關於PCM音頻和g711音頻編碼的轉換。

PCM文件：模擬音頻信號經模數轉換（A/D變換）直接形成的二進制序列，該文件沒有附加的文件頭和文件結束標誌。Windows的Convert工具能夠把PCM音頻格式的文件轉換成Microsoft的WAV格式的文件。    
          將音頻數字化。事實上就是將聲音數字化。最常見的方式是透過脈衝編碼調製PCM(Pulse Code Modulation) 。

運作原理例如以下:首先我們考慮聲音經過麥克風，轉換成一連串電壓變化的信號。例如以下圖所看到的。這張圖的橫座標爲秒。縱座標爲電壓大小。要將這種信號轉爲 PCM 格式的方法，是使用三個參數來表示聲音。它們是：聲道數、採樣位數和採樣頻率。

         採樣頻率：即取樣頻率,指每秒鐘取得聲音樣本的次數。採樣頻率越高,聲音的質量也就越好,聲音的還原也就越真實，但同一時候它佔的資源比較多。因爲人耳的分辨率非常有限,太高的頻率並不能分辨出來。

在16位聲卡中有22KHz、44KHz等幾級,當中，22KHz相當於普通FM廣播的音質，44KHz已相當於CD音質了，眼下的經常使用採樣頻率都不超過48KHz。 
        採樣位數：即採樣值或取樣值（就是將採樣樣本幅度量化）。它是用來衡量聲音波動變化的一個參數。也能夠說是聲卡的分辨率。

它的數值越大，分辨率也就越高。所發出聲音的能力越強。
         聲道數：非常好理解，有單聲道和立體聲之分，單聲道的聲音僅僅能使用一個喇叭發聲（有的也處理成兩個喇叭輸出同一個聲道的聲音）。立體聲的PCM 能夠使兩個喇叭都發聲（一般左右聲道有分工） ，更能感受到空間效果。

 

以下再用圖解來看看採樣位數和採樣頻率的概念。讓我們來看看這幾幅圖。圖中的黑色曲線表示的是PCM 文件錄製的自然界的聲波，紅色曲線表示的是PCM 文件輸出的聲波。橫座標便是採樣頻率；縱座標便是採樣位數。

這幾幅圖中的格子從左到右，逐漸加密，先是加大橫座標的密度，然後加大縱座標的密度。顯然，當橫座標的單位越小即兩個採樣時刻的間隔越小。則越有利於保持原始聲音的真實情況，換句話說，採樣的頻率越大則音質越有保證；同理，當縱座標的單位越小則越有利於音質的提高。即採樣的位數越大越好。

 

在計算機中採樣位數一般有8位和16位之分。但有一點請大家注意，8位不是說把縱座標分成8份，而是分成2的8次方即256份； 同理16位是把縱座標分成2的16次方65536份； 而採樣頻率一般有11025HZ（11KHz），22050HZ（22KHz）、44100Hz（44KHz）三種。

 

那麼，如今我們就能夠得到PCM文件所佔容量的公式：存儲量 = (採樣頻率*採樣位數*聲道)*時間/8(單位：字節數).
比如，數字激光唱盤(CD－DA。紅皮書標準)的標準採樣頻率爲44.lkHz。採樣數位爲16位，立體聲(2聲道)，能夠差點兒無失真地播出頻率高達22kHz的聲音，這也是人類所能聽到的最高頻率聲音。

激光唱盤一分鐘音樂須要的存儲量爲：　　　　　

(44.1*1000*l6*2)*60/8=10。584。000(字節)=10.584MBytes

這個數值就是PCM聲音文件在硬盤中所佔磁盤空間的存儲量。
計算機音頻文件的格式決定了其聲音的品質，日常生活中電話、收音機等均爲模擬音頻信號。即不存在採樣頻率和採樣位數的概念，我們能夠這樣比較一下：

• 44KHz，16BIT的聲音稱作：CD音質；
• 22KHz、16Bit的聲音效果近似於立體聲（FM Stereo）廣播。稱作：廣播音質；
• 11kHz、8Bit的聲音，稱作：電話音質。 

 

 

G711格式

G711編碼的聲音清晰度好，語音自然度高，但壓縮效率低，數據量大常在32Kbps以上。常用於電話語音(推薦使用64Kbps)，sampling rate爲8K，壓縮率爲2，即把S16格式的數據壓縮爲8bit，分爲a-law和u-law。

 

