Version 4.2.1

src/Blit.cpp

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+/***************************************************/
+/*! \class Blit
+    \brief STK band-limited impulse train class.
+
+    This class generates a band-limited impulse train using a
+    closed-form algorithm reported by Stilson and Smith in "Alias-Free
+    Digital Synthesis of Classic Analog Waveforms", 1996.  The user
+    can specify both the fundamental frequency of the impulse train
+    and the number of harmonics contained in the resulting signal.
+
+    The signal is normalized so that the peak value is +/-1.0.
+
+    If nHarmonics is 0, then the signal will contain all harmonics up
+    to half the sample rate.  Note, however, that this setting may
+    produce aliasing in the signal when the frequency is changing (no
+    automatic modification of the number of harmonics is performed by
+    the setFrequency() function).
+
+    Original code by Robin Davies, 2005.
+    Revisions by Gary Scavone for STK, 2005.
+*/
+/***************************************************/
+
+#include "Blit.h"
+#include <cmath>
+#include <limits>
+ 
+Blit:: Blit( StkFloat frequency )
+{
+  nHarmonics_ = 0;
+  this->setFrequency( frequency );
+  this->reset();
+}
+
+Blit :: ~Blit()
+{
+}
+
+void Blit :: reset()
+{
+  phase_ = 0.0;
+  lastOutput_ = 0;
+}
+
+void Blit :: setFrequency( StkFloat frequency )
+{
+#if defined(_STK_DEBUG_)
+  errorString_ << "Blit::setFrequency: frequency = " << frequency << '.';
+  handleError( StkError::DEBUG_WARNING );
+#endif
+
+  p_ = Stk::sampleRate() / frequency;
+  rate_ = PI / p_;
+  this->updateHarmonics();
+}
+
+void Blit :: setHarmonics( unsigned int nHarmonics )
+{
+  nHarmonics_ = nHarmonics;
+  this->updateHarmonics();
+}
+
+void Blit :: updateHarmonics( void )
+{
+  if ( nHarmonics_ <= 0 ) {
+    unsigned int maxHarmonics = (unsigned int) floor( 0.5 * p_ );
+    m_ = 2 * maxHarmonics + 1;
+  }
+  else
+    m_ = 2 * nHarmonics_ + 1;
+
+#if defined(_STK_DEBUG_)
+  errorString_ << "Blit::updateHarmonics: nHarmonics_ = " << nHarmonics_ << ", m_ = " << m_ << '.';
+  handleError( StkError::DEBUG_WARNING );
+#endif
+}
+
+StkFloat Blit :: computeSample( void )
+{
+  // The code below implements the SincM algorithm of Stilson and
+  // Smith with an additional scale factor of P / M applied to
+  // normalize the output.
+
+  // A fully optimized version of this code would replace the two sin
+  // calls with a pair of fast sin oscillators, for which stable fast
+  // two-multiply algorithms are well known. In the spirit of STK,
+  // which favors clarity over performance, the optimization has not
+  // been made here.
+
+  // Avoid a divide by zero at the sinc peak, which has a limiting
+  // value of 1.0.
+  StkFloat denominator = sin( phase_ );
+  if ( denominator <= std::numeric_limits<StkFloat>::epsilon() ) {
+    lastOutput_ = 1.0;
+  } else {
+    lastOutput_ =  sin( m_ * phase_ );
+    lastOutput_ /= m_ * denominator;
+  }
+
+  phase_ += rate_;
+  if ( phase_ >= PI ) phase_ -= PI;
+    
+	return lastOutput_;
+}
+