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;
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;
if ( dac.openStream( &parameters, NULL, format, (unsigned int)Stk::sampleRate(), &bufferFrames, &tick, (void *)&data ) ) {
std::cout << dac.getErrorText() << std::endl;
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 );
if ( dac.startStream() ) {
std::cout << dac.getErrorText() << std::endl;
goto cleanup;
}
// Block waiting until callback signals done.
while ( !data.done )
Stk::sleep( 100 );
// Shut down the callback and output stream.
dac.closeStream();
cleanup:
delete data.instrument;
return 0;
}
RtAudio.h
RtAudioStreamStatus
unsigned int RtAudioStreamStatus
RtAudio stream status (over- or underflow) flags.
Definition RtAudio.h:178
RtAudioFormat
unsigned long RtAudioFormat
RtAudio data format type.
Definition RtAudio.h:105
RtAudio
Realtime audio i/o C++ classes.
Definition RtAudio.h:268
RtAudio::getDefaultOutputDevice
unsigned int getDefaultOutputDevice(void)
A function that returns the ID of the default output device.
Definition RtAudio.h:915
RtAudio::getErrorText
const std::string getErrorText(void)
Retrieve the error message corresponding to the last error or warning condition.
Definition RtAudio.h:920
RtAudio::closeStream
void closeStream(void)
A function that closes a stream and frees any associated stream memory.
Definition RtAudio.h:916
RtAudio::openStream
RtAudioErrorType openStream(RtAudio::StreamParameters *outputParameters, RtAudio::StreamParameters *inputParameters, RtAudioFormat format, unsigned int sampleRate, unsigned int *bufferFrames, RtAudioCallback callback, void *userData=NULL, RtAudio::StreamOptions *options=NULL)
A public function for opening a stream with the specified parameters.
RtAudio::startStream
RtAudioErrorType startStream(void)
A function that starts a stream.
Definition RtAudio.h:917
stk::BeeThree
STK Hammond-oid organ FM synthesis instrument.
Definition BeeThree.h:43
stk::Instrmnt
STK instrument abstract base class.
Definition Instrmnt.h:20
stk::StkError
STK error handling class.
Definition Stk.h:87
stk
The STK namespace.
Definition ADSR.h:6
RtAudio::StreamParameters
The structure for specifying input or output stream parameters.
Definition RtAudio.h:302
RtAudio::StreamParameters::nChannels
unsigned int nChannels
Definition RtAudio.h:305
RtAudio::StreamParameters::deviceId
unsigned int deviceId
Definition RtAudio.h:304

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--2023 Perry R. Cook and Gary P. Scavone. All Rights Reserved.