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filter8580new.h
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// ---------------------------------------------------------------------------
// This file is part of reSID, a MOS6581 SID emulator engine.
// Copyright (C) 2010 Dag Lem <[email protected]>
//
// 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_FILTER_H
#define RESID_FILTER_H
#include "resid-config.h"
namespace reSID
{
// ----------------------------------------------------------------------------
// The SID filter is modeled with a two-integrator-loop biquadratic filter,
// which has been confirmed by Bob Yannes to be the actual circuit used in
// the SID chip.
//
// Measurements show that excellent emulation of the SID filter is achieved,
// except when high resonance is combined with high sustain levels.
// In this case the SID op-amps are performing less than ideally and are
// causing some peculiar behavior of the SID filter. This however seems to
// have more effect on the overall amplitude than on the color of the sound.
//
// The theory for the filter circuit can be found in "Microelectric Circuits"
// by Adel S. Sedra and Kenneth C. Smith.
// The circuit is modeled based on the explanation found there except that
// an additional inverter is used in the feedback from the bandpass output,
// allowing the summer op-amp to operate in single-ended mode. This yields
// filter outputs with levels independent of Q, which corresponds with the
// results obtained from a real SID.
//
// We have been able to model the summer and the two integrators of the circuit
// to form components of an IIR filter.
// Vhp is the output of the summer, Vbp is the output of the first integrator,
// and Vlp is the output of the second integrator in the filter circuit.
//
// According to Bob Yannes, the active stages of the SID filter are not really
// op-amps. Rather, simple NMOS inverters are used. By biasing an inverter
// into its region of quasi-linear operation using a feedback resistor from
// input to output, a MOS inverter can be made to act like an op-amp for
// small signals centered around the switching threshold.
//
// In 2008, Michael Huth facilitated closer investigation of the SID 6581
// filter circuit by publishing high quality microscope photographs of the die.
// Tommi Lempinen has done an impressive work on re-vectorizing and annotating
// the die photographs, substantially simplifying further analysis of the
// filter circuit.
//
// The filter schematics below are reverse engineered from these re-vectorized
// and annotated die photographs. While the filter first depicted in reSID 0.9
// is a correct model of the basic filter, the schematics are now completed
// with the audio mixer and output stage, including details on intended
// relative resistor values. Also included are schematics for the NMOS FET
// voltage controlled resistors (VCRs) used to control cutoff frequency, the
// DAC which controls the VCRs, the NMOS op-amps, and the output buffer.
//
//
// SID 6581 filter / mixer / output
// --------------------------------
//
// ---------------------------------------------------
// | |
// | --1R1-- \-- D7 |
// | ---R1-- | | |
// | | | |--2R1-- \--| D6 |
// | ------------<A]-----| | $17 |
// | | |--4R1-- \--| D5 1=open | (3.5R1)
// | | | | |
// | | --8R1-- \--| D4 | (7.0R1)
// | | | |
// $17 | | (CAP2B) | (CAP1B) |
// 0=to mixer | --R8-- ---R8-- ---C---| ---C---|
// 1=to filter | | | | | | | |
// ------R8--|-----[A>--|--Rw-----[A>--|--Rw-----[A>--|
// ve (EXT IN) | | | |
// D3 \ ---------------R8--| | | (CAP2A) | (CAP1A)
// | v3 | | vhp | vbp | vlp
// D2 | \ -----------R8--| ----- | |
// | | v2 | | | |
// D1 | | \ -------R8--| | ---------------- |
// | | | v1 | | | |
// D0 | | | \ ---R8-- | | ---------------------------
// | | | | | | |
// R6 R6 R6 R6 R6 R6 R6
// | | | | $18 | | | $18
// | \ | | D7: 1=open \ \ \ D6 - D4: 0=open
// | | | | | | |
// --------------------------------- 12V
// |
// | D3 --/ --1R4-- |
// | ---R8-- | | ---R3-- |
// | | | D2 |--/ --2R4--| | | ||--
// ------[A>---------| |-----[A>-----||
// D1 |--/ --4R4--| (4.25R2) ||--
// $18 | | |
// 0=open D0 --/ --8R4-- (8.75R2) |
//
// vo (AUDIO
// OUT)
//
//
// v1 - voice 1
// v2 - voice 2
// v3 - voice 3
// ve - ext in
// vhp - highpass output
// vbp - bandpass output
// vlp - lowpass output
// vo - audio out
// [A> - single ended inverting op-amp (self-biased NMOS inverter)
// Rn - "resistors", implemented with custom NMOS FETs
// Rw - cutoff frequency resistor (VCR)
// C - capacitor
//
// Notes:
//
// R2 ~ 2.0*R1
// R6 ~ 6.0*R1
// R8 ~ 8.0*R1
// R24 ~ 24.0*R1
//
// The Rn "resistors" in the circuit are implemented with custom NMOS FETs,
// probably because of space constraints on the SID die. The silicon substrate
// is laid out in a narrow strip or "snake", with a strip length proportional
// to the intended resistance. The polysilicon gate electrode covers the entire
// silicon substrate and is fixed at 12V in order for the NMOS FET to operate
// in triode mode (a.k.a. linear mode or ohmic mode).
