Merge pull request #121 from navin-neu/biquad-filter-types

add support for different filter types in biquad

include/BiQuad.h

@@ -13,10 +13,15 @@ namespace stk {
     Methods are provided for creating a resonance or notch in the
     frequency response while maintaining a constant filter gain.
 
+    Formulae used calculate coefficients for lowpass, highpass,
+    bandpass, bandreject and allpass are found on pg. 55 of
+    Udo Zölzer's "DAFX - Digital Audio Effects" (2011 2nd ed).   
+
     by Perry R. Cook and Gary P. Scavone, 1995--2021.
 */
 /***************************************************/
 
+const StkFloat RECIP_SQRT_2 = static_cast<StkFloat>( M_SQRT1_2 );  
 class BiQuad : public Filter
 {
 public:
@@ -74,12 +79,72 @@ public:
   */
   void setNotch( StkFloat frequency, StkFloat radius );
 
+  //! Set the filter coefficients for a low-pass with cutoff frequency \e fc (in Hz) and Q-factor \e Q.
+  /*!
+    This method determines the filter coefficients corresponding to a 
+    low-pass filter with cutoff placed at \e fc, where sloping behaviour 
+    and resonance are determined by \e Q. The default value for \e Q is 
+    1/sqrt(2), resulting in a gradual attenuation of frequencies higher than
+    \e fc without added resonance. Values greater than this will more 
+    aggressively attenuate frequencies above \e fc while also adding a 
+    resonance at \e fc. Values less than this will result in a more gradual
+    attenuation of frequencies above \e fc, but will also attenuate 
+    frequencies below \e fc as well. Both \e fc and \e Q must be positive.
+  */
+  void setLowPass( StkFloat fc, StkFloat Q=RECIP_SQRT_2 );
+
+  //! Set the filter coefficients for a high-pass with cutoff frequency \e fc (in Hz) and Q-factor \e Q.
+  /*!
+    This method determines the filter coefficients corresponding to a high-pass 
+    filter with cutoff placed at \e fc, where sloping behaviour and resonance 
+    are determined by \e Q. The default value for \e Q is 1/sqrt(2), resulting 
+    in a gradual attenuation of frequencies lower than \e fc without added 
+    resonance. Values greater than this will more aggressively attenuate 
+    frequencies below \e fc while also adding a resonance at \e fc. Values less 
+    than this will result in a more gradual attenuation of frequencies below 
+    \e fc, but will also attenuate frequencies above \e fc as well. 
+    Both \e fc and \e Q must be positive.
+  */
+  void setHighPass( StkFloat fc, StkFloat Q=RECIP_SQRT_2 );
+
+  //! Set the filter coefficients for a band-pass centered at \e fc (in Hz) with Q-factor \e Q.
+  /*!
+    This method determines the filter coefficients corresponding to a band-pass
+    filter with pass-band centered at \e fc, where band width and slope a 
+    determined by \e Q. Values for \e Q that are less than 1.0 will attenuate
+    frequencies above and below \e fc more gradually, resulting in a convex 
+    slope and a wider band. Values for \e Q greater than 1.0 will attenuate 
+    frequencies above and below \e fc more aggressively, resulting in a 
+    concave slope and a narrower band. Both \e fc and \e Q must be positive.
+  */
+  void setBandPass( StkFloat fc, StkFloat Q );
+
+  //! Set the filter coefficients for a band-reject centered at \e fc (in Hz) with Q-factor \e Q.
+  /*!
+    This method determines the filter coefficients corresponding to a 
+    band-reject filter with stop-band centered at \e fc, where band width 
+    and slope are determined by \e Q. Values for \e Q that are less than 1.0 
+    will yield a wider band with greater attenuation of \e fc. Values for \e Q
+    greater than 1.0 will yield a narrower band with less attenuation of \e fc. 
+    Both \e fc and \e Q must be positive.
+  */
+  void setBandReject( StkFloat fc, StkFloat Q );
+
+  //! Set the filter coefficients for an all-pass centered at \e fc (in Hz) with Q-factor \e Q.
+  /*!
+    This method determines the filter coefficients corresponding to
+    an all-pass filter whose phase response crosses -pi radians at \e fc.
+    High values for \e Q will result in a more instantaenous shift in phase 
+    response at \e fc. Lower values will result in a more gradual shift in
+    phase response around \e fc. Both \e fc and \e Q must be positive.
+  */
+  void setAllPass( StkFloat fc, StkFloat Q );
+
   //! Sets the filter zeroes for equal resonance gain.
   /*!
     When using the filter as a resonator, zeroes places at z = 1, z
     = -1 will result in a constant gain at resonance of 1 / (1 - R),
     where R is the pole radius setting.
-
   */
   void setEqualGainZeroes( void );
 
