ref: 13b6db91928fea27f956cba68ee4169b7604a752
dir: /src/pt2_amigafilters.c/
/* Amiga 500 / Amiga 1200 filter implementation.
**
** Route:
** Paula output -> low-pass filter -> LED filter (if turned on) -> high-pass filter (centering of waveform)
*/
#include <stdint.h>
#include <stdbool.h>
#include "pt2_structs.h"
#include "pt2_audio.h"
#include "pt2_paula.h"
#include "pt2_rcfilter.h"
#include "pt2_math.h"
#include "pt2_textout.h"
typedef struct ledFilter_t
{
double LIn1, LIn2, LOut1, LOut2;
double RIn1, RIn2, ROut1, ROut2;
double a1, a2, a3, b1, b2;
} ledFilter_t;
static int32_t filterModel;
static bool ledFilterEnabled, useA1200LowPassFilter;
static rcFilter_t filterLoA500, filterHiA500, filterLoA1200, filterHiA1200;
static ledFilter_t filterLED;
void (*processAmigaFilters)(double *, double *, int32_t); // globalized
static void processFiltersA1200_NoLED(double *dBufferL, double *dBufferR, int32_t numSamples);
static void processFiltersA1200_LED(double *dBufferL, double *dBufferR, int32_t numSamples);
static void processFiltersA500_NoLED(double *dBufferL, double *dBufferR, int32_t numSamples);
static void processFiltersA500_LED(double *dBufferL, double *dBufferR, int32_t numSamples);
// --------------------------------------------------------
// Crude LED filter implementation
// --------------------------------------------------------
void clearLEDFilterState(ledFilter_t *f)
{
f->LIn1 = f->LIn2 = f->LOut1 = f->LOut2 = 0.0;
f->RIn1 = f->RIn2 = f->ROut1 = f->ROut2 = 0.0;
}
static void calcLEDFilterCoeffs(double sr, double hz, double qfactor, ledFilter_t *filter)
{
const double c = 1.0 / pt2_tan((PT2_PI * hz) / sr);
const double r = 1.0 / qfactor;
filter->a1 = 1.0 / (1.0 + r * c + c * c);
filter->a2 = 2.0 * filter->a1;
filter->a3 = filter->a1;
filter->b1 = 2.0 * (1.0 - c*c) * filter->a1;
filter->b2 = (1.0 - r * c + c * c) * filter->a1;
}
static void LEDFilter(ledFilter_t *f, const double *in, double *out)
{
const double LOut = (f->a1 * in[0]) + (f->a2 * f->LIn1) + (f->a3 * f->LIn2) - (f->b1 * f->LOut1) - (f->b2 * f->LOut2);
const double ROut = (f->a1 * in[1]) + (f->a2 * f->RIn1) + (f->a3 * f->RIn2) - (f->b1 * f->ROut1) - (f->b2 * f->ROut2);
// shift states
f->LIn2 = f->LIn1;
f->LIn1 = in[0];
f->LOut2 = f->LOut1;
f->LOut1 = LOut;
f->RIn2 = f->RIn1;
f->RIn1 = in[1];
f->ROut2 = f->ROut1;
f->ROut1 = ROut;
// set output
out[0] = LOut;
out[1] = ROut;
}
// --------------------------------------------------------
// --------------------------------------------------------
void setupAmigaFilters(double dAudioFreq)
