = (N + 1) / 2;
for (int i = 1; i <= half; ++i)
{
win[i - 1] = 0.54 - 0.46*cos(TWOPI*i / (N + 1.0));
}
int index = half + 1;
for (int i = half-1; i >= 1; i--)
{
win[index - 1] = win[i - 1];
index++;
}
}
}
void apply_window(float* data, float* window, int window_len)
{
for(int i = 0; i< window_len; i++)
{
data[i] = data[i] * window[i];
}
}
测试代码：
#include "mfcc.h"
#include <malloc.h>
int main(int argc, char* argv[])
{
int samples = 1024*4;
float *data = (float*)malloc(samples * sizeof(float));
for(int i = 0; i < samples; i++)
{
data[i] = i;
}
AudioInfo audioctx;
audioctx.data_in = data;
audioctx.data_len = samples;
audioctx.frame_len = 512;
audioctx.frame_shift = 128;
audioctx.sample_rate = 8000;
int nfft = audioctx.frame_len;
int low = 0;
int high = audioctx.sample_rate/2;
int nfilters = 24;
int ndcts = 12;
#if 0
MFCC_TYPE type = MFCC_STD;
MfccInfo* p = MfccInit( audioctx, nfft, low, high, nfilters, ndcts, type);
int len = ndcts * 1;
float *out = (float*) malloc( sizeof(float)* (len));
for(int j = 0; j < p->out_nframes; j++)//能指定输出第i帧的mfcc
{
Mfcc_Frame_std(p, j+1, out, len);
printf("第 %d 帧:\n", j+1);
for(int i = 0; i < len; i++)
{
printf("%f ", out[i]);
}
printf("\n");
}
#endif
#if 0
MFCC_TYPE type = MFCC_DIFF_1;
MfccInfo* p = MfccInit( audioctx, nfft, low, high, nfilters, ndcts, type);
int len = ndcts * 2;
float *out = (float*) malloc( sizeof(float)* (len));
for(int j = 0; j < p->out_nframes; j++)//必须按顺序输出，不能直接获取指定帧的mfcc差分
{
Mfcc_Frame_diff1(p, j+1, out, len);
printf("第 %d 帧:\n", j+1);
for(int i = 0; i < len; i++)
{
printf("%f ", out[i]);
}
printf("\n");
}
#endif
#if 1
MFCC_TYPE type = MFCC_DIFF_2;
MfccInfo* p = MfccInit( audioctx, nfft, low, high, nfilters, ndcts, type);
int len = ndcts * 3;
float *out = (float*) malloc( sizeof(float)* (len));
for(int j = 0; j < p->out_nframes; j++)//必须按顺序输出，不能直接获取指定帧的mfcc差分
{
Mfcc_Frame_diff2(p, j+1, out, len);
printf("第 %d 帧:\n", j+1);
for(int i = 0; i < len; i++)
{
printf("%f ", out[i]);
}
printf("\n");
}
#endif
free(data);
data = NULL;
free(out);
out = NULL;
MfccDestroy(p);
return 0;
}