G.711 u律；A律 压缩算法

G.711  u律；A律 压缩算法

 

 /**********************************************************************
  * g711.c
  * 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. */

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

 /* copy from CCITT G.711 specifications */
 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 int 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 Code       Compressed Code
  *  -----------------       ------------------
  *  0000000wxyza            000wxyz
  *  0000001wxyza            001wxyz
  *  000001wxyzab            010wxyz
  *  00001wxyzabc            011wxyz
  *  0001wxyzabcd            100wxyz
  *  001wxyzabcde            101wxyz
  *  01wxyzabcdef            110wxyz
  *  1wxyzabcdefg            111wxyz
  *
  * 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;

     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);
     }
 }

 /*********************************************************************
  *    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);
 }

 #define BIAS        (0x84)      /* Bias for linear code. */

 /*********************************************************************
  * 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 Code    Compressed Code
  *  ------------------------    ---------------
  *  00000001wxyza               000wxyz
  *  0000001wxyzab               001wxyz
  *  000001wxyzabc               010wxyz
  *  00001wxyzabcd               011wxyz
  *  0001wxyzabcde               100wxyz
  *  001wxyzabcdef               101wxyz
  *  01wxyzabcdefg               110wxyz
  *  1wxyzabcdefgh               111wxyz
  *
  * 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(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);
     }

 }

 /*********************************************************************
  * 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.
  *********************************************************************/
 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));
 }

 /* A-law to u-law conversion */
 unsigned char alaw2ulaw(unsigned char   aval)
 {
     aval &= 0xff;
     return ((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 ((uval & 0x80) ? (0xD5 ^ (_u2a[0xFF ^ uval] - 1)):
            (0x55 ^ (_u2a[0x7F ^ uval] - 1)));
 }