如何从 PCM 获取缓冲区？

Question

我想在 Linux 上使用 ALSA 播放 PCM 文件，这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <alsa/asoundlib.h>

main (int argc, char *argv[])
{
    int i;
    int err;
    short buf[128];
    snd_pcm_t *playback_handle;
    snd_pcm_hw_params_t *hw_params;

    if ((err = snd_pcm_open (&playback_handle, argv[1], SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
        fprintf (stderr, "cannot open audio device %s (%s)\n", 
             argv[1],
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params_malloc (&hw_params)) < 0) {
        fprintf (stderr, "cannot allocate hardware parameter structure (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params_any (playback_handle, hw_params)) < 0) {
        fprintf (stderr, "cannot initialize hardware parameter structure (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params_set_access (playback_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
        fprintf (stderr, "cannot set access type (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params_set_format (playback_handle, hw_params, SND_PCM_FORMAT_S16_LE)) < 0) {
        fprintf (stderr, "cannot set sample format (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params_set_rate_near (playback_handle, hw_params, 44100, 0)) < 0) {
        fprintf (stderr, "cannot set sample rate (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params_set_channels (playback_handle, hw_params, 2)) < 0) {
        fprintf (stderr, "cannot set channel count (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    if ((err = snd_pcm_hw_params (playback_handle, hw_params)) < 0) {
        fprintf (stderr, "cannot set parameters (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    snd_pcm_hw_params_free (hw_params);

    if ((err = snd_pcm_prepare (playback_handle)) < 0) {
        fprintf (stderr, "cannot prepare audio interface for use (%s)\n",
             snd_strerror (err));
        exit (1);
    }

    for (i = 0; i < 10; ++i) {
        if ((err = snd_pcm_writei (playback_handle, buf, 128)) != 128) {
            fprintf (stderr, "write to audio interface failed (%s)\n",
                 snd_strerror (err));
            exit (1);
        }
    }

    snd_pcm_close (playback_handle);
    exit (0);
}

我现在的问题是，我想播放一个解码的 MP3 文件，它现在是 PCM 格式。但是函数snd_pcm_writei的参数是“pcm句柄、缓冲区、大小”。 PCM给了，size就是buffer的大小，但是怎么来buffer参数呢？

Answer 1

您必须先将文件加载到内存中才能播放。

使用libsndfile 或audiofile 之类的库是最简单的。

Answer 2

snd_pcm_writei 有 3 个参数，你已经知道了：

• PCM 句柄
• 包含缓冲区的帧
• 要写入的帧

您必须将 PCM 中的帧read() 放入缓冲区，然后播放它们

这是一个播放原始 PCM 数据的短程序（从 here 重新格式化，清单 3）：

/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

int main() {
    long loops;
    int rc;
    int size;
    snd_pcm_t *handle;
    snd_pcm_hw_params_t *params;
    unsigned int val;
    int dir;
    snd_pcm_uframes_t frames;
    char *buffer;

    /* Open PCM device for playback. */
    rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
    if (rc < 0) 
    {
        fprintf(stderr, "Unable to open PCM device: %s\n", snd_strerror(rc));
        exit(1);
    }

    /* Allocate a hardware parameters object. */
    snd_pcm_hw_params_alloca(&params);

    /* Fill it in with default values. */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */

    /* Interleaved mode */
    snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);

    /* Signed 16-bit little-endian format */
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);

    /* Two channels (stereo) */
    snd_pcm_hw_params_set_channels(handle, params, 2);

    /* 44100 bits/second sampling rate (CD quality) */
    val = 44100;
    snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);

    /* Set period size to 32 frames. */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);

    /* Write the parameters to the driver */
    rc = snd_pcm_hw_params(handle, params);
    if (rc < 0) 
    {
        fprintf(stderr, "Unable to set HW parameters: %s\n", snd_strerror(rc));
        exit(1);
    }

    /* Use a buffer large enough to hold one period */
    snd_pcm_hw_params_get_period_size(params, &frames, &dir);
    size = frames * 4; /* 2 bytes/sample, 2 channels */
    buffer = (char *) malloc(size);

    /* We want to loop for 5 seconds */
    snd_pcm_hw_params_get_period_time(params, &val, &dir);
    /* 5 seconds in microseconds divided by period time */
    loops = 5000000 / val;

    while (loops > 0) 
    {
        loops--;
        rc = read(0, buffer, size);  /* ### Retrieve the buffer ### */
        if (rc == 0) 
        {
            fprintf(stderr, "End of file on input\n");
            break;
        } else if (rc != size) 
        {
            fprintf(stderr, "Short read: read %d bytes\n", rc);
        }
        rc = snd_pcm_writei(handle, buffer, frames);
        if (rc == -EPIPE) 
        {
            /* EPIPE means underrun */
            fprintf(stderr, "Underrun occurred\n");
            snd_pcm_prepare(handle);
        } else if (rc < 0) 
        {
            fprintf(stderr, "Error from writei: %s\n", snd_strerror(rc));
        }  else if (rc != (int)frames) 
        {
           fprintf(stderr, "Short write, write %d frames\n", rc);
        }
    }

    snd_pcm_drain(handle);
    snd_pcm_close(handle);
    free(buffer);

    return 0;
}