G711编码原理及代码

G711编码的声音清晰度好，语音自然度高，但压缩效率低，数据量大常在32Kbps以上。常用于电话语音(推荐使用64Kbps)，sampling rate为8K，压缩率为2，即把S16格式的数据压缩为8bit，分为a-law和u-law。

a-law也叫g711a,输入的是13位（其实是S16的高13位），使用在欧洲和其他地区，这种格式是经过特别设计的，便于数字设备进行快速运算。

运算过程如下：

（1）      取符号位并取反得到s，

（2）      获取强度位eee，获取方法如图所示

（3）      获取高位样本位wxyz

（4）      组合为seeewxyz，将seeewxyz逢偶数为取补数，编码完毕

示例：

输入pcm数据为3210，二进制对应为（0000 1100 1000 1010）

二进制变换下排列组合方式（0 0001 1001 0001010）

（1）      获取符号位最高位为0，取反，s=1

（2）      获取强度位0001，查表，编码制应该是eee=100

（3）      获取高位样本wxyz=1001

（4）      组合为11001001，逢偶数为取反为10011100

编码完毕。

u-law也叫g711u,使用在北美和日本，输入的是14位，编码算法就是查表，没啥复杂算法，就是基础值+平均偏移值，具体示例如下：

pcm=2345

（1）取得范围值

+4062 to +2015 in 16 intervals of 128

（2）得到基础值0x90，

（3）间隔数128，

（4）区间基本值4062，

（5）当前值2345和区间基本值差异4062-2345=1717，

（6）偏移值=1717/间隔数=1717/128，取整得到13，

（7）输出为0x90+13=0x9D

Code如下

#include 
 
#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. */
#define		CLIP            8159

#define		G711_A_LAW	(0)
#define		G711_MU_LAW	(1)
#define		DATA_LEN	(16)
 
static short seg_aend[8] = {
	0x1F, 0x3F, 0x7F, 0xFF,
	0x1FF, 0x3FF, 0x7FF, 0xFFF
};

static short seg_uend[8] = {
	0x3F, 0x7F, 0xFF, 0x1FF,
	0x3FF, 0x7FF, 0xFFF, 0x1FFF
};
 
unsigned char _u2a[128] = {
	/* u- to A-law conversions */
	1,1,2,2,3,3,4,4,
	5,5,6,6,7,7,8,8,
	9,10,11,12,13,14,15,16,
	17,18,19,20,21,22,23,24,
	25,27,29,31,33,34,35,36,
	37,38,39,40,41,42,43,44,
	46,48,49,50,51,52,53,54,
	55,56,57,58,59,60,61,62,
	64,65,66,67,68,69,70,71,
	72,73,74,75,76,77,78,79,
	81,82,83,84,85,86,87,88, 
	89,90,91,92,93,94,95,96,
	97,98,99,100,101,102,103,104,
	105,106,107,108,109,110,111,112,
	113,114,115,116,117,118,119,120,
	121,122,123,124,125,126,127,128
};
 
unsigned char _a2u[128] = {
	/* A- to u-law conversions */
	1,3,5,7,9,11,13,15,
	16,17,18,19,20,21,22,23,
	24,25,26,27,28,29,30,31,
	32,32,33,33,34,34,35,35,
	36,37,38,39,40,41,42,43,
	44,45,46,47,48,48,49,49,
	50,51,52,53,54,55,56,57,
	58,59,60,61,62,63,64,64,
	65,66,67,68,69,70,71,72,
	73,74,75,76,77,78,79,79,
	80,81,82,83,84,85,86,87,
	88,89,90,91,92,93,94,95,
	96,97,98,99,100,101,102,103,
	104,105,106,107,108,109,110,111,
	112,113,114,115,116,117,118,119,
	120,121,122,123,124,125,126,127
};
 
static short search(int val, short *table, int size)
{
	int i;
	for (i = 0; i < size; i++) {
		if (val <= *table++)
			return (i);
 
