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Large Pack of OldSkool DOS MOD Trackers
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goattracker_2.68.zip
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resid-fp
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extfilt.h
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C/C++ Source or Header
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2009-01-03
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121 lines
// ---------------------------------------------------------------------------
// 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_FP_H__
#define __EXTFILT_FP_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_FP_H__