plugins/sid/extfilt.h

   1 //  ---------------------------------------------------------------------------
   2 //  This file is part of reSID, a MOS6581 SID emulator engine.
   3 //  Copyright (C) 2004  Dag Lem <resid@nimrod.no>
   4 //
   5 //  This program is free software; you can redistribute it and/or modify
   6 //  it under the terms of the GNU General Public License as published by
   7 //  the Free Software Foundation; either version 2 of the License, or
   8 //  (at your option) any later version.
   9 //
  10 //  This program is distributed in the hope that it will be useful,
  11 //  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 //  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13 //  GNU General Public License for more details.
  14 //
  15 //  You should have received a copy of the GNU General Public License
  16 //  along with this program; if not, write to the Free Software
  17 //  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  18 //  ---------------------------------------------------------------------------
  19
  20 #ifndef __EXTFILT_H__
  21 #define __EXTFILT_H__
  22
  23 #include "siddefs.h"
  24
  25 // ----------------------------------------------------------------------------
  26 // The audio output stage in a Commodore 64 consists of two STC networks,
  27 // a low-pass filter with 3-dB frequency 16kHz followed by a high-pass
  28 // filter with 3-dB frequency 16Hz (the latter provided an audio equipment
  29 // input impedance of 1kOhm).
  30 // The STC networks are connected with a BJT supposedly meant to act as
  31 // a unity gain buffer, which is not really how it works. A more elaborate
  32 // model would include the BJT, however DC circuit analysis yields BJT
  33 // base-emitter and emitter-base impedances sufficiently low to produce
  34 // additional low-pass and high-pass 3dB-frequencies in the order of hundreds
  35 // of kHz. This calls for a sampling frequency of several MHz, which is far
  36 // too high for practical use.
  37 // ----------------------------------------------------------------------------
  38 class ExternalFilter
  39 {
  40 public:
  41   ExternalFilter();
  42
  43   void enable_filter(bool enable);
  44   void set_chip_model(chip_model model);
  45
  46   RESID_INLINE void clock(sound_sample Vi);
  47   RESID_INLINE void clock(cycle_count delta_t, sound_sample Vi);
  48   void reset();
  49
  50   // Audio output (20 bits).
  51   RESID_INLINE sound_sample output();
  52
  53 protected:
  54   // Filter enabled.
  55   bool enabled;
  56
  57   // Maximum mixer DC offset.
  58   sound_sample mixer_DC;
  59
  60   // State of filters.
  61   sound_sample Vlp; // lowpass
  62   sound_sample Vhp; // highpass
  63   sound_sample Vo;
  64
  65   // Cutoff frequencies.
  66   sound_sample w0lp;
  67   sound_sample w0hp;
  68
  69 friend class cSID;
  70 };
  71
  72
  73 // ----------------------------------------------------------------------------
  74 // Inline functions.
  75 // The following functions are defined inline because they are called every
  76 // time a sample is calculated.
  77 // ----------------------------------------------------------------------------
  78
  79 #if RESID_INLINING || defined(__EXTFILT_CC__)
  80
  81 // ----------------------------------------------------------------------------
  82 // SID clocking - 1 cycle.
  83 // ----------------------------------------------------------------------------
  84 RESID_INLINE
  85 void ExternalFilter::clock(sound_sample Vi)
  86 {
  87   // This is handy for testing.
  88   if (!enabled) {
  89     // Remove maximum DC level since there is no filter to do it.
  90     Vlp = Vhp = 0;
  91     Vo = Vi - mixer_DC;
  92     return;
  93   }
  94
  95   // delta_t is converted to seconds given a 1MHz clock by dividing
  96   // with 1 000 000.
  97
  98   // Calculate filter outputs.
  99   // Vo  = Vlp - Vhp;
 100   // Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
 101   // Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
 102
 103   sound_sample dVlp = (w0lp >> 8)*(Vi - Vlp) >> 12;
 104   sound_sample dVhp = w0hp*(Vlp - Vhp) >> 20;
 105   Vo = Vlp - Vhp;
 106   Vlp += dVlp;
 107   Vhp += dVhp;
 108 }
 109
 110 // ----------------------------------------------------------------------------
 111 // SID clocking - delta_t cycles.
 112 // ----------------------------------------------------------------------------
 113 RESID_INLINE
 114 void ExternalFilter::clock(cycle_count delta_t,
 115                            sound_sample Vi)
 116 {
 117   // This is handy for testing.
 118   if (!enabled) {
 119     // Remove maximum DC level since there is no filter to do it.
 120     Vlp = Vhp = 0;
 121     Vo = Vi - mixer_DC;
 122     return;
 123   }
 124
 125   // Maximum delta cycles for the external filter to work satisfactorily
 126   // is approximately 8.
 127   cycle_count delta_t_flt = 8;
 128
 129   while (delta_t) {
 130     if (delta_t < delta_t_flt) {
 131       delta_t_flt = delta_t;
 132     }
 133
 134     // delta_t is converted to seconds given a 1MHz clock by dividing
 135     // with 1 000 000.
 136
 137     // Calculate filter outputs.
 138     // Vo  = Vlp - Vhp;
 139     // Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
 140     // Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
 141
 142     sound_sample dVlp = (w0lp*delta_t_flt >> 8)*(Vi - Vlp) >> 12;
 143     sound_sample dVhp = w0hp*delta_t_flt*(Vlp - Vhp) >> 20;
 144     Vo = Vlp - Vhp;
 145     Vlp += dVlp;
 146     Vhp += dVhp;
 147
 148     delta_t -= delta_t_flt;
 149   }
 150 }
 151
 152
 153 // ----------------------------------------------------------------------------
 154 // Audio output (19.5 bits).
 155 // ----------------------------------------------------------------------------
 156 RESID_INLINE
 157 sound_sample ExternalFilter::output()
 158 {
 159   return Vo;
 160 }
 161
 162 #endif // RESID_INLINING || defined(__EXTFILT_CC__)
 163
 164 #endif // not __EXTFILT_H__