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