An Audio Synthesis Textbook For Musicians, Digital Artists and Programmers by Mike Krzyzaniak
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  1. //QuadrophonicPanning.c
  2. //gcc MKAiff.c QuadrophonicPanning.c -o QuadrophonicPanning
  5. #include "MKAiff.h"
  6. #include <math.h>
  8. #define SAMPLE_RATE 44100
  9. #define NUM_CHANNELS 4
  10. #define BITS_PER_SAMPLE 16
  11. #define NUM_SECONDS 3
  12. const int numSamples = NUM_SECONDS * NUM_CHANNELS * SAMPLE_RATE;
  13. #define PI 3.141592653589793
  14. const double TWO_PI_OVER_SAMPLE_RATE = 2*PI/SAMPLE_RATE;
  16. int main()
  17. {
  18. MKAiff* aiff = aiffWithDurationInSeconds(NUM_CHANNELS, SAMPLE_RATE, BITS_PER_SAMPLE, NUM_SECONDS);
  19. if(aiff == NULL) return 0;
  21. float audioBuffer[numSamples];
  22. double frequency = 440, angle = 0;
  23. double volumeFrequency = 1, volumeAngle = 0;
  24. float nextSample;
  26. int i;
  27. for(i=0; i<numSamples; i+=NUM_CHANNELS)
  28. {
  29. nextSample = sin(angle);
  30. audioBuffer[i] = nextSample * ((sin(volumeAngle) + 1) / 2);
  31. audioBuffer[i+1] = nextSample * ((cos(volumeAngle) + 1) / 2);
  32. audioBuffer[i+2] = nextSample * ((sin(volumeAngle+PI) + 1) / 2);
  33. audioBuffer[i+3] = nextSample * ((cos(volumeAngle+PI) + 1) / 2);
  35. angle += frequency * TWO_PI_OVER_SAMPLE_RATE;
  36. volumeAngle += volumeFrequency * TWO_PI_OVER_SAMPLE_RATE;
  37. }
  39. aiffAppendFloatingPointSamples(aiff, audioBuffer, numSamples, aiffFloatSampleType);
  40. aiffSaveWithFilename(aiff, "QuadrophonicPanning.aif");
  42. aiffDestroy(aiff);
  44. return 0;
  45. }

Output:

Downloads

Download Code Download Audio Download MKAiff

Builds On

AIFF Template Stereo Panning

Explanation of the Concepts

In the previous chapter we saw how to move a sound back and forth in a stereo field. Here, we will move a sound in an orbit around a quadrophonic field. The process is the same, except that the volume of each channel will be only 90 degrees (PI/2 radians) behind the next. The mathematical function cosine is the same as sine, except that it is advanced 90 degrees, so we can save a couple of additions by using it. Note that the above audio will not play properly unless you happen to have 4 speakers connected to your laptop!

Explanation of the Code

Line 9 sets the number of audio channels to 4. From there, this is identical to the previous chapter on stereo panning, except that here we have 2 extra intermediate channels whose volume will be 90 degrees (PI/2) out of phase with the other channels. This is accomplished using cosine. Notice that

cos(volumeAngle)

is identical to

sin(volumeAngle+PI/2)

and

cos(volumeAngle+PI)

is identical to

sin(volumeAngle+3*PI/2)

It is therefore better to use cosine, because it saves many multiplications and divisions.

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