/*
 * This file is part of libsidplayfp, a SID player engine.
 *
 * Copyright 2011-2024 Leandro Nini <drfiemost@users.sourceforge.net>
 * Copyright 2007-2010 Antti Lankila
 * Copyright 2004,2010 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

#ifndef FILTER6581_H
#define FILTER6581_H

#include "Filter.h"
#include "FilterModelConfig6581.h"
#include "Integrator6581.h"

#include "sidcxx11.h"

namespace reSIDfp
{

class Integrator6581;

/**
 * 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 filter / mixer / output
 * ---------------------------
 * ~~~
 *               +---------------------------------------------------+
 *               |                                                   |
 *               |                        +--1R1-- \--+ D7           |
 *               |             +---R1--+  |           |              |
 *               |             |       |  o--2R1-- \--o D6           |
 *               |   +---------o--<A]--o--o           |     $17      |
 *               |   |                    o--4R1-- \--o D5  1=open   | (3.5R1)
 *               |   |                    |           |              |
 *               |   |                    +--8R1-- \--o D4           | (7.0R1)
 *               |   |                                |              |
 * $17           |   |                    (CAP2B)     |  (CAP1B)     |
 * 0=to mixer    |   +--R8--+  +---R8--+      +---C---o      +---C---o
 * 1=to filter   |          |  |       |      |       |      |       |
 *                ------R8--o--o--[A>--o--Rw--o--[A>--o--Rw--o--[A>--o
 *     ve (EXT IN)          |          |              |              |
 * D3  \ ---------------R8--o          |              | (CAP2A)      | (CAP1A)
 *     |   v3               |          | vhp          | vbp          | vlp
 * D2  |   \ -----------R8--o    +-----+              |              |
 *     |   |   v2           |    |                    |              |
 * D1  |   |   \ -------R8--o    |   +----------------+              |
 *     |   |   |   v1       |    |   |                               |
 * D0  |   |   |   \ ---R8--+    |   |   +---------------------------+
 *     |   |   |   |             |   |   |
 *     R6  R6  R6  R6            R6* R6* R6*
 *     |   |   |   | $18         |   |   |  $18
 *     |    \  |   | D7: 1=open   \   \   \ D6 - D4: 0=open
 *     |   |   |   |             |   |   |
 *     +---o---o---o-------------o---o---+                         12V
 *                 |
 *                 |               D3 +--/ --1R2--+                 |
 *                 |   +---R8--+      |           |  +---R2--+      |
 *                 |   |       |   D2 o--/ --2R2--o  |       |  ||--+
 *                 +---o--[A>--o------o           o--o--[A>--o--||
 *                                 D1 o--/ --4R2--o (4.25R2)    ||--+
 *                        $18         |           |                 |
 *                        0=open   D0 +--/ --8R2--+ (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
 *     R6* ~  1.07*R6
 *     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--o
 *         |    __|__
 *         |    -----
 *         |    |   |
 * vi -----o----+   +--o----- 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 --o-R-o-R-o-R-o-R-o-R-o-R-o-R-o-R-o-R-o-R-o-R-o-   -+
 *
 *
 * 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
 *
 *                         |
 *             +-----------o
 *             |           |
 *             |    +------o
 *             |    |      |
 *             |    |  ||--+
 *             |    +--||
 *             |       ||--+
 *         ||--+           |
 * vi -----||              o---o----- vo
 *         ||--+           |   |
 *             |       ||--+   |
 *             |-------||      |
 *             |       ||--+   |
 *         ||--+           |   |
 *      +--||              |   |
 *      |  ||--+           |   |
 *      |      |           |   |
 *      |      +-----------o   |
 *      |                  |   |
 *      |                      |
 *      |                 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 -----||
 *         ||--+
 *             |
 *             o------ 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.
 */
class Filter6581 final : public Filter
{
private:
    /// VCR + associated capacitor connected to highpass output.
    Integrator6581 hpIntegrator;

    /// VCR + associated capacitor connected to bandpass output.
    Integrator6581 bpIntegrator;

    const unsigned short* f0_dac;

protected:
    /**
     * Set filter cutoff frequency.
     */
    void updateCenterFrequency() override;

    int solveIntegrators() override;

public:
    Filter6581() :
        Filter(*FilterModelConfig6581::getInstance()),
        hpIntegrator(*FilterModelConfig6581::getInstance()),
        bpIntegrator(*FilterModelConfig6581::getInstance()),
        f0_dac(FilterModelConfig6581::getInstance()->getDAC(0.5))
    {}

    ~Filter6581() override;

    /**
     * Set filter curve type based on single parameter.
     *
     * @param curvePosition 0 .. 1, where 0 sets center frequency high ("bright") and 1 sets it low ("dark").
     *                      Default is 0.5
     */
    void setFilterCurve(double curvePosition);

    /**
     * Set filter offset and range based on single parameter.
     *
     * @param adjustment 0 .. 1, where 0 sets center frequency low ("dark"), 1 sets it high ("bright").
     *                   This also affects the range. Default is 0.5
     */
    void setFilterRange(double adjustment);
};

} // namespace reSIDfp

#endif