//
// Even in "linear mode", an NMOS FET is only an approximation of a resistor,
// as the apparant resistance increases with increasing drain-to-source
// voltage. If the drain-to-source voltage should approach the gate voltage
// of 12V, the NMOS FET will enter saturation mode (a.k.a. active mode), and
// the NMOS FET will not operate anywhere like a resistor.
//
//
//
// NMOS FET voltage controlled resistor (VCR)
// ------------------------------------------
//
// Vw
//
// |
// |
// R1
// |
// --R1--|
// | __|__
// | -----
// | | |
// vi ---------- -------- vo
// | |
// ----R24----
//
//
// vi - input
// vo - output
// Rn - "resistors", implemented with custom NMOS FETs
// Vw - voltage from 11-bit DAC (frequency cutoff control)
//
// Notes:
//
// An approximate value for R24 can be found by using the formula for the
// filter cutoff frequency:
//
// FCmin = 1/(2*pi*Rmax*C)
//
// Assuming that a the setting for minimum cutoff frequency in combination with
// a low level input signal ensures that only negligible current will flow
// through the transistor in the schematics above, values for FCmin and C can
// be substituted in this formula to find Rmax.
// Using C = 470pF and FCmin = 220Hz (measured value), we get:
//
// FCmin = 1/(2*pi*Rmax*C)
// Rmax = 1/(2*pi*FCmin*C) = 1/(2*pi*220*470e-12) ~ 1.5MOhm
//
// From this it follows that:
// R24 = Rmax ~ 1.5MOhm
// R1 ~ R24/24 ~ 64kOhm
// R2 ~ 2.0*R1 ~ 128kOhm
// R6 ~ 6.0*R1 ~ 384kOhm
// R8 ~ 8.0*R1 ~ 512kOhm
//
// Note that these are only approximate values for one particular SID chip,
// due to process variations the values can be substantially different in
// other chips.
//
//
//
// Filter frequency cutoff DAC
// ---------------------------
//
//
// 12V 10 9 8 7 6 5 4 3 2 1 0 VGND
// | | | | | | | | | | | | | Missing
// 2R 2R 2R 2R 2R 2R 2R 2R 2R 2R 2R 2R 2R termination
// | | | | | | | | | | | | |
// Vw ----R---R---R---R---R---R---R---R---R---R---R-- ---
//
// Bit on: 12V
// Bit off: 5V (VGND)
//
// As is the case with all MOS 6581 DACs, the termination to (virtual) ground
// at bit 0 is missing.
//
// Furthermore, the control of the two VCRs imposes a load on the DAC output
// which varies with the input signals to the VCRs. This can be seen from the
// VCR figure above.
