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filter
function in Matlab. In this example, we create a Filter instance and initialize it with specific numerator and denominator coefficients. We then compute its impulse response for 20 samples.
#include "Filter.h" int main() { StkFrames output( 20, 1 ); // initialize StkFrames to 20 frames and 1 channel (default: interleaved) output[0] = 1.0; std::vector<StkFloat> numerator( 5, 0.1 ); // create and initialize numerator coefficients std::vector<StkFloat> denominator; // create empty denominator coefficients denominator.push_back( 1.0 ); // populate our denomintor values denominator.push_back( 0.3 ); denominator.push_back( -0.5 ); Filter filter( numerator, denominator ); filter.tick( output ); for ( unsigned int i=0; i<output.size(); i++ ) { std::cout << "i = " << i << " : output = " << output[i] << std::endl; } return 0; }
The Filter class implements the standard difference equation
a[0]*y[n] = b[0]*x[n] + ... + b[nb]*x[n-nb] - a[1]*y[n-1] - ... - a[na]*y[n-na],
where "b" values are numerator coefficients and "a" values are denominator coefficients. Note that if the first denominator coefficient is not 1.0, the Filter class automatically normalizes all filter coefficients by that value. The coefficient values are passed to the Filter class via a C++ vector, a container object provided by the C++ Standard Library.
Most STK classes use more specific types of digital filters, such as the OneZero, OnePole, TwoPole, or BiQuad varieties. These classes inherit from the Filter class and provide specific functionality particular to their use, as well as functions to independently control individual coefficient values.
#include "BiQuad.h" #include "Noise.h" int main() { StkFrames output( 20, 1 ); // initialize StkFrames to 20 frames and 1 channel (default: interleaved) Noise noise; BiQuad biquad; biquad.setResonance( 440.0, 0.98, true ); // automatically normalize for unity peak gain for ( unsigned int i=0; i<output.size(); i++ ) { output[i] = biquad.tick( noise.tick() ); // single-sample computations std::cout << "i = " << i << " : output = " << output[i] << std::endl; } return 0; }
By passing a boolian value of true
as the third argument to the BiQuad::setResonance() function, the filter coefficients are automatically scaled to achieve unity gain at the resonance peak frequency. The previous code could be easily modified for "vector-based" calculations:
#include "BiQuad.h" #include "Noise.h" int main() { StkFrames output( 20, 1 ); // initialize StkFrames to 20 frames and 1 channel (default: interleaved) Noise noise; BiQuad biquad; biquad.setResonance( 440.0, 0.98, true ); // automatically normalize for unity peak gain biquad.tick( noise.tick( output ) ); // vector-based computations for ( unsigned int i=0; i<output.size(); i++ ) { std::cout << "i = " << i << " : output = " << output[i] << std::endl; } return 0; }
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