Innovation SSI-2001 emulation. Using ReSID-FP.

src/resid-fp/extfilt.h

@@ -0,0 +1,120 @@
+//  ---------------------------------------------------------------------------
+//  This file is part of reSID, a MOS6581 SID emulator engine.
+//  Copyright (C) 2004  Dag Lem <resid@nimrod.no>
+//
+//  This program is free software; you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation; either version 2 of the License, or
+//  (at your option) any later version.
+//
+//  This program is distributed in the hope that it will be useful,
+//  but WITHOUT ANY WARRANTY; without even the implied warranty of
+//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//  GNU General Public License for more details.
+//
+//  You should have received a copy of the GNU General Public License
+//  along with this program; if not, write to the Free Software
+//  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+//  ---------------------------------------------------------------------------
+
+#ifndef __EXTFILT_H__
+#define __EXTFILT_H__
+
+#include <math.h>
+
+#include "siddefs-fp.h"
+
+// ----------------------------------------------------------------------------
+// The audio output stage in a Commodore 64 consists of two STC networks,
+// a low-pass filter with 3-dB frequency 16kHz followed by a high-pass
+// filter with 3-dB frequency 16Hz (the latter provided an audio equipment
+// input impedance of 1kOhm).
+// The STC networks are connected with a BJT supposedly meant to act as
+// a unity gain buffer, which is not really how it works. A more elaborate
+// model would include the BJT, however DC circuit analysis yields BJT
+// base-emitter and emitter-base impedances sufficiently low to produce
+// additional low-pass and high-pass 3dB-frequencies in the order of hundreds
+// of kHz. This calls for a sampling frequency of several MHz, which is far
+// too high for practical use.
+// ----------------------------------------------------------------------------
+class ExternalFilterFP
+{
+public:
+  ExternalFilterFP();
+
+  void enable_filter(bool enable);
+  void set_sampling_parameter(float pass_freq);
+  void set_chip_model(chip_model model);
+  void set_clock_frequency(float);
+
+  RESID_INLINE void clock(float Vi);
+  void reset();
+
+  // Audio output (20 bits).
+  RESID_INLINE float output();
+
+private:
+  void _set_sampling_parameter();
+  void nuke_denormals();
+
+  // Filter enabled.
+  bool enabled;
+
+  // Maximum mixer DC offset.
+  float mixer_DC;
+
+  // Relevant clocks
+  float clock_frequency, pass_frequency;
+
+  // State of filters.
+  float Vlp; // lowpass
+  float Vhp; // highpass
+  float Vo;
+
+  // Cutoff frequencies.
+  float w0lp;
+  float w0hp;
+
+friend class SIDFP;
+};
+
+// ----------------------------------------------------------------------------
+// SID clocking - 1 cycle.
+// ----------------------------------------------------------------------------
+RESID_INLINE
+void ExternalFilterFP::clock(float Vi)
+{
+  // This is handy for testing.
+  if (! enabled) {
+    // Remove maximum DC level since there is no filter to do it.
+    Vlp = Vhp = 0.f;
+    Vo = Vi - mixer_DC;
+    return;
+  }
+
+  float dVlp = w0lp * (Vi - Vlp);
+  float dVhp = w0hp * (Vlp - Vhp);
+  Vo = Vlp - Vhp;
+  Vlp += dVlp;
+  Vhp += dVhp;
+}
+
+// ----------------------------------------------------------------------------
+// Audio output (19.5 bits).
+// ----------------------------------------------------------------------------
+RESID_INLINE
+float ExternalFilterFP::output()
+{
+  return Vo;
+}
+
+RESID_INLINE
+void ExternalFilterFP::nuke_denormals()
+{
+    if (Vhp > -1e-12f && Vhp < 1e-12f)
+        Vhp = 0;
+    if (Vlp > -1e-12f && Vlp < 1e-12f)
+        Vlp = 0;
+}
+
+#endif // not __EXTFILT_H__