 

a-law也叫g711a,輸入的是13位（其實是S16的高13位），使用在歐洲和其他地區，這種格式是經過特別設計的，便於數字設備進行快速運算。

運算過程如下：

（1）      取符號位並取反得到s，

（2）      獲取強度位eee，獲取方法如圖所示

（3）      獲取高位樣本位wxyz

（4）      組合爲seeewxyz，將seeewxyz逢偶數爲取補數，編碼完畢

 

示例：

輸入pcm數據爲3210，二進制對應爲（0000 1100 1000 1010）

二進制變換下排列組合方式（0 0001 1001 0001010）

（1）      獲取符號位最高位爲0，取反，s=1

（2）      獲取強度位0001，查表，編碼制應該是eee=100

（3）      獲取高位樣本wxyz=1001

（4）      組合爲11001001，逢偶數爲取反爲10011100

編碼完畢。

 

u-law也叫g711u,使用在北美和日本，輸入的是14位，編碼算法就是查表，沒啥複雜算法，就是基礎值+平均偏移值，具體示例如下：

pcm=2345

（1）取得範圍值

+4062 to +2015 in 16 intervals of 128

（2）得到基礎值0x90，

（3）間隔數128，

（4）區間基本值4062，

（5）當前值2345和區間基本值差異4062-2345=1717，

（6）偏移值=1717/間隔數=1717/128，取整得到13，

（7）輸出爲0x90+13=0x9D

代碼如下：

g711codec.h

/*
 * G711 encode decode HEADER.
 */

#ifndef	__G711CODEC_H__
#define	__G711CODEC_H__

/*
* u-law, A-law and linear PCM conversions.
*/
#define	SIGN_BIT	(0x80)		/* Sign bit for a A-law byte. */
#define	QUANT_MASK	(0xf)		/* Quantization field mask. */
#define	NSEGS		(8)			/* Number of A-law segments. */
#define	SEG_SHIFT	(4)			/* Left shift for segment number. */
#define	SEG_MASK	(0x70)		/* Segment field mask. */
#define	BIAS		(0x84)		/* Bias for linear code. */

int PCM2G711a( char *InAudioData, char *OutAudioData, int DataLen, int reserve );
int PCM2G711u( char *InAudioData, char *OutAudioData, int DataLen, int reserve );

int G711a2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve );
int G711u2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve );

int g711a_decode(short amp[], const unsigned char g711a_data[], int g711a_bytes);

int g711u_decode(short amp[], const unsigned char g711u_data[], int g711u_bytes);

int g711a_encode(unsigned char g711_data[], const short amp[], int len);

int g711u_encode(unsigned char g711_data[], const short amp[], int len);

#endif  /* g711codec.h */

g711codec.c

#include "g711codec.h"

static short seg_end[8] = {0xFF, 0x1FF, 0x3FF, 0x7FF,
			    0xFFF, 0x1FFF, 0x3FFF, 0x7FFF};

static int search(int val, short	*table, int	size)
{
	int	i;

	for (i = 0; i < size; i++) {
		if (val <= *table++)
			return (i);
	}
	return (size);
}

/*
* alaw2linear() - Convert an A-law value to 16-bit linear PCM
*
*/
static int alaw2linear( unsigned char a_val )
{
	int	t;
	int	seg;

	a_val ^= 0x55;

	t = (a_val & QUANT_MASK) << 4;
	seg = ( (unsigned)a_val & SEG_MASK ) >> SEG_SHIFT;
	switch (seg) 
	{
		case 0:
			t += 8;
			break;
		case 1:
			t += 0x108;
			break;
		default:
			t += 0x108;
			t <<= seg - 1;
	}
	return ((a_val & SIGN_BIT) ? t : -t);
}


/*
* ulaw2linear() - Convert a u-law value to 16-bit linear PCM
*
* First, a biased linear code is derived from the code word. An unbiased
* output can then be obtained by subtracting 33 from the biased code.
*
* Note that this function expects to be passed the complement of the
* original code word. This is in keeping with ISDN conventions.
*/
static int ulaw2linear(unsigned char u_val)
{
	int	t;

	/* Complement to obtain normal u-law value. */
	u_val = ~u_val;

	/*
	* Extract and bias the quantization bits. Then
	* shift up by the segment number and subtract out the bias.
	*/
	t = ((u_val & QUANT_MASK) << 3) + BIAS;
	t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;

	return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
}


/*
 * linear2alaw() - Convert a 16-bit linear PCM value to 8-bit A-law
 *
 */
unsigned char linear2alaw(int pcm_val)	/* 2's complement (16-bit range) */
{
	int		mask;
	int		seg;
	unsigned char	aval;

	if (pcm_val >= 0) {
		mask = 0xD5;		/* sign (7th) bit = 1 */
	} else {
		mask = 0x55;		/* sign bit = 0 */
		pcm_val = -pcm_val - 8;
	}

