AM Examples

• The Matlab script modulate.m demonstrates both AM and ring modulation. The resulting plot for AM is shown below:

  Figure 5: A time- and frequency-domain plot of the AM example from the modulate.m script.

  

• The Max/MSP AM patch modulate.maxpat, shown below, also demonstrates both AM and ring modulation.
  

• The following Synthesis ToolKit (STK) program implements ring modulation with sinusoids:

  // am.cpp STK example ring modulation program using blocking audio output.
  //
  // compile with: g++ -std=c++11 -Istk/include/ -Lstk/src/ -D__MACOSX_CORE__ am.cpp -lstk -lpthread -framework CoreAudio -framework CoreMIDI -framework CoreFoundation
  
  
  #include "SineWave.h"
  #include "Envelope.h"
  #include "RtWvOut.h"
  
  using namespace stk;
  
  int main( int argc, char *argv[] )
  {
    if ( argc != 2 ) {
      std::cout << "usage: " << argv[0] << " duration" << std::endl;
      std::cout << "    where 'duration' is in seconds.\n" << std::endl;
      exit(0);
    }
  
    int nSeconds = atof( argv[1] ); // convert ascii to float
    if ( nSeconds <= 0.0 ) {
      std::cout << "Duration must be greater than zero!\n" << std::endl;
      exit(0);
    }
    
    // Set the global sample rate before creating class instances.
    Stk::setSampleRate( 44100.0 );
    Stk::showWarnings( true );
  
    int nFrames = (int) nSeconds * Stk::sampleRate();
    StkFloat temp;
  
    try {
  
      SineWave carrier;
      SineWave modulator;
      Envelope env;
  
      // Define and open the default realtime output device for one-channel playback
      RtWvOut dac( 1 );
  
      carrier.setFrequency( 440.0 );
  
      // Use envelope to sweep the modulator frequency from 4 to 100 Hz
      // over duration of the program.
      env.keyOn(); // sets target to 1.0
      env.setTime( nFrames / Stk::sampleRate() );
  
      // Single-sample computations
      for ( int i=0; i<nFrames; i++ ) {
        modulator.setFrequency( 4.0 + 96.0*env.tick() ); // 4 to 100 Hz
        temp = 0.4 * (1.0 + modulator.tick()) * carrier.tick();
        dac.tick( temp );
      }
    }
    catch ( StkError & ) {
      exit( 1 );
    }
  
    return 0;
  }