	}
	return (size);
}
 
/*
 * linear2alaw() - Convert a 16-bit linear PCM value to 8-bit A-law
 *
 * linear2alaw() accepts an 16-bit integer and encodes it as A-law data.
 *
 *Linear Input CodeCompressed Code
 *---------------------------------------
 *0000000wxyza000wxyz
 *0000001wxyza001wxyz
 *000001wxyzab010wxyz
 *00001wxyzabc011wxyz
 *0001wxyzabcd100wxyz
 *001wxyzabcde101wxyz
 *01wxyzabcdef110wxyz
 *1wxyzabcdefg111wxyz
 *
 * For further information see John C. Bellamy's Digital Telephony, 1982,
 * John Wiley & Sons, pps 98-111 and 472-476.
 */
unsigned char linear2alaw(int pcm_val)/* 2's complement (16-bit range) */
{
 
	int mask;
	int seg;
	unsigned char aval;
 
	pcm_val = pcm_val >> 3;
 
	if (pcm_val >= 0) {
		mask = 0xD5;/* sign (7th) bit = 1 */
	} else {
		mask = 0x55;/* sign bit = 0 */
		pcm_val = -pcm_val - 1;
	}
 
	/* Convert the scaled magnitude to segment number. */
	seg = search(pcm_val, seg_aend, 8);
 
	/* Combine the sign, segment, and quantization bits. */
 
	if (seg >= 8)/* out of range, return maximum value. */
		return (unsigned char) (0x7F ^ mask);
	else {
		aval = (unsigned char) seg << SEG_SHIFT;
		if (seg < 2)
			aval |= (pcm_val >> 1) & QUANT_MASK;
		else
			aval |= (pcm_val >> seg) & QUANT_MASK;
		return (aval ^ mask);
	}
 
}
 
/*
 * alaw2linear() - Convert an A-law value to 16-bit linear PCM
 *
 */
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);
}
 
 
/*
 * linear2ulaw() - Convert a linear PCM value to u-law
 *
 * In order to simplify the encoding process, the original linear magnitude
 * is biased by adding 33 which shifts the encoding range from (0 - 8158) to
 * (33 - 8191). The result can be seen in the following encoding table:
 *
 *Biased Linear Input CodeCompressed Code
 *---------------------------------------
 *00000001wxyza000wxyz
 *0000001wxyzab001wxyz
 *000001wxyzabc010wxyz
 *00001wxyzabcd011wxyz
 *0001wxyzabcde100wxyz
 *001wxyzabcdef101wxyz
 *01wxyzabcdefg110wxyz
 *1wxyzabcdefgh111wxyz
 *
 * Each biased linear code has a leading 1 which identifies the segment
 * number. The value of the segment number is equal to 7 minus the number
 * of leading 0's. The quantization interval is directly available as the
 * four bits wxyz.  * The trailing bits (a - h) are ignored.
 *
 * Ordinarily the complement of the resulting code word is used for
 * transmission, and so the code word is complemented before it is returned.
 *
 * For further information see John C. Bellamy's Digital Telephony, 1982,
 * John Wiley & Sons, pps 98-111 and 472-476.
 */
unsigned char linear2ulaw(short pcm_val)/* 2's complement (16-bit range) */
{
	short mask;
	short seg;
	unsigned char uval;
 
	/* Get the sign and the magnitude of the value. */
	pcm_val = pcm_val >> 2;
	if (pcm_val < 0) {
		pcm_val = -pcm_val;
		mask = 0x7F;
	} else {
		mask = 0xFF;
	}
        if (pcm_val > CLIP)
		pcm_val = CLIP;/* clip the magnitude */
	pcm_val += (BIAS >> 2);
 
	/* Convert the scaled magnitude to segment number. */
	seg = search(pcm_val, seg_uend, 8);
 