	/* Convert the scaled magnitude to segment number. */
	seg = search(pcm_val, seg_end, 8);

	/* Combine the sign, segment, and quantization bits. */

	if (seg >= 8)		/* out of range, return maximum value. */
		return (0x7F ^ mask);
	else {
		aval = seg << SEG_SHIFT;
		if (seg < 2)
			aval |= (pcm_val >> 4) & QUANT_MASK;
		else
			aval |= (pcm_val >> (seg + 3)) & QUANT_MASK;
		return (aval ^ mask);
	}
}


/*
 * linear2ulaw() - Convert a linear PCM value to u-law
 *
 */
unsigned char linear2ulaw(int pcm_val)	/* 2's complement (16-bit range) */
{
	int		mask;
	int		seg;
	unsigned char	uval;

	/* Get the sign and the magnitude of the value. */
	if (pcm_val < 0) {
		pcm_val = BIAS - pcm_val;
		mask = 0x7F;
	} else {
		pcm_val += BIAS;
		mask = 0xFF;
	}

	/* Convert the scaled magnitude to segment number. */
	seg = search(pcm_val, seg_end, 8);

	/*
	 * Combine the sign, segment, quantization bits;
	 * and complement the code word.
	 */
	if (seg >= 8)		/* out of range, return maximum value. */
		return (0x7F ^ mask);
	else {
		uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0xF);
		return (uval ^ mask);
	}
}


int g711a_decode( short amp[], const unsigned char g711a_data[], int g711a_bytes )
{
	int i;
	int samples;
	unsigned char code;
	int sl;

	for ( samples = i = 0; ; )
	{
		if (i >= g711a_bytes)
			break;
		code = g711a_data[i++];

		sl = alaw2linear( code );

		amp[samples++] = (short) sl;
	}
	return samples*2;
}

int g711u_decode(short amp[], const unsigned char g711u_data[], int g711u_bytes)
{
	int i;
	int samples;
	unsigned char code;
	int sl;

	for (samples = i = 0;;)
	{
		if (i >= g711u_bytes)
			break;
		code = g711u_data[i++];

		sl = ulaw2linear(code);

		amp[samples++] = (short) sl;
	}
	return samples*2;
}

int g711a_encode(unsigned char g711_data[], const short amp[], int len)
{
    int i;

    for (i = 0;  i < len;  i++)
	{
        g711_data[i] = linear2alaw(amp[i]);
    }

    return len;
}

int g711u_encode(unsigned char g711_data[], const short amp[], int len)
{
    int i;

    for (i = 0;  i < len;  i++)
	{
        g711_data[i] = linear2ulaw(amp[i]);
    }

    return len;
}

decode.c

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "g711codec.h"

int main( int argc, char *argv[] )
{
	if(argc < 3)
	{
		printf("==> Usage:\n\tdecode [src.g711a] [dest.pcm]\n");	
		//printf("==> Usage:\n\tdecode [src.g711u] [dest.pcm]\n");	
		return 0;
	}

	FILE *pInFile = fopen(argv[1], "rb");
	FILE *pOutFile = fopen(argv[2], "wb");
	if (NULL == pInFile || NULL == pOutFile)
	{
		printf("open file failed\n");
		return 0;
	}

    struct stat s_buf;
    int status = 0;
    status = stat( argv[1], &s_buf );
    printf("file_size = %d\n", s_buf.st_size);
	
	int Ret = 0;
	int Read = 0;
    int DataLen = s_buf.st_size;
    printf("datalen = %d, %s, %d\n", DataLen, __func__, __LINE__);

	unsigned char ucInBuff[ DataLen + 1 ];
	unsigned char ucOutBuff[ 2*DataLen + 1 ];
	memset( ucInBuff, 0, sizeof(ucInBuff) );
    memset( ucOutBuff, 0, sizeof(ucOutBuff) );

	Read = fread( ucInBuff, 1, DataLen, pInFile );
	printf("Read = %d, Ret = %d\n", Read, Ret);
	if (Read)
	{
		Ret = G711a2PCM( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
		//Ret = G711u2PCM( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
		printf("Read = %d, Ret = %d, %s, %d\n", Read, Ret, __func__, __LINE__);
		fwrite( ucOutBuff, 1, Ret, pOutFile );
		memset( ucInBuff, 0, sizeof(ucInBuff) );
		memset( ucOutBuff, 0, sizeof(ucOutBuff) );
	}
	else
	{
		printf("fread error !\n");
		return -1;
	}
	
	fclose(pInFile);
	fclose(pOutFile);
	return 0;
}

encode.c

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "g711codec.h"

int main(int argc, char *argv[])
{
	if(argc < 3)
	{
		printf("==> Usage:\n\tencode [src.pcm] [dest.g711a]\n");	
		//printf("==> Usage:\n\tencode [src.pcm] [dest.g711u]\n");	
		return 0;
	}