{
/* Amiga 500/1200 filter emulation
**
** aciddose:
** First comes a static low-pass 6dB formed by the supply current
** from the Paula's mixture of channels A+B / C+D into the opamp with
** 0.1uF capacitor and 360 ohm resistor feedback in inverting mode biased by
** dac vRef (used to center the output).
**
** R = 360 ohm
** C = 0.1uF
** Low Hz = 4420.97~ = 1 / (2pi * 360 * 0.0000001)
**
** Under spice simulation the circuit yields -3dB = 4400Hz.
** In the Amiga 1200, the low-pass cutoff is ~34kHz, so the
** static low-pass filter is disabled in the mixer in A1200 mode.
**
** Next comes a bog-standard Sallen-Key filter ("LED") with:
** R1 = 10K ohm
** R2 = 10K ohm
** C1 = 6800pF
** C2 = 3900pF
** Q ~= 1/sqrt(2)
**
** This filter is optionally bypassed by an MPF-102 JFET chip when
** the LED filter is turned off.
**
** Under spice simulation the circuit yields -3dB = 2800Hz.
** 90 degrees phase = 3000Hz (so, should oscillate at 3kHz!)
**
** The buffered output of the Sallen-Key passes into an RC high-pass with:
** R = 1.39K ohm (1K ohm + 390 ohm)
** C = 22uF (also C = 330nF, for improved high-frequency)
**
** High Hz = 5.2~ = 1 / (2pi * 1390 * 0.000022)
** Under spice simulation the circuit yields -3dB = 5.2Hz.
**
** 8bitbubsy:
** Keep in mind that many of the Amiga schematics that are floating around on
** the internet have wrong RC values! They were most likely very early schematics
** that didn't change before production (or changes that never reached production).
** This has been confirmed by measuring the components on several Amiga motherboards.
**
** Correct values for A500, >rev3 (?) (A500_R6.pdf):
** - 1-pole RC 6dB/oct low-pass: R=360 ohm, C=0.1uF
** - Sallen-key low-pass ("LED"): R1/R2=10k ohm, C1=6800pF, C2=3900pF
** - 1-pole RC 6dB/oct high-pass: R=1390 ohm (1000+390), C=22.33uF (22+0.33)
**
** Correct values for A1200, all revs (A1200_R2.pdf):
** - 1-pole RC 6dB/oct low-pass: R=680 ohm, C=6800pF
** - Sallen-key low-pass ("LED"): R1/R2=10k ohm, C1=6800pF, C2=3900pF (same as A500)
** - 1-pole RC 6dB/oct high-pass: R=1390 ohm (1000+390), C=22uF
*/
double R, C, R1, R2, C1, C2, cutoff, qfactor;
if (audio.oversamplingFlag)
dAudioFreq *= 2.0; // 2x oversampling
const bool audioWasntLocked = !audio.locked;
if (audioWasntLocked)
lockAudio();
// A500 1-pole (6db/oct) static RC low-pass filter:
R = 360.0; // R321 (360 ohm)
C = 1e-7; // C321 (0.1uF)
cutoff = 1.0 / (PT2_TWO_PI * R * C); // ~4420.971Hz
calcRCFilterCoeffs(dAudioFreq, cutoff, &filterLoA500);
// (optional) A1200 1-pole (6db/oct) static RC low-pass filter:
R = 680.0; // R321 (680 ohm)
C = 6.8e-9; // C321 (6800pF)
cutoff = 1.0 / (PT2_TWO_PI * R * C); // ~34419.322Hz
useA1200LowPassFilter = false;
if (dAudioFreq/2.0 > cutoff)
{
calcRCFilterCoeffs(dAudioFreq, cutoff, &filterLoA1200);
useA1200LowPassFilter = true;
}
// Sallen-Key low-pass filter ("LED" filter, same values on A500/A1200):
R1 = 10000.0; // R322 (10K ohm)
R2 = 10000.0; // R323 (10K ohm)
C1 = 6.8e-9; // C322 (6800pF)
C2 = 3.9e-9; // C323 (3900pF)
cutoff = 1.0 / (PT2_TWO_PI * pt2_sqrt(R1 * R2 * C1 * C2)); // ~3090.533Hz
qfactor = pt2_sqrt(R1 * R2 * C1 * C2) / (C2 * (R1 + R2)); // ~0.660225
calcLEDFilterCoeffs(dAudioFreq, cutoff, qfactor, &filterLED);
// A500 1-pole (6dB/oct) static RC high-pass filter:
R = 1390.0; // R324 (1K ohm) + R325 (390 ohm)
C = 2.233e-5; // C334 (22uF) + C335 (0.33uF)
cutoff = 1.0 / (PT2_TWO_PI * R * C); // ~5.128Hz
calcRCFilterCoeffs(dAudioFreq, cutoff, &filterHiA500);
// A1200 1-pole (6dB/oct) static RC high-pass filter:
R = 1390.0; // R324 (1K ohm resistor) + R325 (390 ohm resistor)
C = 2.2e-5; // C334 (22uF capacitor)
cutoff = 1.0 / (PT2_TWO_PI * R * C); // ~5.205Hz
calcRCFilterCoeffs(dAudioFreq, cutoff, &filterHiA1200);
if (audioWasntLocked)
unlockAudio();
}
void resetAmigaFilterStates(void)
{
