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(¶ms);
/* 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