alsa 無噪音播放pcm

之前的代碼播放各種pcm始終有噪音，後來修改以後播放沒有噪音了。
代碼是從標準輸入讀取數據，記得要重定向到pcm文件。
（有個奇怪的問題，代碼改回有噪音的版本卻不能夠播放了，有時間再分析解決）
簡易代碼：

/*

This example reads standard from input and writes
to the default PCM device for 5 seconds of data.

*/

/* 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 = 0;
    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);

#if 0 //this will play with noise
    /* 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 */


    /* We want to loop for 5 seconds */
    snd_pcm_hw_params_get_period_time(params,&val, &dir);

#else //this will play without noise
    /* 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);
    }
    size = 1024;//or any 4*n
    frames = size/4;
#endif

    buffer = (char *) malloc(size);
    printf("size = %d\n",size);




    while(rc = read(0, buffer, size))//Redirect pcmFile
    {
        //loops--;
        //rc = read(0, buffer, size);
        if (rc == 0) //沒有讀取到數據
        {
            fprintf(stderr, "end of file on input\n");
            break;
        }
        else if (rc != size)//實際讀取 的數據 小於 要讀取的數據
        {
            fprintf(stderr,"short read: read %d bytes\n", rc);
        }

        rc = snd_pcm_writei(handle, buffer, frames);//寫入聲卡  （放音）
        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;
}

編譯加 -lasound