ADPCM C语音示例

下面是一个简单的 ADPCM C语音示例，它可以将一个 WAV 文件压缩为 ADPCM 格式：

#include <stdio.h>
#include <stdlib.h>

#define BLOCK_SIZE 256

typedef struct adpcm_state {
    short valprev;
    char index;
} adpcm_state;

static short int stepsizeTable[] = {
    7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 19, 21, 23, 25, 28, 31,
    34, 37, 41, 45, 50, 55, 60, 66, 73, 80, 88, 97, 107, 118, 130, 143
};

static char indexTable[] = {
    -1, -1, -1, -1, 2, 4, 6, 8, -1, -1, -1, -1, 2, 4, 6, 8
};

void adpcm_coder(short int *indata, char *outdata, int len, adpcm_state *state)
{
    int valprev = state->valprev;
    int index = state->index;
    int sign, delta, step, vpdiff, outputbuffer = 0, bufferstep = 1;

    for (int i = 0; i < len; i++) {
        /* Step 1 - compute difference with previous value */
        delta = indata[i] - valprev;
        sign = (delta < 0) ? 8 : 0;
        if (sign) delta = (-delta);

        /* Step 2 - divide and clamp */
        step = stepsizeTable[index];
        vpdiff = step >> 3;
        if (delta > step) {
            outputbuffer |= sign | 7;
            if ((delta >>= 1) > step) {
                outputbuffer |= 8;
                vpdiff += step;
            } else {
                vpdiff += delta;
            }
        } else {
            if ((delta >>= 1) > step) {
                outputbuffer |= sign;
                vpdiff -= step;
            } else {
                vpdiff -= delta;
            }
        }

        /* Step 3 - update previous value */
        valprev += (sign) ? -vpdiff : vpdiff;

        /* Step 4 - clamp previous value to 16 bits */
        if (valprev > 32767) {
            valprev = 32767;
        } else if (valprev < -32768) {
            valprev = -32768;
        }

        /* Step 5 - update step value */
        index += indexTable[outputbuffer & 0x0f];
        if (index < 0) {
            index = 0;
        } else if (index > 15) {
            index = 15;
        }

        /* Step 6 - output value */
        if (bufferstep) {
            outputbuffer = (outputbuffer << 4) & 0xf0;
        } else {
            *outdata++ = outputbuffer | ((outputbuffer >> 4) & 0x0f);
        }
        bufferstep = !bufferstep;
    }

    /* Output last step, if needed */
    if (!bufferstep) {
        *outdata++ = outputbuffer | ((outputbuffer >> 4) & 0x0f);
    }

    /* Save state */
    state->valprev = valprev;
    state->index = index;
}

int main(int argc, char *argv[])
{
    if (argc != 3) {
        printf("Usage: %s input_file output_file\n", argv[0]);
        return 1;
    }

    FILE *fin = fopen(argv[1], "rb");
    if (!fin) {
        printf("Cannot open input file %s\n", argv[1]);
        return 1;
    }

    fseek(fin, 0L, SEEK_END);
    int input_size = ftell(fin);
    fseek(fin, 0L, SEEK_SET);

    short int *input_data = (short int *) malloc(input_size);
    if (!input_data) {
        printf("Out of memory\n");
        return 1;
    }

    fread(input_data, 1, input_size, fin);
    fclose(fin);

    adpcm_state state;
    state.valprev = 0;
    state.index = 0;

    char *output_data = (char *) malloc(input_size / 4);
    if (!output_data) {
        printf("Out of memory\n");
        return 1;
    }

    int blocks = input_size / BLOCK_SIZE;
    for (int i = 0; i < blocks; i++) {
        adpcm_coder(input_data + i * BLOCK_SIZE, output_data + i * BLOCK_SIZE / 4, BLOCK_SIZE, &state);
    }

    FILE *fout = fopen(argv[2], "wb");
    if (!fout) {
        printf("Cannot open output file %s\n", argv[2]);
        return 1;
    }

    fwrite(output_data, 1, input_size / 4, fout);
    fclose(fout);

    free(input_data);
    free(output_data);

    return 0;
}

此示例使用了一个 adpcm_state 结构体来保存编码器的状态。adpcm_coder() 函数将输入数据块压缩为 ADPCM 格式，并更新状态。main() 函数读取 WAV 文件，将其分成块，压缩每个块并将结果写入输出文件。注意，此示例仅处理单声道 16 位 PCM WAV 文件。