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Diffstat (limited to 'plugins/sid/sidplay-libs/resid/filter.cpp')
| -rw-r--r-- | plugins/sid/sidplay-libs/resid/filter.cpp | 281 |
1 files changed, 281 insertions, 0 deletions
diff --git a/plugins/sid/sidplay-libs/resid/filter.cpp b/plugins/sid/sidplay-libs/resid/filter.cpp new file mode 100644 index 00000000..9ca793e0 --- /dev/null +++ b/plugins/sid/sidplay-libs/resid/filter.cpp @@ -0,0 +1,281 @@ +// --------------------------------------------------------------------------- +// 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 +// --------------------------------------------------------------------------- + +#define __FILTER_CC__ +#include "filter.h" + +RESID_NAMESPACE_START + +// Maximum cutoff frequency is specified as +// FCmax = 2.6e-5/C = 2.6e-5/2200e-12 = 11818. +// +// Measurements indicate a cutoff frequency range of approximately +// 220Hz - 18kHz on a MOS6581 fitted with 470pF capacitors. The function +// mapping FC to cutoff frequency has the shape of the tanh function, with +// a discontinuity at FCHI = 0x80. +// In contrast, the MOS8580 almost perfectly corresponds with the +// specification of a linear mapping from 30Hz to 12kHz. +// +// The mappings have been measured by feeding the SID with an external +// signal since the chip itself is incapable of generating waveforms of +// higher fundamental frequency than 4kHz. It is best to use the bandpass +// output at full resonance to pick out the cutoff frequency at any given +// FC setting. +// +// The mapping function is specified with spline interpolation points and +// the function values are retrieved via table lookup. +// +// NB! Cutoff frequency characteristics may vary, we have modeled two +// particular Commodore 64s. + +fc_point Filter::f0_points_6581[] = +{ + // FC f FCHI FCLO + // ---------------------------- + { 0, 220 }, // 0x00 - repeated end point + { 0, 220 }, // 0x00 + { 128, 230 }, // 0x10 + { 256, 250 }, // 0x20 + { 384, 300 }, // 0x30 + { 512, 420 }, // 0x40 + { 640, 780 }, // 0x50 + { 768, 1600 }, // 0x60 + { 832, 2300 }, // 0x68 + { 896, 3200 }, // 0x70 + { 960, 4300 }, // 0x78 + { 992, 5000 }, // 0x7c + { 1008, 5400 }, // 0x7e + { 1016, 5700 }, // 0x7f + { 1023, 6000 }, // 0x7f 0x07 + { 1023, 6000 }, // 0x7f 0x07 - discontinuity + { 1024, 4600 }, // 0x80 - + { 1024, 4600 }, // 0x80 + { 1032, 4800 }, // 0x81 + { 1056, 5300 }, // 0x84 + { 1088, 6000 }, // 0x88 + { 1120, 6600 }, // 0x8c + { 1152, 7200 }, // 0x90 + { 1280, 9500 }, // 0xa0 + { 1408, 12000 }, // 0xb0 + { 1536, 14500 }, // 0xc0 + { 1664, 16000 }, // 0xd0 + { 1792, 17100 }, // 0xe0 + { 1920, 17700 }, // 0xf0 + { 2047, 18000 }, // 0xff 0x07 + { 2047, 18000 } // 0xff 0x07 - repeated end point +}; + +fc_point Filter::f0_points_8580[] = +{ + // FC f FCHI FCLO + // ---------------------------- + { 0, 0 }, // 0x00 - repeated end point + { 0, 0 }, // 0x00 + { 128, 800 }, // 0x10 + { 256, 1600 }, // 0x20 + { 384, 2500 }, // 0x30 + { 512, 3300 }, // 0x40 + { 640, 4100 }, // 0x50 + { 768, 4800 }, // 0x60 + { 896, 5600 }, // 0x70 + { 1024, 6500 }, // 0x80 + { 1152, 7500 }, // 0x90 + { 1280, 8400 }, // 0xa0 + { 1408, 9200 }, // 0xb0 + { 1536, 9800 }, // 0xc0 + { 1664, 10500 }, // 0xd0 + { 1792, 11000 }, // 0xe0 + { 1920, 11700 }, // 0xf0 + { 2047, 12500 }, // 0xff 0x07 + { 2047, 12500 } // 0xff 0x07 - repeated end point +}; + + +// ---------------------------------------------------------------------------- +// Constructor. +// ---------------------------------------------------------------------------- +Filter::Filter() +{ + enable_filter(true); + + // Create mappings from FC to cutoff frequency. + set_chip_model(MOS8580); + interpolate(f0_points, f0_points + f0_count - 1, fc_plotter(), 1.0); + set_chip_model(MOS6581); + interpolate(f0_points, f0_points + f0_count - 1, fc_plotter(), 1.0); + + reset(); +} + + +// ---------------------------------------------------------------------------- +// Enable filter. +// ---------------------------------------------------------------------------- +void