Multi-Channel I/O

The ToolKit stk::WvIn and stk::WvOut classes (and their subclasses) support multi-channel audio data input and output. Several other abstract base classes, such as stk::Instrmnt, stk::Generator, and stk::Effect, also support multi-channel computations though not all of their subclasses produce or take multi-channel data. A set of interleaved audio samples representing a single time "slice" is referred to as a sample frame. At a sample rate of 44.1 kHz, a four-channel audio stream will have 44100 sample frames per second and a total of 176400 individual samples per second.

Most STK classes process single-sample data streams via their tick() function. For classes supporting multi-channel data, one must distinguish the tick() functions taking or producing single StkFloat arguments from those taking stk::StkFrames& arguments. If a single-sample version of the tick() function is called for these classes, a full sample frame is computed but only a single value is either input and/or output. For example, if the single-sample tick() function is called for subclasses of WvOut, the sample argument is written to all channels in the one computed frame. For classes returning values, an optional channel argument specifies which channel value is returned from the computed frame (the default is always channel 0). To input and/or output multichannel data to these classes, the overloaded tick() functions taking StkFrames reference arguments should be used.

Multi-channel support for realtime audio input and output is dependent on the audio device(s) available on your system.

The following example demonstrates the use of the stk::FileWvOut class for creating a four channel, 16-bit AIFF formatted audio file. We will use four sinewaves of different frequencies for the first two seconds and then a single sinewave for the last two seconds.

// foursine.cpp STK tutorial program
 
#include "SineWave.h"
#include "FileWvOut.h"
#include <cstdlib>
 
using namespace stk;
 
int main()
{
  // Set the global sample rate before creating class instances.
  Stk::setSampleRate( 44100.0 );
 
  int i;
  FileWvOut output;
  SineWave inputs[4];
 
  // Set the sine wave frequencies.
  for ( i=0; i<4; i++ )
    inputs[i].setFrequency( 220.0 * (i+1) );
 
  // Define and open a 16-bit, four-channel AIFF formatted output file
  try {
    output.openFile( "foursine.aif", 4, FileWrite::FILE_AIF, Stk::STK_SINT16 );
  }
  catch (StkError &) {
    exit( 1 );
  }
 
  // Write two seconds of four sines to the output file
  StkFrames frames( 88200, 4 );
  for ( i=0; i<4; i++ )
    inputs[i].tick( frames, i );
 
  output.tick( frames );
 
  // Now write the first sine to all four channels for two seconds
  for ( i=0; i<88200; i++ ) {
    output.tick( inputs[0].tick() );
  }
 
  return 0;
}

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