音頻錄放框架 下面給出一個利用聲卡上的DSP設(shè)備進(jìn)行聲音錄制和回放的基本框架，它的功能是先錄制幾秒種音頻數(shù)據(jù)，將其存放在內(nèi)存緩沖區(qū)中，然后再進(jìn)行回放，其所有的功能都是通過讀寫/dev/dsp設(shè)備文件來完成的：

?

/*

 * sound.c

 */

#include 

#include 

#include 

#include 

#include 

#include 

#include 

?

#define LENGTH 3??? /* 存儲秒數(shù) */

#define RATE 8000?? /* 采樣頻率 */

#define SIZE 8????? /* 量化位數(shù) */

#define CHANNELS 1?/* 聲道數(shù)目 */

?

/* 用于保存數(shù)字音頻數(shù)據(jù)的內(nèi)存緩沖區(qū) */

unsigned char buf[LENGTH*RATE*SIZE*CHANNELS/8];

?

int main()

{

?int fd;?/* 聲音設(shè)備的文件描述符 */

?int arg; /* 用于ioctl調(diào)用的參數(shù) */

?int status;?? /* 系統(tǒng)調(diào)用的返回值 */

?

?/* 打開聲音設(shè)備 */

?fd = open("/dev/dsp", O_RDWR);

?if (fd < 0) {

??? perror("open of /dev/dsp failed");

??? exit(1);

?}

?

?/* 設(shè)置采樣時的量化位數(shù) */

?arg = SIZE;

?status = ioctl(fd, SOUND_PCM_WRITE_BITS, &arg);

?if (status == -1)

??? perror("SOUND_PCM_WRITE_BITS ioctl failed");

?if (arg != SIZE)

??? perror("unable to set sample size");

?

?/* 設(shè)置采樣時的聲道數(shù)目 */

?arg = CHANNELS; 

??status = ioctl(fd, SOUND_PCM_WRITE_CHANNELS, &arg);

?if (status == -1)

??? perror("SOUND_PCM_WRITE_CHANNELS ioctl failed");

?if (arg != CHANNELS)

??? perror("unable to set number of channels");

?

?/* 設(shè)置采樣時的采樣頻率 */

?arg = RATE;

?status = ioctl(fd, SOUND_PCM_WRITE_RATE, &arg);

?if (status == -1)

??? perror("SOUND_PCM_WRITE_WRITE ioctl failed");

?

?/* 循環(huán)，直到按下Control-C */

?while (1) {

??? printf("Say something:n");

??? status = read(fd, buf, sizeof(buf)); /* 錄音 */

??? if (status != sizeof(buf))

????? perror("read wrong number of bytes");

?

??? printf("You said:n");

??? status = write(fd, buf, sizeof(buf)); /* 回放 */

??? if (status != sizeof(buf))

????? perror("wrote wrong number of bytes");

?

??? /* 在繼續(xù)錄音前等待回放結(jié)束 */

??? status = ioctl(fd, SOUND_PCM_SYNC, 0); 

????if (status == -1)

????? perror("SOUND_PCM_SYNC ioctl failed");

?}

}

? 4.4 混音器框架 下面再給出一個對混音器進(jìn)行編程的基本框架，利用它可以對各種混音通道的增益進(jìn)行調(diào)節(jié)，其所有的功能都是通過讀寫/dev/mixer設(shè)備文件來完成的：

?

/*

 * mixer.c

 */

#include 

#include 

#include 

#include 

#include 

#include 

?

/* 用來存儲所有可用混音設(shè)備的名稱 */

const char *sound_device_names[] = SOUND_DEVICE_NAMES;

?

int fd;????????????????? /* 混音設(shè)備所對應(yīng)的文件描述符 */

int devmask, stereodevs; /* 混音器信息對應(yīng)的位圖掩碼 */

char *name;

?

/* 顯示命令的使用方法及所有可用的混音設(shè)備 */

void usage()

{

?int i;

?