//
//
//
// "Op-amp" (self-biased NMOS inverter)
// ------------------------------------
//
//
// 12V
//
// |
// -----------|
// | |
// | ------|
// | | |
// | | ||--
// | --||
// | ||--
// ||-- |
// vi -----|| |--------- vo
// ||-- | |
// | ||-- |
// |-------|| |
// | ||-- |
// ||-- | |
// --|| | |
// | ||-- | |
// | | | |
// | -----------| |
// | | |
// | |
// | GND |
// | |
// ----------------------
//
//
// vi - input
// vo - output
//
// Notes:
//
// The schematics above are laid out to show that the "op-amp" logically
// consists of two building blocks; a saturated load NMOS inverter (on the
// right hand side of the schematics) with a buffer / bias input stage
// consisting of a variable saturated load NMOS inverter (on the left hand
// side of the schematics).
//
// Provided a reasonably high input impedance and a reasonably low output
// impedance, the "op-amp" can be modeled as a voltage transfer function
// mapping input voltage to output voltage.
//
//
//
// Output buffer (NMOS voltage follower)
// -------------------------------------
//
//
// 12V
//
// |
// |
// ||--
// vi -----||
// ||--
// |
// |------ vo
// | (AUDIO
// Rext OUT)
// |
// |
//
// GND
//
// vi - input
// vo - output
// Rext - external resistor, 1kOhm
//
// Notes:
//
// The external resistor Rext is needed to complete the NMOS voltage follower,
// this resistor has a recommended value of 1kOhm.
//
// Die photographs show that actually, two NMOS transistors are used in the
// voltage follower. However the two transistors are coupled in parallel (all
// terminals are pairwise common), which implies that we can model the two
// transistors as one.
//
// ----------------------------------------------------------------------------
//
// SID 8580 filter / mixer / output
// --------------------------------
//
// +---------------------------------------------------+
// | $17 +----Rf-+ |
// | | | |
// | D4&!D5 o- \-R3-o |
// | | | $17 |
// | !D4&!D5 o- \-R2-o |
// | | | +---R8-- \--+ !D6&D7 |
// | D4&!D5 o- \-R1-o | | |
// | | | o---RC-- \--o D6&D7 |
// | +---------o--<A]--o--o | |
// | | o---R4-- \--o D6&!D7 |
// | | | | |
// | | +---Ri-- \--o !D6&!D7 |
// | | | |
// $17 | | (CAP2B) | (CAP1B) |
// 0=to mixer | +--R7--+ +---R7--+ +---C---o +---C---o
// 1=to filter | | | | | | | |
// +------R7--o--o--[A>--o--Rfc-o--[A>--o--Rfc-o--[A>--o
// ve (EXT IN) | | | |
// D3 \ --------------R12--o | | (CAP2A) | (CAP1A)
// | v3 | | vhp | vbp | vlp
// D2 | \ -----------R7--o +-----+ | |
// | | v2 | | | |
// D1 | | \ -------R7--o | +----------------+ |
// | | | v1 | | | |
// D0 | | | \ ---R7--+ | | +---------------------------+
// | | | | | | |
// R9 R5 R5 R5 R5 R5 R5
// | | | | $18 | | | $18
// | \ | | D7: 1=open \ \ \ D6 - D4: 0=open
// | | | | | | |
// +---o---o---o-------------o---o---+
// |
// | D3 +--/ --1R4--+
// | +---R8--+ | | +---R2--+
// | | | D2 o--/ --2R4--o | |
// +---o--[A>--o------o o--o--[A>--o-- vo (AUDIO OUT)
// D1 o--/ --4R4--o
// $18 | |
// 0=open D0 +--/ --8R4--+
//
//
//
//
// R1 = 15.3*Ri
// R2 = 7.3*Ri
// R3 = 4.7*Ri
// Rf = 1.4*Ri
// R4 = 1.4*Ri
// R8 = 2.0*Ri
// RC = 2.8*Ri
//
//
//
// Op-amps
// -------
// Unlike the 6581, the 8580 has real OpAmps.
