示例：

输入pcm数据为1234，二进制对应为（0000 0100 1101 0010）
      二进制变换下排列组合方式（0 00001 0011 010010）
    1、获取符号位最高位为0，取反，s=1
    2、获取强度位00001，查表，编码制应该是eee=011
    3、获取高位样本wxyz=0011
    4、组合为10110011，逢偶数为取反为11100110，得到E6

#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. */
static int seg_aend[8] = {0x1F, 0x3F, 0x7F, 0xFF,0x1FF, 0x3FF, 0x7FF, 0xFFF};
static int seg_uend[8] = {0x3F, 0x7F, 0xFF, 0x1FF,0x3FF, 0x7FF, 0xFFF, 0x1FFF};static int search(int val,  /* changed from "short" *drago* */int *   table,int size) /* changed from "short" *drago* */
{int i;     /* changed from "short" *drago* */for (i = 0; i < size; i++) {if (val <= *table++)return (i);}return (size);
}int linear2alaw(int    pcm_val)        /* 2's complement (16-bit range) *//* changed from "short" *drago* */
{int        mask;   /* changed from "short" *drago* */int     seg;    /* changed from "short" *drago* */int     aval;pcm_val = pcm_val >> 3;//这里右移3位，因为采样值是16bit，而A-law是13bit，存储在高13位上，低3位被舍弃if (pcm_val >= 0) {mask = 0xD5;        /* sign (7th) bit = 1 二进制的11010101*/} else {mask = 0x55;      /* sign bit = 0  二进制的01010101*/pcm_val = -pcm_val - 1; //负数转换为正数计算}/* Convert the scaled magnitude to segment number. */seg = search(pcm_val, seg_aend, 8); //查找采样值对应哪一段折线/* Combine the sign, segment, and quantization bits. */if (seg >= 8)       /* out of range, return maximum value. */return (0x7F ^ mask);else {
//以下按照表格第一二列进行处理，低4位是数据，5~7位是指数，最高位是符号aval = seg << SEG_SHIFT;if (seg < 2)aval |= (pcm_val >> 1) & QUANT_MASK;elseaval |= (pcm_val >> seg) & QUANT_MASK;return (aval ^ mask);}
}

示例：

输入pcm数据为1234
    1、取得范围值，查表得 +2014 to +991 in 16 intervals of 64
    2、得到基础值为0xA0
    3、得到间隔数为64
    4、得到区间基本值2014
    5、当前值1234和区间基本值差异2014-1234=780
    6、偏移值=780/间隔数=780/64，取整得到12
    7、输出为0xA0+12=0xAC

#define BIAS        (0x84)      /* Bias for linear code. 线性码偏移值*/
#define CLIP            8159    //最大量化级数量int linear2ulaw( int   pcm_val)    /* 2's complement (16-bit range) */
{int        mask;int        seg;int     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 削波*/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 (0x7F ^ mask);else {uval = (seg << 4) | ((pcm_val >> (seg + 1)) & 0xF);return (uval ^ mask);}}int ulaw2linear( int    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 <<= (u_val & SEG_MASK) >> SEG_SHIFT;return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
}

和A-law画在同一个坐标轴中就能发现A-law在低强度信号下，精度要稍微高一些。