1 // ---------------------------------------------------------------------------
2 // This file is part of reSID, a MOS6581 SID emulator engine.
3 // Copyright (C) 2004 Dag Lem <resid@nimrod.no>
4 //
5 // This program is free software; you can redistribute it and/or modify
6 // it under the terms of the GNU General Public License as published by
7 // the Free Software Foundation; either version 2 of the License, or
8 // (at your option) any later version.
9 //
10 // This program is distributed in the hope that it will be useful,
11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 // GNU General Public License for more details.
14 //
15 // You should have received a copy of the GNU General Public License
16 // along with this program; if not, write to the Free Software
17 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
18 // ---------------------------------------------------------------------------
19
20 #ifndef __EXTFILT_H__
21 #define __EXTFILT_H__
22
23 #include "siddefs.h"
24
25 // ----------------------------------------------------------------------------
26 // The audio output stage in a Commodore 64 consists of two STC networks,
27 // a low-pass filter with 3-dB frequency 16kHz followed by a high-pass
28 // filter with 3-dB frequency 16Hz (the latter provided an audio equipment
29 // input impedance of 1kOhm).
30 // The STC networks are connected with a BJT supposedly meant to act as
31 // a unity gain buffer, which is not really how it works. A more elaborate
32 // model would include the BJT, however DC circuit analysis yields BJT
33 // base-emitter and emitter-base impedances sufficiently low to produce
34 // additional low-pass and high-pass 3dB-frequencies in the order of hundreds
35 // of kHz. This calls for a sampling frequency of several MHz, which is far
36 // too high for practical use.
37 // ----------------------------------------------------------------------------
38 class ExternalFilter
39 {
40 public:
41 ExternalFilter();
42
43 void enable_filter(bool enable);
44 void set_chip_model(chip_model model);
45
46 RESID_INLINE void clock(sound_sample Vi);
47 RESID_INLINE void clock(cycle_count delta_t, sound_sample Vi);
48 void reset();
49
50 // Audio output (20 bits).
51 RESID_INLINE sound_sample output();
52
53 protected:
54 // Filter enabled.
55 bool enabled;
56
57 // Maximum mixer DC offset.
58 sound_sample mixer_DC;
59
60 // State of filters.
61 sound_sample Vlp; // lowpass
62 sound_sample Vhp; // highpass
63 sound_sample Vo;
64
65 // Cutoff frequencies.
66 sound_sample w0lp;
67 sound_sample w0hp;
68
69 friend class SID;
70 };
71
72
73 // ----------------------------------------------------------------------------
74 // Inline functions.
75 // The following functions are defined inline because they are called every
76 // time a sample is calculated.
77 // ----------------------------------------------------------------------------
78
79 #if RESID_INLINING || defined(__EXTFILT_CC__)
80
81 // ----------------------------------------------------------------------------
82 // SID clocking - 1 cycle.
83 // ----------------------------------------------------------------------------
84 RESID_INLINE
clock(sound_sample Vi)85 void ExternalFilter::clock(sound_sample Vi)
86 {
87 // This is handy for testing.
88 if (!enabled) {
89 // Remove maximum DC level since there is no filter to do it.
90 Vlp = Vhp = 0;
91 Vo = Vi - mixer_DC;
92 return;
93 }
94
95 // delta_t is converted to seconds given a 1MHz clock by dividing
96 // with 1 000 000.
97
98 // Calculate filter outputs.
99 // Vo = Vlp - Vhp;
100 // Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
101 // Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
102
103 sound_sample dVlp = (w0lp >> 8)*(Vi - Vlp) >> 12;
104 sound_sample dVhp = w0hp*(Vlp - Vhp) >> 20;
105 Vo = Vlp - Vhp;
106 Vlp += dVlp;
107 Vhp += dVhp;
108 }
109
110 // ----------------------------------------------------------------------------
111 // SID clocking - delta_t cycles.
112 // ----------------------------------------------------------------------------
113 RESID_INLINE
clock(cycle_count delta_t,sound_sample Vi)114 void ExternalFilter::clock(cycle_count delta_t,
115 sound_sample Vi)
116 {
117 // This is handy for testing.
118 if (!enabled) {
119 // Remove maximum DC level since there is no filter to do it.
120 Vlp = Vhp = 0;
121 Vo = Vi - mixer_DC;
122 return;
123 }
124
125 // Maximum delta cycles for the external filter to work satisfactorily
126 // is approximately 8.
127 cycle_count delta_t_flt = 8;
128
129 while (delta_t) {
130 if (delta_t < delta_t_flt) {
131 delta_t_flt = delta_t;
132 }
133
134 // delta_t is converted to seconds given a 1MHz clock by dividing
135 // with 1 000 000.
136
137 // Calculate filter outputs.
138 // Vo = Vlp - Vhp;
139 // Vlp = Vlp + w0lp*(Vi - Vlp)*delta_t;
140 // Vhp = Vhp + w0hp*(Vlp - Vhp)*delta_t;
141
142 sound_sample dVlp = (w0lp*delta_t_flt >> 8)*(Vi - Vlp) >> 12;
143 sound_sample dVhp = w0hp*delta_t_flt*(Vlp - Vhp) >> 20;
144 Vo = Vlp - Vhp;
145 Vlp += dVlp;
146 Vhp += dVhp;
147
148 delta_t -= delta_t_flt;
149 }
150 }
151
152
153 // ----------------------------------------------------------------------------
154 // Audio output (19.5 bits).
155 // ----------------------------------------------------------------------------
156 RESID_INLINE
output()157 sound_sample ExternalFilter::output()
158 {
159 return Vo;
160 }
161
162 #endif // RESID_INLINING || defined(__EXTFILT_CC__)
163
164 #endif // not __EXTFILT_H__
165