const bool audioWasntLocked = !audio.locked;
if (audioWasntLocked)
lockAudio();
clearRCFilterState(&filterLoA500);
clearRCFilterState(&filterLoA1200);
clearRCFilterState(&filterHiA500);
clearRCFilterState(&filterHiA1200);
clearLEDFilterState(&filterLED);
if (audioWasntLocked)
unlockAudio();
}
static void processFiltersA1200_NoLED(double *dBufferL, double *dBufferR, int32_t numSamples)
{
if (useA1200LowPassFilter)
{
for (int32_t i = 0; i < numSamples; i++)
{
double dOut[2];
dOut[0] = dBufferL[i];
dOut[1] = dBufferR[i];
// low-pass filter
RCLowPassFilterStereo(&filterLoA1200, dOut, dOut);
// high-pass RC filter
RCHighPassFilterStereo(&filterHiA1200, dOut, dOut);
dBufferL[i] = dOut[0];
dBufferR[i] = dOut[1];
}
}
else
{
for (int32_t i = 0; i < numSamples; i++)
{
double dOut[2];
dOut[0] = dBufferL[i];
dOut[1] = dBufferR[i];
// high-pass RC filter
RCHighPassFilterStereo(&filterHiA1200, dOut, dOut);
dBufferL[i] = dOut[0];
dBufferR[i] = dOut[1];
}
}
}
static void processFiltersA1200_LED(double *dBufferL, double *dBufferR, int32_t numSamples)
{
if (useA1200LowPassFilter)
{
for (int32_t i = 0; i < numSamples; i++)
{
double dOut[2];
dOut[0] = dBufferL[i];
dOut[1] = dBufferR[i];
// low-pass filter
RCLowPassFilterStereo(&filterLoA1200, dOut, dOut);
// "LED" Sallen-Key filter
LEDFilter(&filterLED, dOut, dOut);
// high-pass RC filter
RCHighPassFilterStereo(&filterHiA1200, dOut, dOut);
dBufferL[i] = dOut[0];
dBufferR[i] = dOut[1];
}
}
else
{
for (int32_t i = 0; i < numSamples; i++)
{
double dOut[2];
dOut[0] = dBufferL[i];
dOut[1] = dBufferR[i];
// "LED" Sallen-Key filter
LEDFilter(&filterLED, dOut, dOut);
// high-pass RC filter
RCHighPassFilterStereo(&filterHiA1200, dOut, dOut);
dBufferL[i] = dOut[0];
dBufferR[i] = dOut[1];
}
}
}
static void processFiltersA500_NoLED(double *dBufferL, double *dBufferR, int32_t numSamples)
{
for (int32_t i = 0; i < numSamples; i++)
{
double dOut[2];
dOut[0] = dBufferL[i];
dOut[1] = dBufferR[i];
// low-pass RC filter
RCLowPassFilterStereo(&filterLoA500, dOut, dOut);
// high-pass RC filter
RCHighPassFilterStereo(&filterHiA500, dOut, dOut);
dBufferL[i] = dOut[0];
dBufferR[i] = dOut[1];
}
}
static void processFiltersA500_LED(double *dBufferL, double *dBufferR, int32_t numSamples)
{
for (int32_t i = 0; i < numSamples; i++)
{
double dOut[2];
dOut[0] = dBufferL[i];
dOut[1] = dBufferR[i];
// low-pass RC filter
RCLowPassFilterStereo(&filterLoA500, dOut, dOut);
// "LED" Sallen-Key filter
LEDFilter(&filterLED, dOut, dOut);
// high-pass RC filter
RCHighPassFilterStereo(&filterHiA500, dOut, dOut);
dBufferL[i] = dOut[0];
dBufferR[i] = dOut[1];
}
}
static void updateAmigaFilterFunctions(void)
{
if (filterModel == FILTERMODEL_A500)
{
if (ledFilterEnabled)
processAmigaFilters = processFiltersA500_LED;
else
processAmigaFilters = processFiltersA500_NoLED;
}
else // A1200
{
if (ledFilterEnabled)
processAmigaFilters = processFiltersA1200_LED;
else
processAmigaFilters = processFiltersA1200_NoLED;
}
}
void setAmigaFilterModel(uint8_t model)
{
const bool audioWasntLocked = !audio.locked;
if (audioWasntLocked)
lockAudio();
filterModel = model;
updateAmigaFilterFunctions();
if (audioWasntLocked)
unlockAudio();
}
void setLEDFilter(bool state)
{
if (ledFilterEnabled == state)
return; // same state as before!
const bool audioWasntLocked = !audio.locked;
if (audioWasntLocked)
lockAudio();
clearLEDFilterState(&filterLED);
ledFilterEnabled = editor.useLEDFilter;
updateAmigaFilterFunctions();
if (audioWasntLocked)
unlockAudio();
}
void toggleAmigaFilterModel(void)
{
const bool audioWasntLocked = !audio.locked;
if (audioWasntLocked)
lockAudio();
resetAmigaFilterStates();
filterModel ^= 1;
updateAmigaFilterFunctions();
if (audioWasntLocked)
unlockAudio();
if (filterModel == FILTERMODEL_A500)
displayMsg("AUDIO: AMIGA 500");
else
displayMsg("AUDIO: AMIGA 1200");
}