The Synthesis ToolKit in C++ (STK)

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Instruments

The ToolKit comes with a wide variety of synthesis algorithms, all of which inherit from the stk::Instrmnt class. In this example, we'll fire up an instance of the stk::BeeThree FM synthesis class and show how its frequency can be modified over time.

// bethree.cpp STK tutorial program
#include "BeeThree.h"
#include "RtAudio.h"
using namespace stk;
// The TickData structure holds all the class instances and data that
// are shared by the various processing functions.
struct TickData {
Instrmnt *instrument;
StkFloat frequency;
StkFloat scaler;
long counter;
bool done;
// Default constructor.
TickData()
: instrument(0), scaler(1.0), counter(0), done( false ) {}
};
// This tick() function handles sample computation only. It will be
// called automatically when the system needs a new buffer of audio
// samples.
int tick( void *outputBuffer, void *inputBuffer, unsigned int nBufferFrames,
double streamTime, RtAudioStreamStatus status, void *userData )
{
TickData *data = (TickData *) userData;
register StkFloat *samples = (StkFloat *) outputBuffer;
for ( unsigned int i=0; i<nBufferFrames; i++ ) {
*samples++ = data->instrument->tick();
if ( ++data->counter % 2000 == 0 ) {
data->scaler += 0.025;
data->instrument->setFrequency( data->frequency * data->scaler );
}
}
if ( data->counter > 80000 )
data->done = true;
return 0;
}
int main()
{
// Set the global sample rate and rawwave path before creating class instances.
Stk::setSampleRate( 44100.0 );
Stk::setRawwavePath( "../../rawwaves/" );
TickData data;
RtAudio dac;
// Figure out how many bytes in an StkFloat and setup the RtAudio stream.
RtAudio::StreamParameters parameters;
parameters.deviceId = dac.getDefaultOutputDevice();
parameters.nChannels = 1;
RtAudioFormat format = ( sizeof(StkFloat) == 8 ) ? RTAUDIO_FLOAT64 : RTAUDIO_FLOAT32;
unsigned int bufferFrames = RT_BUFFER_SIZE;
try {
dac.openStream( &parameters, NULL, format, (unsigned int)Stk::sampleRate(), &bufferFrames, &tick, (void *)&data );
}
catch ( RtAudioError& error ) {
error.printMessage();
goto cleanup;
}
try {
// Define and load the BeeThree instrument
data.instrument = new BeeThree();
}
catch ( StkError & ) {
goto cleanup;
}
data.frequency = 220.0;
data.instrument->noteOn( data.frequency, 0.5 );
try {
dac.startStream();
}
catch ( RtAudioError &error ) {
error.printMessage();
goto cleanup;
}
// Block waiting until callback signals done.
while ( !data.done )
Stk::sleep( 100 );
// Shut down the callback and output stream.
try {
dac.closeStream();
}
catch ( RtAudioError &error ) {
error.printMessage();
}
cleanup:
delete data.instrument;
return 0;
}

We have used an Instrmnt pointer when referencing the BeeThree instance above, so it would be simple to replace the BeeThree class with any other STK instrument class. It should be noted, however, that a few classes do not respond to the setFrequency() function (e.g., Shakers, Drummer).

The noteOn() function initiates an instrument attack. Instruments that are continuously excited (e.g., stk::Clarinet, stk::BeeThree) will continue to sound until stopped with a noteOff(). Impulsively excited instrument sounds (e.g., stk::Plucked, stk::Wurley) typically decay within a few seconds time, requiring subsequent noteOn() messages for re-attack.

Instrument parameters can be precisely controlled as demonstrated above. A more flexible approach to instrument control, allowing arbitrary scorefile or realtime updates, is described in the next tutorial chapter.

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The Synthesis ToolKit in C++ (STK)
©1995--2019 Perry R. Cook and Gary P. Scavone. All Rights Reserved.