//
// Temperature compensated differential amplifier:
//
// 9V
//
// |
// +-------o-o-o-------+
// | | | |
// | R R |
// +--|| | | ||--+
// ||---o o---||
// +--|| | | ||--+
// | | | |
// o-----+ | | o--- Va
// | | | | |
// +--|| | | | ||--+
// ||-o-+---+---||
// +--|| | | ||--+
// | | | |
// | |
// GND | | GND
// ||--+ +--||
// in- -----|| ||------ in+
// ||----o----||
// |
// 8 Current sink
// |
//
// GND
//
// Inverter + non-inverting output amplifier:
//
// Va ---o---||-------------------o--------------------+
// | | 9V |
// | +----------+----------+ | |
// | 9V | | 9V | ||--+ |
// | | | 9V | | +-|| |
// | R | | | ||--+ ||--+ |
// | | | ||--+ +--|| o---o--- Vout
// | o---o---|| ||--+ ||--+
// | | ||--+ o-----||
// | ||--+ | ||--+ ||--+
// +-----|| o-----|| |
// ||--+ | ||--+
// | R | GND
// |
// GND GND
// GND
//
//
//
// Virtual ground
// --------------
// A PolySi resitive voltage divider provides the voltage
// for the non-inverting input of the filter op-amps.
//
// 5V
// +----------+
// | | |\ |
// R1 +---|-\ |
// 5V | |A >---o--- Vref
// o-------|+/
// | | |/
// R10 R4
// | |
// o---+
// |
// R10
// |
//
// GND
//
// Rn = n*R1
//
//
//
// Rfc - freq control DAC resistance ladder
// ----------------------------------------
// The resistance for the bandpass and lowpass integrator stages of the filter are determined
// by an 11 bit DAC driven by the FC register.
// If all 11 bits are '0', the impedance of the DAC would be "infinitely high".
// To get around this, there is an 11 input NOR gate below the DAC sensing those 11 bits.
// If they are all 0, the NOR gate gives the gate control voltage to the 12 bit DAC LSB.
//
//
//
// Crystal stabilized precision switched capacitor voltage divider
// ---------------------------------------------------------------
// There is a FET working as a temperature sensor close to the DACs which changes the gate voltage
// of the frequency control DACs according to the temperature, to reduce its effects on the filter curve.
// An asynchronous 3 bit binary counter, running at the speed of PHI2, drives two big capacitors
// which AC resistance is then used as a voltage divider.
// This implicates that frequency difference between PAL and NTSC might shift the filter curve by 4% or such.
//
// https://en.wikipedia.org/wiki/Switched_capacitor
//
// |\ OpAmp has a smaller capacitor
// Vref ---|+\ than the other OPs
// |A >---o--- Vdac
// o-------|-/ |
// | |/ |
// | |
// C1 | C2 |
// +---||---o---+ +---o-----||-------o
// | | | | | |
// o----+ | ----- | |
// | | | ----- +----+ +-----+
// | ----- | | | |
// | ----- | ----- |
// | | | ----- |
// | +-----------+ | |
// | /Q Q | +-------+
// GND +-----------+ FET close to DAC
// | clk/8 | working as temperature sensor
// +-----------+
// | |
// clk1 clk2
//
// Compile-time computation of op-amp summer and mixer table offsets.
// The highpass summer has 2 - 6 inputs (bandpass, lowpass, and 0 - 4 voices).
template<int i>
struct summer_offset
{
enum { value = summer_offset<i - 1>::value + ((2 + i - 1) << 16) };
};
template<>
struct summer_offset<0>
{
enum { value = 0 };
};
// The mixer has 0 - 7 inputs (0 - 4 voices and 0 - 3 filter outputs).
template<int i>
struct mixer_offset
{
enum { value = mixer_offset<i - 1>::value + ((i - 1) << 16) };
};
template<>
struct mixer_offset<1>
{
enum { value = 1 };
};
template<>
struct mixer_offset<0>
{
enum { value = 0 };
};
class Filter
{
public:
Filter();
void enable_filter(bool enable);
void adjust_filter_bias(double dac_bias);
void set_chip_model(chip_model model);
void set_voice_mask(reg4 mask);
void clock(int voice1, int voice2, int voice3);
void clock(cycle_count delta_t, int voice1, int voice2, int voice3);
void reset();
// Write registers.
void writeFC_LO(reg8);
void writeFC_HI(reg8);
void writeRES_FILT(reg8);
void writeMODE_VOL(reg8);
// SID audio input (16 bits).
void input(short sample);
// SID audio output (16 bits).
short output();
protected:
void set_sum_mix();
void set_w0();
// Filter enabled.
bool enabled;
// Filter cutoff frequency.
reg12 fc;
// Filter resonance.
reg8 res;
// Selects which voices to route through the filter.
reg8 filt;
// Selects which filter outputs to route into the mixer.
reg4 mode;
// Output master volume.
reg4 vol;
// Used to mask out EXT IN if not connected, and for test purposes
// (voice muting).
reg8 voice_mask;
// Select which inputs to route into the summer / mixer.
