Linux中的声音编程。捕获原始声音数据

Question

我正在阅读ALSA教程，并提出了有关捕获PCM数据的问题。我编写了以下示例（类似于本教程中的示例）。就是这里（省略错误检查和打印消息）：

#include <alsa/asoundlib.h>

int rate = 44100;
int periods = 2;
snd_pcm_uframes_t period_size = 8192;
int exact_rate;
int direction;

int main(int argc, char ** argv){
    snd_pcm_t *pcm_handle;
    snd_pcm_stream_t stream = SND_PCM_STREAM_CAPTURE; // <--- for capturin PCM data
    snd_pcm_hw_params_t *hwparams;
    snd_pcm_hw_params_alloca(&hwparams);
    snd_pcm_open(&pcm_handle, pcm_name, stream, 0)
    snd_pcm_hw_params_any(pcm_handle, hwparams)
    snd_pcm_hw_params_set_access(pcm_handle, hwparams, SND_PCM_ACCESS_RW_INTERLEAVED);
    snd_pcm_hw_params_set_format(pcm_handle, hwparams, SND_PCM_FORMAT_S16_LE);
    snd_pcm_hw_params_set_rate_near(pcm_handle, hwparams, &exact_rate, &direction);
    snd_pcm_hw_params_set_channels(pcm_handle, hwparams, 2);
    snd_pcm_hw_params_set_periods(pcm_handle, hwparams, periods, direction);
    snd_pcm_hw_params_set_buffer_size(pcm_handle, hwparams, (period_size * periods) >> 2);
    snd_pcm_hw_params(pcm_handle, hwparams);

    //reading PCM data, microphone is not plugged
    char *buffer = calloc(sizeof(char), period_size * periods);
    int pcm_read_return = 0;
    pcm_read_return = snd_pcm_readi(pcm_handle, buffer, period_size >> 2);

    printf("Read %d frames\n", pcm_read_return); //always 2048
    printf("Buffer: %s\n", buffer);  //Some data,not all bytes are 0

    free(buffer);

    return 0;
}

我不太了解这种行为。始终读取的帧数为2048。 它们始终为0 有时会产生一些非零字节。我预计不会读取任何帧，因为拔出了麦克风并且应该没有PCM数据。