软件混音的实现

原

软件混音的实现

2005年11月07日 17:36:00

阅读数：8508

  Symbian 6.1上实现的混音是个比较麻烦的问题，因为程序只能同时播放一个音乐，实现混音就需要程序自己来实现。下面是我从newlc上找到的一个关于PCM脉冲编码的音频信 的混音实现，其中包含了一个关键的混音算法！

Hi !!!!

I am not sure weather I have fully understood your question or not, I persume that you are asking
”How can we mix two or more audio stream”, If this is the question then I am explaning below the
mixing of the two audio stream (You Can Mix More Audio Stream),

Step 1,

Get the Raw data of the two files, (Example, of the sample 8bit and 8Kh, means one sample is of
8bit)

Step 2

Let the two audio signal be A and B respectively, the range is between 0 and 255.  Where A and B are the
Sample Values (Each raw data) And store the resultant into the Y

If Both the samples Values are possitve

Y = A  +  B – A * B / 255

Where Y is the resultant signal which contains both signal A and B, merging two audio streams into single
stream by this method solves the  problem of overflow and information loss to an extent.

   If the range of 8-bit sampling is between -127 to 128

   If both A and B are negative       Y = A +B – (A * B / (-127))
   Else                                       Y = A + B – A * B / 128

Similarly for the nbit (ex 16bit data)

   For  n-bit sampling audio signal

   If both A and B are negative       Y = A + B – (A * B  /  (-(2 pow(n-1) -1)))
   Else                                       Y = A + B – (A * B /  (2 pow(n-1))

Step 3.

Add the Header to the Resultant (mixed) data and play back.

If some thing is unclear and ambigious let me know.

Regards
Ranjeet Gupta.

还有简单C程序示意代码，但是其中包含了核心算法：

#include
#include
#include
#include

int main(int argc,char argv[]) {
  char mixname[255];
  FILE *pcm1, *pcm2, *mix;
  char sample1, sample2;
  int value;

  pcm1 = fopen(argv[1],”r”);
  pcm2 = fopen(argv[2],”r”);
 
  strcpy (mixname, argv[1]);
  strcat (mixname, “_temp.wav”);
  mix = fopen(mixname, “w”);

  while(!feof(pcm1)) {

    sample1 = fgetc(pcm1);
    sample2 = fgetc(pcm2);
   
    if ((sample1
      value = sample1 + sample2 – (sample1 sample2 / -(pow(2,16-1)-1));
    }else{
      value = sample1 + sample2 – (sample1 * sample2 / (pow(2,16-1)-1));
    }

    fputc(value, mix);
  }

  fclose(pcm1);
  fclose(pcm2);
  fclose(mix);

  return 0;
}

另外，如果只是在Symbian上实现混音，在Nokia 的官方 站上，也有一个SoundMixer的examples
http://www.forum.nokia.com/info/sw.nokia.com/id/70a2bde5-9b14-41b3-89ae-198b0d8d380d/SoundMixer_Example_v1_0.zip.html