?fprintf(stderr, "usage: %s   n"

????? ?"?????? %s  nn"

????? ?"Where  is one of:n", name, name);

?for (i = 0 ; i < SOUND_MIXER_NRDEVICES ; i++)

??? if ((1 << i) & devmask) /* 只顯示有效的混音設(shè)備 */

????? fprintf(stderr, "%s ", sound_device_names[i]);

?fprintf(stderr, "n");

?exit(1);

}

?

int main(int argc, char *argv[])

{

?int left, right, level;?/* 增益設(shè)置 */

?int status;????????????? /* 系統(tǒng)調(diào)用的返回值 */

?int device;????????????? /* 選用的混音設(shè)備 */

?char *dev;?????????????? /* 混音設(shè)備的名稱 */

?int i;

?

?name = argv[0];

?

?/* 以只讀方式打開混音設(shè)備 */

?fd = open("/dev/mixer", O_RDONLY);

?if (fd == -1) {

??? perror("unable to open /dev/mixer");

??? exit(1);

?}

?

??/* 獲得所需要的信息 */

?status = ioctl(fd, SOUND_MIXER_READ_DEVMASK, &devmask);

?if (status == -1)

??? perror("SOUND_MIXER_READ_DEVMASK ioctl failed");

?status = ioctl(fd, SOUND_MIXER_READ_STEREODEVS, &stereodevs);

?if (status == -1)

??? perror("SOUND_MIXER_READ_STEREODEVS ioctl failed");

?

?/* 檢查用戶輸入 */

?if (argc != 3 && argc != 4)

??? usage();

?

?/* 保存用戶輸入的混音器名稱 */

?dev = argv[1];

?

?/* 確定即將用到的混音設(shè)備 */

?for (i = 0 ; i < SOUND_MIXER_NRDEVICES ; i++)

??? if (((1 << i) & devmask) && !strcmp(dev, sound_device_names[i]))

????? break;

?if (i == SOUND_MIXER_NRDEVICES) { /* 沒有找到匹配項 */

??? fprintf(stderr, "%s is not a valid mixer devicen", dev);

??? usage();

?}

?

?/* 查找到有效的混音設(shè)備 */

?device = i;

?

?/* 獲取增益值 */

?if (argc == 4) {

??? /* 左、右聲道均給定 */

??? left?= atoi(argv[2]);

??? right = atoi(argv[3]);

?} else {

??? /* 左、右聲道設(shè)為相等 */

??? left?= atoi(argv[2]);

??? right = atoi(argv[2]);

?}

?

??/* 對非立體聲設(shè)備給出警告信息 */

?if ((left != right) && !((1 << i) & stereodevs)) {

??? fprintf(stderr, "warning: %s is not a stereo devicen", dev);

?}

?

??/* 將兩個聲道的值合到同一變量中 */

?level = (right << 8) + left;

?

??/* 設(shè)置增益 */

?status = ioctl(fd, MIXER_WRITE(device), &level);

?if (status == -1) {

??? perror("MIXER_WRITE ioctl failed");

??? exit(1);

?}

?

?/* 獲得從驅(qū)動返回的左右聲道的增益 */

?left?= level & 0xff;

?right = (level & 0xff00) >> 8;

?

?/* 顯示實際設(shè)置的增益 */

?fprintf(stderr, "%s gain set to %d%% / %d%%n", dev, left, right);

?

?/* 關(guān)閉混音設(shè)備 */

?close(fd);

?return 0;

}

? 編譯好上面的程序之后，先不帶任何參數(shù)執(zhí)行一遍，此時會列出聲卡上所有可用的混音通道：

?

[xiaowp@linuxgam sound]$ ./mixer

usage: ./mixer   

?????? ./mixer  

Where  is one of:

vol pcm speaker line mic cd igain line1 phin video

? 之后就可以很方便地設(shè)置各個混音通道的增益大小了，例如下面的命令就能夠?qū)D輸入的左、右聲道的增益分別設(shè)置為80%和90%：

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[xiaowp@linuxgam sound]$ ./mixer cd 80 90

cd gain set to 80% / 90%