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Diffstat (limited to 'sid/sidplay-libs-2.1.0/resid/extfilt.h')
| -rw-r--r-- | sid/sidplay-libs-2.1.0/resid/extfilt.h | 169 |
1 files changed, 169 insertions, 0 deletions
diff --git a/sid/sidplay-libs-2.1.0/resid/extfilt.h b/sid/sidplay-libs-2.1.0/resid/extfilt.h new file mode 100644 index 00000000..84c204ef --- /dev/null +++ b/sid/sidplay-libs-2.1.0/resid/extfilt.h @@ -0,0 +1,169 @@ +// --------------------------------------------------------------------------- +// This file is part of reSID, a MOS6581 SID emulator engine. +// Copyright (C) 2002 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" + +RESID_NAMESPACE_START + +// ---------------------------------------------------------------------------- +// 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_sampling_parameter(double pass_freq); + 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 SID; +}; + + +// ---------------------------------------------------------------------------- +// 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__) + +RESID_NAMESPACE_STOP + +#endif // not __EXTFILT_H__ |