Filter::enable_filter(bool enable) +{ + enabled = enable; +} + + +// ---------------------------------------------------------------------------- +// Set chip model. +// ---------------------------------------------------------------------------- +void Filter::set_chip_model(chip_model model) +{ + if (model == MOS6581) { + // The mixer has a small input DC offset. This is found as follows: + // + // The "zero" output level of the mixer measured on the SID audio + // output pin is 5.50V at zero volume, and 5.44 at full + // volume. This yields a DC offset of (5.44V - 5.50V) = -0.06V. + // + // The DC offset is thus -0.06V/1.05V ~ -1/18 of the dynamic range + // of one voice. See voice.cc for measurement of the dynamic + // range. + + mixer_DC = -0xfff*0xff/18 >> 7; + + f0 = f0_6581; + f0_points = f0_points_6581; + f0_count = sizeof(f0_points_6581)/sizeof(*f0_points_6581); + } + else { + // No DC offsets in the MOS8580. + mixer_DC = 0; + + f0 = f0_8580; + f0_points = f0_points_8580; + f0_count = sizeof(f0_points_8580)/sizeof(*f0_points_8580); + } +} + + +// ---------------------------------------------------------------------------- +// SID reset. +// ---------------------------------------------------------------------------- +void Filter::reset() +{ + fc = 0; + + res = 0; + + filtex = 0; + + filt3_filt2_filt1 = 0; + + voice3off = 0; + + hp_bp_lp = 0; + + vol = 0; + + // State of filter. + Vhp = 0; + Vbp = 0; + Vlp = 0; + Vnf = 0; + + set_w0(); + set_Q(); +} + + +// ---------------------------------------------------------------------------- +// Register functions. +// ---------------------------------------------------------------------------- +void Filter::writeFC_LO(reg8 fc_lo) +{ + fc = fc & 0x7f8 | fc_lo & 0x007; + set_w0(); +} + +void Filter::writeFC_HI(reg8 fc_hi) +{ + fc = (fc_hi << 3) & 0x7f8 | fc & 0x007; + set_w0(); +} + +void Filter::writeRES_FILT(reg8 res_filt) +{ + res = (res_filt >> 4) & 0x0f; + set_Q(); + + filtex = res_filt & 0x08; + filt3_filt2_filt1 = res_filt & 0x07; +} + +void Filter::writeMODE_VOL(reg8 mode_vol) +{ + voice3off = mode_vol & 0x80; + + hp_bp_lp = (mode_vol >> 4) & 0x07; + + vol = mode_vol & 0x0f; +} + +// Set filter cutoff frequency. +void Filter::set_w0() +{ + const double pi = 3.1415926535897932385; + + // Multiply with 1.048576 to facilitate division by 1 000 000 by right- + // shifting 20 times (2 ^ 20 = 1048576). + w0 = static_cast<sound_sample>(2*pi*f0[fc]*1.048576); +} + +// Set filter resonance. +void Filter::set_Q() +{ + // Q is controlled linearly by res. Q has approximate range [0.707, 1.7]. + // As resonance is increased, the filter must be clocked more often to keep + // stable. + + // The coefficient 1024 is dispensed of later by right-shifting 10 times + // (2 ^ 10 = 1024). + _1024_div_Q = static_cast<sound_sample>(1024.0/(0.707 + 1.0*res/0x0f)); +} + +// ---------------------------------------------------------------------------- +// Spline functions. +// ---------------------------------------------------------------------------- + +// ---------------------------------------------------------------------------- +// Return the array of spline interpolation points used to map the FC register +// to filter cutoff frequency. +// ---------------------------------------------------------------------------- +void Filter::fc_default(const fc_point*& points, int& count) +{ + points = f0_points; + count = f0_count; +} + +// ---------------------------------------------------------------------------- +// Given an array of interpolation points p with n points, the following +// statement will specify a new FC mapping: +// interpolate(p, p + n - 1, filter.fc_plotter(), 1.0); +// Note that the x range of the interpolation points *must* be [0, 2047], +// and that additional end points *must* be present since the end points +// are not interpolated. +// ---------------------------------------------------------------------------- +PointPlotter<sound_sample> Filter::fc_plotter() +{ + return PointPlotter<sound_sample>(f0); +} + +RESID_NAMESPACE_STOP |