108 lines
2.7 KiB
Plaintext
108 lines
2.7 KiB
Plaintext
/*
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* This file is part of libsidplayfp, a SID player engine.
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*
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* Copyright 2011-2013 Leandro Nini <drfiemost@users.sourceforge.net>
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* Copyright 2007-2010 Antti Lankila
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* Copyright 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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*/
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#ifndef EXTERNALFILTER_H
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#define EXTERNALFILTER_H
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#include "siddefs-fp.h"
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namespace reSIDfp
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{
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/**
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* The audio output stage in a Commodore 64 consists of two STC networks, a
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* low-pass filter with 3-dB frequency 16kHz followed by a high-pass filter with
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* 3-dB frequency 16Hz (the latter provided an audio equipment input impedance
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* of 1kOhm).
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*
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* The STC networks are connected with a [BJT] supposedly meant to act
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* as a unity gain buffer, which is not really how it works.
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* A more elaborate model would include the BJT, however DC circuit analysis
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* yields BJT base-emitter and emitter-base impedances sufficiently low
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* to produce additional low-pass and high-pass 3dB-frequencies
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* in the order of hundreds of kHz. This calls for a sampling frequency
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* of several MHz, which is far too high for practical use.
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*
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* [BJT]: https://en.wikipedia.org/wiki/Bipolar_junction_transistor
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*/
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class ExternalFilter
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{
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private:
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/// Lowpass filter voltage
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int Vlp;
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/// Highpass filter voltage
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int Vhp;
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int w0lp_1_s7;
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int w0hp_1_s17;
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public:
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/**
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* SID clocking.
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*
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* @param Vi
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*/
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int clock(int Vi);
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/**
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* Constructor.
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*/
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ExternalFilter();
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/**
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* Setup of the external filter sampling parameters.
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*
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* @param frequency
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*/
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void setClockFrequency(double frequency);
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/**
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* SID reset.
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*/
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void reset();
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};
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} // namespace reSIDfp
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#if RESID_INLINING || defined(EXTERNALFILTER_CPP)
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namespace reSIDfp
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{
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RESID_INLINE
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int ExternalFilter::clock(int Vi)
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{
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const int dVlp = (w0lp_1_s7 * ((Vi << 11) - Vlp) >> 7);
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const int dVhp = (w0hp_1_s17 * (Vlp - Vhp) >> 17);
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Vlp += dVlp;
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Vhp += dVhp;
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return (Vlp - Vhp) >> 11;
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}
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} // namespace reSIDfp
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#endif
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#endif
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