extfilt.h
// ---------------------------------------------------------------------------
// This file is part of reSID, a MOS6581 SID emulator engine.
// Copyright (C) 2010 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 RESID_EXTFILT_H
#define RESID_EXTFILT_H
#include "resid-config.h"
namespace reSID
{
// ----------------------------------------------------------------------------
// 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.
//
// 9/12V
// -----+
// audio| 10k |
// +----+---R---+--------+-----(K) +-----
// out | | | | | |audio
// -----+ R 1k C 1000 | | 10 uF |
// | | pF +-C----+-----C-----+ 1K
// 470 | |
// GND GND pF R 1K | amp
// | +-----
//
// GND
//
// ----------------------------------------------------------------------------
class ExternalFilter
{
public:
ExternalFilter();
void enable_filter(bool enable);
void clock(short Vi);
void clock(cycle_count delta_t, short Vi);
void reset();
// Audio output (16 bits).
int output();
protected:
// Filter enabled.
bool enabled;
// State of filters (27 bits).
int Vlp; // lowpass
int Vhp; // highpass
// Cutoff frequencies.
int w0lp_1_s7;
int w0hp_1_s17;
friend class SID;
};
// ----------------------------------------------------------------------------
// Inline functions.
// The following functions are defined inline because they are called every
// time a sample is calculated.
// ----------------------------------------------------------------------------
#if RESID_INLINING || defined(RESID_EXTFILT_CC)
// ----------------------------------------------------------------------------
// SID clocking - 1 cycle.
// ----------------------------------------------------------------------------
RESID_INLINE
void ExternalFilter::clock(short Vi)
{
// This is handy for testing.
if (unlikely(!enabled)) {
// Vo = Vlp - Vhp;
Vlp = Vi << 11;
Vhp = 0;
return;
}
// Calculate filter outputs.
// Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
// Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
// Vo = Vlp - Vhp;
int dVlp = w0lp_1_s7*int((unsigned(Vi) << 11) - unsigned(Vlp)) >> 7;
int dVhp = w0hp_1_s17*(Vlp - Vhp) >> 17;
Vlp += dVlp;
Vhp += dVhp;
}
// ----------------------------------------------------------------------------
// SID clocking - delta_t cycles.
// ----------------------------------------------------------------------------
RESID_INLINE
void ExternalFilter::clock(cycle_count delta_t, short Vi)
{
// This is handy for testing.
if (unlikely(!enabled)) {
// Vo = Vlp - Vhp;
Vlp = Vi << 11;
Vhp = 0;
return;
}
// Maximum delta cycles for the external filter to work satisfactorily
// is approximately 8.
cycle_count delta_t_flt = 8;
while (delta_t) {
if (unlikely(delta_t < delta_t_flt)) {
delta_t_flt = delta_t;
}
// Calculate filter outputs.
// Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
// Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
// Vo = Vlp - Vhp;
int dVlp = (w0lp_1_s7*delta_t_flt >> 3)*((Vi << 11) - Vlp) >> 4;
int dVhp = (w0hp_1_s17*delta_t_flt >> 3)*(Vlp - Vhp) >> 14;
Vlp += dVlp;
Vhp += dVhp;
delta_t -= delta_t_flt;
}
}
// ----------------------------------------------------------------------------
// Audio output (16 bits).
// ----------------------------------------------------------------------------
RESID_INLINE
int ExternalFilter::output()
{
return (Vlp - Vhp) >> 11;
}
#endif // RESID_INLINING || defined(RESID_EXTFILT_CC)
} // namespace reSID
#endif // not RESID_EXTFILT_H
reSID::ExternalFilter
Definition: extfilt.h:56
reSID::SID
Definition: sid.h:39