G.711标准的C语言实现。

以下是G.711标准的C语言实现：

#include <stdio.h>
#include <stdint.h>

#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. */

int16_t alaw2linear(uint8_t a_val)
{
    int16_t t;
    uint8_t 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);
}

uint8_t linear2alaw(int16_t pcm_val)
{
    int16_t mask;
    int16_t seg;
    uint8_t aval;

    if (pcm_val >= 0) {
        mask = 0xD5;
    } else {
        mask = 0x55;
        pcm_val = -pcm_val - 8;
    }

    /* Convert the scaled magnitude to segment number. */
    for (seg = 0; seg < NSEGS; seg++) {
        if (pcm_val <= (int16_t) _aLawTbl[seg]) {
            break;
        }
    }

    /* Combine the sign, segment, and quantization bits. */
    if (seg >= NSEGS) { /* Out of range, return maximum value. */
        return (uint8_t) (mask | (QUANT_MASK << 4));
    } else {
        aval = (uint8_t) (seg << SEG_SHIFT);
        if (seg < 2) {
            aval |= (pcm_val >> 4) & QUANT_MASK;
        } else {
            aval |= (pcm_val >> (seg + 3)) & QUANT_MASK;
        }
        return (uint8_t) (aval ^ mask);
    }
}

这段代码实现了将G.711 A-law编码转换为线性PCM值以及将线性PCM值转换为G.711 A-law编码。