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)));
}