@@ -114,6 +179,14 @@ public:
  protected:
 
   virtual void sampleRateChanged( StkFloat newRate, StkFloat oldRate );
+  
+  // Helper function to update the three intermediate values for the predefined filter types
+  // along with the feedback filter coefficients. Performs the debug check for fc and Q-factor arguments.
+  void setCommonFilterValues( StkFloat fc, StkFloat Q );
+
+  StkFloat K_;
+  StkFloat kSqr_;
+  StkFloat denom_;
 };
 
 inline StkFloat BiQuad :: tick( StkFloat input )

src/BiQuad.cpp

@@ -24,6 +24,10 @@ BiQuad :: BiQuad() : Filter()
   inputs_.resize( 3, 1, 0.0 );
   outputs_.resize( 3, 1, 0.0 );
 
+  K_ = 0.0;
+  kSqr_ = 0.0;
+  denom_ = 1.0;
+
   Stk::addSampleRateAlert( this );
 }
 
@@ -73,6 +77,11 @@ void BiQuad :: setResonance( StkFloat frequency, StkFloat radius, bool normalize
     b_[1] = 0.0;
     b_[2] = -b_[0];
   }
+  else {
+    b_[0] = 1.0;
+    b_[1] = 0.0;
+    b_[2] = 0.0;
+  }
 }
 
 void BiQuad :: setNotch( StkFloat frequency, StkFloat radius )
@@ -89,8 +98,57 @@ void BiQuad :: setNotch( StkFloat frequency, StkFloat radius )
 #endif
 
   // This method does not attempt to normalize the filter gain.
-  b_[2] = radius * radius;
+  b_[0] = 1.0; 
   b_[1] = (StkFloat) -2.0 * radius * cos( TWO_PI * (double) frequency / Stk::sampleRate() );
+  b_[2] = radius * radius;
+  
+  a_[1] = 0.0; 
+  a_[2] = 0.0;
+}
+
+void BiQuad :: setLowPass( StkFloat fc, StkFloat Q )
+{
+  setCommonFilterValues(fc, Q);
+
+  b_[0] = kSqr_ * Q * denom_;
+  b_[1] = 2 * b_[0];
+  b_[2] = b_[0];
+}
+
+void BiQuad :: setHighPass( StkFloat fc, StkFloat Q )
+{
+  setCommonFilterValues(fc, Q);
+
+  b_[0] = Q * denom_;
+  b_[1] = -2 * b_[0];
+  b_[2] = b_[0];
+}
+
+void BiQuad :: setBandPass( StkFloat fc, StkFloat Q )
+{
+  setCommonFilterValues(fc, Q);
+
+  b_[0] = K_ * denom_;
+  b_[1] = 0.0;
+  b_[2] = -b_[0];
+}
+
+void BiQuad :: setBandReject( StkFloat fc, StkFloat Q )
+{
+  setCommonFilterValues(fc, Q);
+
+  b_[0] = Q * (kSqr_ + 1) * denom_;
+  b_[1] = 2 * Q * (kSqr_ - 1) * denom_;
+  b_[2] = b_[0];
+}
+
+void BiQuad :: setAllPass( StkFloat fc, StkFloat Q )
+{
+  setCommonFilterValues(fc, Q);
+
+  b_[0] = a_[2];
+  b_[1] = a_[1];
+  b_[2] = 1;
 }
 
 void BiQuad :: setEqualGainZeroes( void )
@@ -100,4 +158,25 @@ void BiQuad :: setEqualGainZeroes( void )
   b_[2] = -1.0;
 }
 
+void BiQuad :: setCommonFilterValues( StkFloat fc, StkFloat Q)
+{
+#if defined(_STK_DEBUG_)
+  if ( fc < 0.0 ) {
+    oStream_ << "BiQuad::updateKValues: fc argument (" << fc << ") is negative!";
+    handleError( StkError::WARNING ); return;
+  }
+   if ( Q < 0.0 ) {
+    oStream_ << "BiQuad::updateKValues: Q argument (" << Q << ") is negative!";
+    handleError( StkError::WARNING ); return;
+  }
+#endif
+
+  K_ = tan(PI * fc / Stk::sampleRate());
+  kSqr_ = K_ * K_;
+  denom_ = 1 / (kSqr_ * Q + K_ + Q);
+
+  a_[1] = 2 * Q * (kSqr_ - 1) * denom_;
+  a_[2] = (kSqr_ * Q - K_ + Q) * denom_;
+}
+
 } // stk namespace