DSP

Linux 音频 驱动 接口 简介 之 OSS 编程 方法 采样频率 量化 位数 实例 driver

2019-07-13 20:26发布

音频器件与DSP间的接口 两个概念:
* 采样频率:每秒采多少次;
* 量化精度(位数):每采一次采出的电压值分为多少份,如:量化精度为8bit,则分为2的8次方等份。
PCM接口:
由时钟脉冲(BCLK)、帧同步(FS)、数据接收(DR)、数据发送(DX)组成,每个音频通道获得一条独立的数据队列;

IIS(Inter-IC Sound)接口:
1980s-LRCLK(Left/Right Clock) ,其高位为左声道,低位为右声道;

AC97接口:
是一种数据格式,用于音频编码的内部架构规格,具有控制功能;AC-Link接口:位时钟(BITCLK),同步校正(SYNC)、编码到处理器(SDATAIN)与CPU到编码器(SDATAOUT)。


OSS驱动的组成与用户空间编程

OSS驱动实现两个基本音频设备:

mixer(混音器)
分为 input mixer 和 output mixer
打开设备 /dev/mixer 后,
可使用 ioctl(mixerfd, SOUND_MIXER_READ(SOUND_MIXER_MIC), &value);
及 ioctl(mixerfd, SOUND_MIXER_WRITE(SOUND_MIXER_VOLUME), &value);
来读取和写入麦克及喇叭的输入、输出增益;

dsp(数字信号处理器)
首先打开 /dev/dsp (声卡若支持全双工模式,则可以 O_RDWR 模式打开)后,
使用 ioctl(dspfd, SOUND_PCM_WRITE_CHANNELS, &value); 设置通道数,
ioctl(dspfd, SOUND_PCM_WRITE_RATE, &value); 设置采样频率,
ioctl(dspfd, SOUND_PCM_WRITE_BITS, &value); 设置量化位数,
使用 read 和 write 对 dspfd 设备操作,可实现录音放音,
ioctl(dspfd, SOUND_PCM_SYNC, 0); 函数可实现录音前等待放音结束。

简单的录音放音程序实例:
/*
* ===================================================================
*
*       Filename: oss.c
*
*    Description:
*
*        Version: 1.0
*        Created: Monday, May 24, 2010 11:07:27 HKT HKT
*       Revision: none
*       Compiler: gcc
*
*         Author: Wang Cheng (Dancy), dancesimply@gmail.com
*        Company: Kunming Lang Ju Inc.
*
* ===================================================================
*/
#include
#include
#include
#include
#include
#include
#include
#include
#define RATE 8000
#define SIZE 8
#define CHANNELS 1
unsigned char buf[RATE*SIZE*CHANNELS/(8)]; char voice_name[] = "test.wav";//record file name
volatile int file_voice_fd=-1;//record file fd;
volatile int fd; /* 声音设备的文件描述符 */ //关闭声卡设备
void file_close()
{
    if(fd >0)
       {
           close(fd);
           fd=-1;
       }
}
int file_open()
{
int arg; /* 用于ioctl调用的参数 */
int status;   /* 系统调用的返回值 */
status = 1;
int ret;
ret = 0;
    //判断声音设备是否打开
if (fd > 0) return fd;
/* 打开声音设备 */
fd = open("/dev/dsp", O_RDWR,0);
if (fd < 0) {
    perror("open of /dev/dsp failed");
    ret = -1;
    goto quit;
}
ret = fd;
/* 设置采样时的量化位数 */
arg = SIZE;
status = ioctl(fd, SOUND_PCM_WRITE_BITS, &arg);
if (status == -1)
{
    perror("SOUND_PCM_WRITE_BITS ioctl failed");
    ret = -2;
    goto fd_err;
}
if (arg != SIZE)
{
   perror("unable to set sample size");
   ret = -3;
   goto fd_err;
}
/* 设置采样时的声道数目 */
arg = CHANNELS;
status = ioctl(fd, SOUND_PCM_WRITE_CHANNELS, &arg);
if (status == -1)
{
   perror("SOUND_PCM_WRITE_CHANNELS ioctl failed");
   ret = -4;
     file_close();
    goto fd_err;
}
if (arg != CHANNELS)
{
      perror("unable to set number of channels");
      ret = -5;
     goto fd_err;
}
/* 设置采样时的采样频率 */
arg = RATE;
status = ioctl(fd, SOUND_PCM_WRITE_RATE, &arg);
if (status == -1)
{
   perror("SOUND_PCM_WRITE_WRITE ioctl failed");
   ret = -6;
   }
fd_err:
if(ret < 0)
{
     file_close();
}
quit:
    printf("ret = %d ",ret);
return ret;
} int create_voice_file()
{
    file_voice_fd = open(voice_name , O_RDWR | O_CREAT | O_TRUNC,S_IRUSR|S_IWUSR|S_IXUSR);
    if(file_voice_fd < 0)
   {
        printf("create new file failure");
       return -1;
    }
    printf("voice_name = %s, file fd is %d ",voice_name, file_voice_fd);
    return 0;
}
int open_voice_file()
{
    file_voice_fd = open(voice_name , O_RDWR | O_CREAT | O_APPEND,S_IRUSR|S_IWUSR|S_IXUSR);
    if(file_voice_fd < 0)
   {
        printf("create new file failure");
       return -1;
    }
    printf("voice_name = %s, file fd is %d ",voice_name, file_voice_fd);
    return 0;
}
int close_record_file()
{
    if(file_voice_fd >0 )
    {
        close(file_voice_fd);
        file_voice_fd = -1;
        return 0;
    }
    else
    {
        return -1;
    }
}
int main(int agrc, char **argv)
{
file_open();
if ('c' == **(argv+1))
{
   printf("voice record! ");
   create_voice_file();
   int i = 0;
   while (i<5)
   {
    read(fd, buf, sizeof(buf));
    write(file_voice_fd, buf, sizeof(buf));
    printf("No. %d ", i++);
   }
   close_record_file();
   goto play;
}
else if ('p' == **(argv+1))
{
play:
   printf("voice play! ");
   open_voice_file();
   char playbuf[RATE*SIZE*CHANNELS/8*5];
   read(file_voice_fd, playbuf, sizeof(playbuf));
   write(fd, playbuf, sizeof(playbuf));
   close_record_file();
}
else
{
   printf("please input args... c for record and p for play. ");
}
file_close();
}
参考文献:宋宝华 《Linux设备驱动开发详解》