使用 libao 播放音频文件

Question

我正在尝试让一个简单的 C 程序播放 AIFF 或 WAV 文件。根据我在http://www.xiph.org/ao/doc/ 看到的内容，这应该可以工作，但无论我提供什么文件，它都会发出嗡嗡声。这有什么问题？

/* compile with "gcc -o playme playme.c -lao -ldl -lm" */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ao/ao.h>
#include <math.h>

ao_device *device;
ao_sample_format format;

int main(int argc, char *argv[])
{
    int default_driver;
    char *buffer;
    unsigned long count;

    FILE *fp;

    if (argc != 2) {
    printf("usage: %s <filename>\n", argv[0]);
    exit(1);
    }

    ao_initialize();
    default_driver = ao_default_driver_id();
    memset(&format, 0, sizeof(format));

    format.bits = 16;
    format.channels = 2;
    format.rate = 44100;
    format.byte_format = AO_FMT_LITTLE;

    device = ao_open_live(default_driver, &format, NULL /* no options */);
    if (device == NULL) {
        printf("Error opening sound device.\n");
        exit(1);
    }

    fp = fopen(argv[1], "rb");
    if (fp == NULL) {
    printf("Cannot open %s.\n", argv[1]);
    exit(2);
    }

    fseek(fp, 0, SEEK_END);
    count = ftell(fp);
    fseek(fp, 0, SEEK_SET);

    fread(buffer, sizeof(char), count, fp);
    ao_play(device, buffer, count);
    ao_close(device);
    ao_shutdown();

    return 0;
}

Answer 1

我没有意识到的一个关键问题是 libao 完全不解码。因此，程序员需要在打开音频设备之前提取样本大小、速率、通道等并将其提供给 libao。 libsndfile 可用于执行此操作，但如果您只是想要快速而肮脏的东西，这里是播放 AIFF 文件的代码：

/* compile with "gcc -o playaiff playaiff.c -lao -ldl -lm" */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ao/ao.h>
#include <math.h>

#define MAXCHAN 8

ao_device *device;
ao_sample_format format;

#define gshort( b) (((int)((b)[0]) << 8) + (int)((b)[1]))
#define glong( b) (((int)((b)[0]) << 24) + ((int)((b)[1]) << 16) +\
        ((int)((b)[2]) << 8) + (int)((b)[3]))

typedef struct {
    short     channels;
    short     samplesize;
    int   samplerate;
    unsigned long samplecount;
    int       valid;
} aiffinfo;

aiffinfo getaiffinfo(FILE *);
static int IeeeExtendedToLong(unsigned char *);

int main(int argc, char *argv[])
{
    int default_driver;
    char *buffer;

    aiffinfo info;

    FILE *fp;

    if (argc != 2) {
    printf("usage: %s <filename>\n", argv[0]);
    exit(1);
    }

    ao_initialize();
    default_driver = ao_default_driver_id();
    memset(&format, 0, sizeof(format));

    fp = fopen(argv[1], "rb");
    if (fp == NULL) {
    printf("Cannot open %s.\n", argv[1]);
    exit(2);
    }

    info = getaiffinfo(fp);

    if (!info.valid) {
    printf("Invalid AIFF file.\n");
    exit(1);
    }

    format.bits = info.samplesize;
    format.channels = info.channels;
    format.rate = info.samplerate;
    format.byte_format = AO_FMT_LITTLE;

    device = ao_open_live(default_driver, &format, NULL /* no options */);
    if (device == NULL) {
        printf("Error opening sound device.\n");
        exit(1);
    }

    buffer = malloc(sizeof(char) * info.samplecount);

    fread(buffer, sizeof(char), info.samplecount, fp);
    ao_play(device, buffer, info.samplecount);
    ao_close(device);
    ao_shutdown();

    return 0;
}


aiffinfo getaiffinfo(FILE *fp)
{
    int size;
    int len;
    int offset;
    int blocksize;
    int found = 0;
    unsigned char chunk[18];
    unsigned char fid[4];
    aiffinfo info;

    info.samplesize = 0;
    info.valid = 0;

    if (fread(chunk, 1, 4, fp) < 4) return info;
    if (memcmp(chunk,"FORM",4)) return info;
    if (fread(chunk, 1, 4, fp) < 4)  return info;
    size = glong(chunk);
    if (size & 1) size++;
    if (size < 20) return info;
    if (fread(chunk, 1, 4, fp) < 4) return info;
    if (memcmp(chunk, "AIFF", 4)) return info;

    size -= 4;
    while (size > 8) {
    if (fread(fid, 1, 4, fp) < 4) return info;    // chunck id
    if (fread(chunk, 1, 4, fp) < 4) return info;    // and len
    size -= 8;
    len = glong(chunk);
    if (len < 0) return info;
    if (len & 1) len++;
    size -= len;
    if (size < 0) return info;
    if (memcmp(fid, "COMM", 4) == 0) {
        if (len != 18) return info;
        if (fread(chunk, 1, 18, fp) < 18) return info;
        info.channels = gshort(chunk);
        if (info.channels < 1) return info;
        if (info.channels > MAXCHAN) return info;
        info.samplecount = glong(chunk+2);
        if (info.samplecount < 1) return info;
        info.samplerate = IeeeExtendedToLong(chunk + 8);
        if (info.samplerate <= 0) return info;
        info.samplesize = gshort(chunk + 6);
        if (info.samplesize < 1 || info.samplesize > 16) return info;
    } else if (memcmp(fid,"SSND",4)==0){
        if (!info.channels) return info;
        if (fread(chunk, 1, 4, fp) < 4) return info;
        offset = glong(chunk);
        if (fread(chunk, 1, 4, fp) < 4) return info;
        blocksize = glong(chunk);
        if (blocksize) return info;
        if (offset) fseek(fp, offset,SEEK_CUR);
        found = 1;
        break;
    } else fseek (fp, len, SEEK_CUR);
    }

    if (!found) return info;
    if (!info.channels) return info;

//    printf("Looks good so far.\n");
    info.valid = 1;
    return info;
}


/****************************************************************
 * Extended precision IEEE floating-point conversion routine.
 ****************************************************************/

#ifndef Uint32
#define Uint32 unsigned int
#endif
#ifndef HUGE_INT32
#define HUGE_INT32 0x7fffffff
#endif                                          /* HUGE_VAL */

static int IeeeExtendedToLong( unsigned char *bytes)
{
    int f = 0;
    int expon;
    Uint32 hiMant;
    Uint32 loMant;

    expon = ((bytes[0] & 0x7F) << 8) | (bytes[1] & 0xFF);
    hiMant = ((Uint32) (bytes[2] & 0xFF) << 24)
    | ((Uint32) (bytes[3] & 0xFF) << 16)
    | ((Uint32) (bytes[4] & 0xFF) << 8)
    | ((Uint32) (bytes[5] & 0xFF));
    loMant = ((Uint32) (bytes[6] & 0xFF) << 24)
    | ((Uint32) (bytes[7] & 0xFF) << 16)
    | ((Uint32) (bytes[8] & 0xFF) << 8)
    | ((Uint32) (bytes[9] & 0xFF));

    if (expon == 0 && hiMant == 0 && loMant == 0) f = 0;
    else if (expon == 0x7FFF) f = HUGE_INT32;
    else {
    expon -= 16382;
    expon = 32-expon;
    if (expon < 0) f = HUGE_INT32;
    else f = hiMant >> expon;
    }

    if (bytes[0] & 0x80)
    return -f;
    else
    return f;
}