// These are derived from filt, mode, and voice_mask.
reg8 sum;
reg8 mix;
// State of filter.
int Vhp; // highpass
int Vbp; // bandpass
int Vbp_x, Vbp_vc;
int Vlp; // lowpass
int Vlp_x, Vlp_vc;
// Filter / mixer inputs.
int ve;
int v3;
int v2;
int v1;
chip_model sid_model;
typedef struct {
unsigned short vx;
short dvx;
} opamp_t;
typedef struct {
int kVddt; // K*(Vdd - Vth)
int voice_scale_s14;
int voice_DC;
int ak;
int bk;
int vc_min;
int vc_max;
double vo_N16; // Fixed point scaling for 16 bit op-amp output.
// Reverse op-amp transfer function.
unsigned short opamp_rev[1 << 16];
// Lookup tables for gain and summer op-amps in output stage / filter.
unsigned short summer[summer_offset<5>::value];
unsigned short gain[16][1 << 16];
unsigned short resonance[16][1 << 16];
unsigned short mixer[mixer_offset<8>::value];
// Cutoff frequency DAC output voltage table. FC is an 11 bit register.
unsigned short f0_dac[1 << 11];
} model_filter_t;
// 6581 only
// Cutoff frequency DAC voltage, resonance.
int Vddt_Vw_2, Vw_bias;
static int n_snake;
// 8580 only
int n_dac;
static int n_param;
// DAC gate voltage
int nVgt;
int solve_gain(opamp_t* opamp, int n, int vi_t, int& x, model_filter_t& mf);
int solve_integrate_6581(int dt, int vi_t, int& x, int& vc, model_filter_t& mf);
int solve_integrate_8580(int dt, int vi_t, int& x, int& vc, model_filter_t& mf);
// VCR - 6581 only.
static unsigned short vcr_kVg[1 << 16];
static unsigned short vcr_n_Ids_term[1 << 16];
// Common parameters.
static model_filter_t model_filter[2];
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_FILTER_CC)
// ----------------------------------------------------------------------------
// SID clocking - 1 cycle.
// ----------------------------------------------------------------------------
RESID_INLINE
void Filter::clock(int voice1, int voice2, int voice3)
{
model_filter_t& f = model_filter[sid_model];
v1 = (voice1*f.voice_scale_s14 >> 18) + f.voice_DC;
v2 = (voice2*f.voice_scale_s14 >> 18) + f.voice_DC;
v3 = (voice3*f.voice_scale_s14 >> 18) + f.voice_DC;
// Sum inputs routed into the filter.
int Vi = 0;
int offset = 0;
switch (sum & 0xf) {
case 0x0:
Vi = 0;
offset = summer_offset<0>::value;
break;
case 0x1:
Vi = v1;
offset = summer_offset<1>::value;
break;
case 0x2:
Vi = v2;
offset = summer_offset<1>::value;
break;
case 0x3:
Vi = v2 + v1;
offset = summer_offset<2>::value;
break;
case 0x4:
Vi = v3;
offset = summer_offset<1>::value;
break;
case 0x5:
Vi = v3 + v1;
offset = summer_offset<2>::value;
break;
case 0x6:
Vi = v3 + v2;
offset = summer_offset<2>::value;
break;
case 0x7:
Vi = v3 + v2 + v1;
offset = summer_offset<3>::value;
break;
case 0x8:
Vi = ve;
offset = summer_offset<1>::value;
break;
case 0x9:
Vi = ve + v1;
offset = summer_offset<2>::value;
break;
case 0xa:
Vi = ve + v2;
offset = summer_offset<2>::value;
break;
case 0xb:
Vi = ve + v2 + v1;
offset = summer_offset<3>::value;
break;
case 0xc:
Vi = ve + v3;
offset = summer_offset<2>::value;
break;
case 0xd:
Vi = ve + v3 + v1;
offset = summer_offset<3>::value;
break;
case 0xe:
Vi = ve + v3 + v2;
offset = summer_offset<3>::value;
break;
case 0xf:
Vi = ve + v3 + v2 + v1;
offset = summer_offset<4>::value;
break;
}
// Calculate filter outputs.
