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// ---------------------------------------------------------------------------
// 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 "siddefs.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 ExternalFilter
{
public:
ExternalFilter();
void enable_filter(bool enable);
void set_chip_model(chip_model model);
RESID_INLINE void clock(sound_sample Vi);
RESID_INLINE void clock(cycle_count delta_t, sound_sample Vi);
void reset();
// Audio output (20 bits).
RESID_INLINE sound_sample output();
protected:
// Filter enabled.
bool enabled;
// Maximum mixer DC offset.
sound_sample mixer_DC;
// State of filters.
sound_sample Vlp; // lowpass
sound_sample Vhp; // highpass
sound_sample Vo;
// Cutoff frequencies.
sound_sample w0lp;
sound_sample w0hp;
friend class cSID;
};
// ----------------------------------------------------------------------------
// Inline functions.
// The following functions are defined inline because they are called every
// time a sample is calculated.
// ----------------------------------------------------------------------------
#if RESID_INLINING || defined(__EXTFILT_CC__)
// ----------------------------------------------------------------------------
// SID clocking - 1 cycle.
// ----------------------------------------------------------------------------
RESID_INLINE
void ExternalFilter::clock(sound_sample Vi)
{
// This is handy for testing.
if (!enabled) {
// Remove maximum DC level since there is no filter to do it.
Vlp = Vhp = 0;
Vo = Vi - mixer_DC;
return;
}
// delta_t is converted to seconds given a 1MHz clock by dividing
// with 1 000 000.
// Calculate filter outputs.
// Vo = Vlp - Vhp;
// Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
// Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
sound_sample dVlp = (w0lp >> 8)*(Vi - Vlp) >> 12;
sound_sample dVhp = w0hp*(Vlp - Vhp) >> 20;
Vo = Vlp - Vhp;
Vlp += dVlp;
Vhp += dVhp;
}
// ----------------------------------------------------------------------------
// SID clocking - delta_t cycles.
// ----------------------------------------------------------------------------
RESID_INLINE
void ExternalFilter::clock(cycle_count delta_t,
sound_sample Vi)
{
// This is handy for testing.
if (!enabled) {
// Remove maximum DC level since there is no filter to do it.
Vlp = Vhp = 0;
Vo = Vi - mixer_DC;
return;
}
// Maximum delta cycles for the external filter to work satisfactorily
// is approximately 8.
cycle_count delta_t_flt = 8;
while (delta_t) {
if (delta_t < delta_t_flt) {
delta_t_flt = delta_t;
}
// delta_t is converted to seconds given a 1MHz clock by dividing
// with 1 000 000.
// Calculate filter outputs.
// Vo = Vlp - Vhp;
// Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
// Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
sound_sample dVlp = (w0lp*delta_t_flt >> 8)*(Vi - Vlp) >> 12;
sound_sample dVhp = w0hp*delta_t_flt*(Vlp - Vhp) >> 20;
Vo = Vlp - Vhp;
Vlp += dVlp;
Vhp += dVhp;
delta_t -= delta_t_flt;
}
}
// ----------------------------------------------------------------------------
// Audio output (19.5 bits).
// ----------------------------------------------------------------------------
RESID_INLINE
sound_sample ExternalFilter::output()
{
return Vo;
}
#endif // RESID_INLINING || defined(__EXTFILT_CC__)
#endif // not __EXTFILT_H__
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