    FILE *pInFile = fopen(argv[1], "rb");
	FILE *pOutFile = fopen(argv[2], "wb");
	if (NULL == pInFile || NULL == pOutFile)
	{
		printf("open file failed\n");
		return 0;
	}

    struct stat s_buf;
    int status = 0;
    status = stat( argv[1], &s_buf);    
    printf("file_size = %d\n", s_buf.st_size);
	
	int Ret = 0;
	int Read = 0;
    int Len = s_buf.st_size;
    printf("datalen = %d\n", s_buf.st_size);

	unsigned char ucInBuff[ Len +1 ];
	unsigned char ucOutBuff[ Len + 1 ];
    memset(ucInBuff, 0, sizeof(ucInBuff));
    memset(ucOutBuff, 0, sizeof(ucOutBuff));
	
	Read = fread(ucInBuff, 1, Len, pInFile);
	printf("Read = %d, Ret = %d\n", Read, Ret);
	if (Read)
	{
		Ret = PCM2G711a( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
		//Ret = PCM2G711u( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
		printf("Read = %d, Ret = %d, %s, %d\n", Read, Ret, __func__, __LINE__);
		fwrite(ucOutBuff, 1, Ret, pOutFile);
		memset(ucInBuff, 0, sizeof(ucInBuff));
		memset(ucOutBuff, 0, sizeof(ucOutBuff));
	}
	else
	{
		printf("fread error !\n");
		return -1;
	}

	fclose(pInFile);
	fclose(pOutFile);
	return 0;	
}

 

g711.c 

#include <stdio.h>
#include "g711codec.h"

/*
 * function: convert PCM audio format to g711 alaw/ulaw.(zqj)
 *	 InAudioData:	PCM data prepared for encoding to g711 alaw/ulaw.
 *   OutAudioData:	encoded g711 alaw/ulaw.
 *   DataLen:		PCM data size.
 *   reserve:		reserved param, no use.
 */

/*alaw*/
int PCM2G711a( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{	
	//check params.
	if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
	{
		printf("Error, empty data or transmit failed, exit !\n");	
		return -1;
	}
	printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);

	int Retaen = 0; 
	printf("G711a encode start......\n");
	Retaen = g711a_encode( (unsigned char *)OutAudioData, (short*)InAudioData, DataLen/2 );
	printf("Retaen = %d, %s, %d\n", Retaen, __func__, __LINE__);

	return Retaen; //index successfully encoded data len.
}

/*ulaw*/
int PCM2G711u( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{	
	//check params.
	if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
	{
		printf("Error, empty data or transmit failed, exit !\n");	
		return -1;
	}
	printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);

	int Retuen = 0; 
	printf("G711u encode start......\n");
	Retuen = g711u_encode( (unsigned char *)OutAudioData, (short*)InAudioData, DataLen/2 );
	printf("Retuen = %d, %s, %d\n", Retuen, __func__, __LINE__);

	return Retuen; 
}

/*
 * function: convert g711 alaw audio format to PCM.(zqj)
 *	 InAudioData:	g711 alaw data prepared for encoding to PCM.
 *   OutAudioData:	encoded PCM audio data.
 *   DataLen:		g711a data size.
 *   reserve:		reserved param, no use.
 */

/*alaw*/
int G711a2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{
	//check param.
	if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
	{
		printf("Error, empty data or transmit failed, exit !\n");	
		return -1;
	}
	printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);

	int Retade = 0;
	printf("G711a decode start......\n");
	Retade = g711a_decode( (short*)OutAudioData, (unsigned char *)InAudioData, DataLen );
	printf("Retade = %d, %s, %d\n", Retade, __func__, __LINE__);

	return Retade;	//index successfully decoded data len.
}

/*ulaw*/
int G711u2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{
	//check param.
	if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
	{
		printf("Error, empty data or transmit failed, exit !\n");	
		return -1;
	}
	printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);

	int Retude = 0;
	printf("G711u decode start......\n");
	Retude = g711u_decode( (short*)OutAudioData, (unsigned char *)InAudioData, DataLen );
	printf("Retude = %d, %s, %d\n", Retude, __func__, __LINE__);

	return Retude;	
}