	/*
	 * Combine the sign, segment, quantization bits;
	 * and complement the code word.
	 */
	if (seg >= 8)/* out of range, return maximum value. */
		return (unsigned char) (0x7F ^ mask);
	else {
 
		uval = (unsigned char) (seg << 4) | ((pcm_val >> (seg + 1)) & 0xF);
		return (uval ^ mask);
	}
}
 
/*
 * 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.
 */
short ulaw2linear(unsigned char u_val)
{
	short 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));
}
 
/* A-law to u-law conversion */
unsigned char alaw2ulaw(unsigned char aval)
{
	aval &= 0xff;
	return (unsigned char) ((aval & 0x80) ? (0xFF ^ _a2u[aval ^ 0xD5]) :
	    (0x7F ^ _a2u[aval ^ 0x55]));
}
 
/* u-law to A-law conversion */
unsigned char ulaw2alaw(unsigned char uval)
{
	uval &= 0xff;
	return (unsigned char) ((uval & 0x80) ? (0xD5 ^ (_u2a[0xFF ^ uval] - 1)) :
	    (unsigned char) (0x55 ^ (_u2a[0x7F ^ uval] - 1)));
}
 
int encode(char *a_psrc, char *a_pdst, int in_data_len, unsigned char type)
{
 
	int i;
	short *psrc = (short *)a_psrc;
	int out_data_len = in_data_len / sizeof(short);
 
	if (a_psrc == NULL || a_pdst == NULL) {
		return (-1);
	}
 
	if (in_data_len <= 0) {
		return (-1);
	}
 
 
	if (type == G711_A_LAW) {
		for (i = 0; i < out_data_len; i++) {
			a_pdst[i] = (char)linear2alaw(psrc[i]);
		}
	} else {
		for (i = 0; i < out_data_len; i++) {
			a_pdst[i] = (char)linear2ulaw(psrc[i]);
		}
	}
	return (i);
}
 
int decode(char *a_psrc, char *a_pdst, int in_data_len, unsigned char type)
{

	int i;
	short *pdst = (short *)a_pdst;
	int out_data_len = in_data_len / sizeof(char);

	if (a_psrc == NULL || a_pdst == NULL) {
		return (-1);
	}

	if (type == G711_A_LAW) {
		for (i = 0; i < out_data_len; i++) {
			pdst[i] = (short)alaw2linear((unsigned char)a_psrc[i]);
		}
	} else {
		for (i = 0; i < out_data_len; i++) {
			pdst[i] = (short)ulaw2linear((unsigned char)a_psrc[i]);
		}
	}

	return (i * sizeof(short));
}

int main(int argc, char **argv)
{
	int i = 0;
	int n = 0;
	unsigned short pcm_buf[DATA_LEN] = {0}; /*store linear pcm data*/
	unsigned short pcm_buf2[DATA_LEN] = {0}; /*store linear pcm data*/
	unsigned char g711_buf[DATA_LEN] = {0};

	FILE * fp_in = fopen("input.wav", "r");
	FILE * fp_out = fopen("pcm.g711_alaw", "w");
	FILE * fp_out_pcm = fopen("pcm2.wav", "w");

	unsigned char header[128] = { 0 };
	fread(header, 1, 0x2c, fp_in);
	fwrite(header, 1, 0x2c, fp_out_pcm);

	while (DATA_LEN * 2 == fread(pcm_buf, 1, DATA_LEN * 2, fp_in)) {

		printf("encode %d was trans\n",
			encode(pcm_buf, g711_buf, sizeof(pcm_buf), G711_A_LAW));

		fwrite(g711_buf, 1, DATA_LEN, fp_out);

		printf("decode %d was trans\n",
			decode(g711_buf, pcm_buf2, sizeof(g711_buf), G711_A_LAW));
	
		fwrite(pcm_buf2, 1, DATA_LEN*2, fp_out_pcm);
	}

	fclose(fp_in);
	fclose(fp_out);
	fclose(fp_out_pcm);
	return 0;
}