if (sid_model == 0) {
// MOS 6581.
Vlp = solve_integrate_6581(1, Vbp, Vlp_x, Vlp_vc, f);
Vbp = solve_integrate_6581(1, Vhp, Vbp_x, Vbp_vc, f);
Vhp = f.summer[offset + f.resonance[res][Vbp] + Vlp + Vi];
}
else {
// MOS 8580.
Vlp = solve_integrate_8580(1, Vbp, Vlp_x, Vlp_vc, f);
Vbp = solve_integrate_8580(1, Vhp, Vbp_x, Vbp_vc, f);
Vhp = f.summer[offset + f.resonance[res][Vbp] + Vlp + Vi];
}
}
// ----------------------------------------------------------------------------
// SID clocking - delta_t cycles.
// ----------------------------------------------------------------------------
RESID_INLINE
void Filter::clock(cycle_count delta_t, int voice1, int voice2, int voice3)
{
model_filter_t& f = model_filter[sid_model];
v1 = (voice1*f.voice_scale_s14 >> 18) + f.voice_DC;
v2 = (voice2*f.voice_scale_s14 >> 18) + f.voice_DC;
v3 = (voice3*f.voice_scale_s14 >> 18) + f.voice_DC;
// Enable filter on/off.
// This is not really part of SID, but is useful for testing.
// On slow CPUs it may be necessary to bypass the filter to lower the CPU
// load.
if (unlikely(!enabled)) {
return;
}
// Sum inputs routed into the filter.
int Vi = 0;
int offset = 0;
switch (sum & 0xf) {
case 0x0:
Vi = 0;
offset = summer_offset<0>::value;
break;
case 0x1:
Vi = v1;
offset = summer_offset<1>::value;
break;
case 0x2:
Vi = v2;
offset = summer_offset<1>::value;
break;
case 0x3:
Vi = v2 + v1;
offset = summer_offset<2>::value;
break;
case 0x4:
Vi = v3;
offset = summer_offset<1>::value;
break;
case 0x5:
Vi = v3 + v1;
offset = summer_offset<2>::value;
break;
case 0x6:
Vi = v3 + v2;
offset = summer_offset<2>::value;
break;
case 0x7:
Vi = v3 + v2 + v1;
offset = summer_offset<3>::value;
break;
case 0x8:
Vi = ve;
offset = summer_offset<1>::value;
break;
case 0x9:
Vi = ve + v1;
offset = summer_offset<2>::value;
break;
case 0xa:
Vi = ve + v2;
offset = summer_offset<2>::value;
break;
case 0xb:
Vi = ve + v2 + v1;
offset = summer_offset<3>::value;
break;
case 0xc:
Vi = ve + v3;
offset = summer_offset<2>::value;
break;
case 0xd:
Vi = ve + v3 + v1;
offset = summer_offset<3>::value;
break;
case 0xe:
Vi = ve + v3 + v2;
offset = summer_offset<3>::value;
break;
case 0xf:
Vi = ve + v3 + v2 + v1;
offset = summer_offset<4>::value;
break;
}
// Maximum delta cycles for filter fixpoint iteration to converge
// is approximately 3.
cycle_count delta_t_flt = 3;
if (sid_model == 0) {
// MOS 6581.
while (delta_t) {
if (unlikely(delta_t < delta_t_flt)) {
delta_t_flt = delta_t;
}
// Calculate filter outputs.
