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diode_ladder_filter.hpp

By: a guest on Apr 3rd, 2012  |  syntax: C++  |  size: 3.56 KB  |  hits: 919  |  expires: Never
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  1.  
  2. // This code is released under the MIT license (see below).
  3. //
  4. // The MIT License
  5. //
  6. // Copyright (c) 2012 Dominique Wurtz (www.blaukraut.info)
  7. //
  8. // Permission is hereby granted, free of charge, to any person obtaining a copy
  9. // of this software and associated documentation files (the "Software"), to deal
  10. // in the Software without restriction, including without limitation the rights
  11. // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  12. // copies of the Software, and to permit persons to whom the Software is
  13. // furnished to do so, subject to the following conditions:
  14. //
  15. // The above copyright notice and this permission notice shall be included in
  16. // all copies or substantial portions of the Software.
  17. //
  18. // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  19. // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  20. // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  21. // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  22. // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  23. // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  24. // THE SOFTWARE.
  25.  
  26. #ifndef __DIODE_LADDER_FILTER_HPP__
  27. #define __DIODE_LADDER_FILTER_HPP__
  28.  
  29. #include <cmath>
  30. #include <algorithm>
  31.  
  32. // Emulation of Diode ladder lowpass filter as found in Roland TB303 or EMS VCS3
  33. // Version 0.1 (04/03/2012)
  34.  
  35. class DiodeLadderFilter
  36. {
  37. public:
  38.  
  39.         DiodeLadderFilter()
  40.         {
  41.                 std::fill(z, z + 4, 0);
  42.                 set_q(0);
  43.         }
  44.  
  45.         void reset()
  46.         {
  47.                 if (k < 17) std::fill(z, z + 4, 0);
  48.         }
  49.  
  50.         // q: resonance in the range [0..1]
  51.         void set_q(const double q)
  52.         {
  53.                 assert(q >= 0 && q <= 1.);
  54.                 k = 20 * q;
  55.                 A = 1 + 0.5*k; // resonance gain compensation
  56.         }
  57.  
  58.         // Process one sample.
  59.         //
  60.         // x: input signal
  61.         // fc: normalized cutoff frequency in the range [0..1] => 0 HZ .. Nyquist
  62.         __forceinline double tick(const double x, const double fc)
  63.         {
  64.                 assert(fc > 0 && fc < 1);
  65.                 const double wc = PI_HALF * fc; // PI is Nyquist frequency
  66.                 // wc = 2 * tan(0.5*wc); // dewarping, not required with 2x oversampling
  67.                 const double wc2 = wc*wc;
  68.                 const double wc3 = wc2*wc;
  69.                 const double wc4 = wc3*wc;
  70.                 const double b = 1 / (1+8*wc+20*wc2+16*wc3+2*wc4);
  71.                 const double g = 2*wc4 * b;
  72.  
  73.                 // current state
  74.                 const double s = (z[0]*wc3 + z[1]*(wc2+2*wc3) + z[2]*(wc+4*wc2+2*wc3) + z[3]*(1+6*wc+9*wc2+2*wc3)) * b;
  75.                
  76.                 // solve feedback loop (linear)
  77.                 double y4 = (g*x + s) / (1 + g*k);
  78.  
  79.                 // input clipping
  80.                 const double y0 = fast_tanh(x - k*y4);
  81.  
  82.                 // Compute all integrator outputs (y1, y2, y3, y4).
  83.                 // Unlike in the well-known Moog transistor ladder, this gets quite nasty due the
  84.                 // inherent coupling between filter stages.
  85.                 const double y1 = (y0*(2*wc+12*wc2+20*wc3+8*wc4) + z[0]*(1+6*wc+10*wc2+4*wc3) +
  86.                         z[1]*(2*wc+8*wc2+6*wc3) + z[2]*(2*wc2+4*wc3) + z[3]*2*wc3)*b;
  87.                 const double y2 = (y0*(2*wc2+8*wc3+6*wc4) + z[0]*(wc+4*wc2+3*wc3) +
  88.                         z[1]*(1+6*wc+11*wc2+6*wc3) + z[2]*(wc+4*wc2+4*wc3) + z[3]*(wc2+2*wc3))*b;
  89.                 const double y3 = (y0*(2*wc3+4*wc4) + z[0]*(wc2+2*wc3) +
  90.                         z[1]*(wc+4*wc2+4*wc3) + z[2]*(1+6*wc+10*wc2+4*wc3) + z[3]*(wc+4*wc2+2*wc3))*b;
  91.                 y4 = g*y0 + s;
  92.  
  93.                 // update filter state
  94.                 z[0] += 4*wc*(y0 - y1 + y2);
  95.                 z[1] += 2*wc*(y1 - 2*y2 + y3);
  96.                 z[2] += 2*wc*(y2 - 2*y3 + y4);
  97.                 z[3] += 2*wc*(y3 - 2*y4);
  98.  
  99.                 return A*y4;
  100.         }
  101.        
  102. private:
  103.         double k, A;
  104.         double z[4];
  105.  
  106.         static __forceinline double fast_tanh(const double x)
  107.         {
  108.                 return x / (1 + abs(x));
  109.         }
  110. };
  111.  
  112. #endif // __DIODE_LADDER_FILTER_HPP__