Vlp = solve_integrate_6581(delta_t_flt, Vbp, Vlp_x, Vlp_vc, f);
Vbp = solve_integrate_6581(delta_t_flt, Vhp, Vbp_x, Vbp_vc, f);
Vhp = f.summer[offset + f.resonance[res][Vbp] + Vlp + Vi];
delta_t -= delta_t_flt;
}
}
else {
// MOS 8580.
while (delta_t) {
if (unlikely(delta_t < delta_t_flt)) {
delta_t_flt = delta_t;
}
// Calculate filter outputs.
Vlp = solve_integrate_8580(delta_t_flt, Vbp, Vlp_x, Vlp_vc, f);
Vbp = solve_integrate_8580(delta_t_flt, Vhp, Vbp_x, Vbp_vc, f);
Vhp = f.summer[offset + f.resonance[res][Vbp] + Vlp + Vi];
delta_t -= delta_t_flt;
}
}
}
// ----------------------------------------------------------------------------
// SID audio input (16 bits).
// ----------------------------------------------------------------------------
RESID_INLINE
void Filter::input(short sample)
{
// Scale to three times the peak-to-peak for one voice and add the op-amp
// "zero" DC level.
// NB! Adding the op-amp "zero" DC level is a (wildly inaccurate)
// approximation of feeding the input through an AC coupling capacitor.
// This could be implemented as a separate filter circuit, however the
// primary use of the emulator is not to process external signals.
// The upside is that the MOS8580 "digi boost" works without a separate (DC)
// input interface.
// Note that the input is 16 bits, compared to the 20 bit voice output.
model_filter_t& f = model_filter[sid_model];
ve = (sample*f.voice_scale_s14*3 >> 14) + f.mixer[0];
}
// ----------------------------------------------------------------------------
// SID audio output (16 bits).
// ----------------------------------------------------------------------------
RESID_INLINE
short Filter::output()
{
model_filter_t& f = model_filter[sid_model];
// Writing the switch below manually would be tedious and error-prone;
// it is rather generated by the following Perl program:
/*
my @i = qw(v1 v2 v3 ve Vlp Vbp Vhp);
for my $mix (0..2**@i-1) {
print sprintf(" case 0x%02x:\n", $mix);
my @sum;
for (@i) {
unshift(@sum, $_) if $mix & 0x01;
$mix >>= 1;
}
my $sum = join(" + ", @sum) || "0";
print " Vi = $sum;\n";
print " offset = mixer_offset<" . @sum . ">::value;\n";
print " break;\n";
}
*/
// Sum inputs routed into the mixer.
int Vi = 0;
int offset = 0;
switch (mix & 0x7f) {
case 0x00:
Vi = 0;
offset = mixer_offset<0>::value;
break;
case 0x01:
Vi = v1;
offset = mixer_offset<1>::value;
break;
case 0x02:
Vi = v2;
offset = mixer_offset<1>::value;
break;
case 0x03:
Vi = v2 + v1;
offset = mixer_offset<2>::value;
break;
case 0x04:
Vi = v3;
offset = mixer_offset<1>::value;
break;
case 0x05:
Vi = v3 + v1;
offset = mixer_offset<2>::value;
break;
case 0x06:
Vi = v3 + v2;
offset = mixer_offset<2>::value;
break;
case 0x07:
Vi = v3 + v2 + v1;
offset = mixer_offset<3>::value;
break;
case 0x08:
Vi = ve;
offset = mixer_offset<1>::value;
break;
case 0x09:
Vi = ve + v1;
offset = mixer_offset<2>::value;
break;
case 0x0a:
Vi = ve + v2;
offset = mixer_offset<2>::value;
break;
case 0x0b:
Vi = ve + v2 + v1;
offset = mixer_offset<3>::value;
break;
case 0x0c:
Vi = ve + v3;
offset = mixer_offset<2>::value;
break;
case 0x0d:
Vi = ve + v3 + v1;
offset = mixer_offset<3>::value;
break;
case 0x0e:
Vi = ve + v3 + v2;
offset = mixer_offset<3>::value;
break;
case 0x0f:
Vi = ve + v3 + v2 + v1;
offset = mixer_offset<4>::value;
break;
case 0x10: