chassis/plugin/digital.cpp

/*
   Chassis polysynth framework

   Copyright 2024 Gordon JC Pearce <gordonjcp@gjcp.net>

   Permission to use, copy, modify, and/or distribute this software for any
   purpose with or without fee is hereby granted, provided that the above
   copyright notice and this permission notice appear in all copies.

   THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
   WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
   MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
   SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
   WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
   OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
   CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/

// contains the emulation of the digital bits

#include <cmath>
#include <cstdio>

#include "voice.hpp"

bool Voice::isFree() {
    return ff10 == false;
}

void Voice::on(uint32_t key, bool reset = 0) {
    // what's with the crazy private variables and all the gotos with crazy labels?
    // this code emulates the 78C11 code directly (probably inefficiently)
    // to allow for documenting what the variables actually do
    // they're really bitfields holding a bit for each voice

    // this current implementation doesn't reset the voice
    (void)reset;

    ff10 = true;  // note held from keyboard
    ff07 = true;  // attack phase
    if (note == key) goto h0144;
    note = key;
    if (ff11) goto h013e;
h0132:
    if (ff33) goto h0149;  // sustained
    ff33 = false;
    goto h0149;

h013e:
    ff00 = true;  // in a real one, voice counter needs programmed
    goto h0149;

h0144:
    if (!ff11) goto h0132;  // unsure, copied from ff10 at start of mainloop

h0149:
    // this is in the wrong place really but is the equivalent of programming the counter
    // and VCO ramp DAC
    omega = (261.63 * powf(2, (note - 60) / 12.0f)) / 48000.0f;
}

void Voice::off() {
    bool sustain = false;
    ff10 = false;
    if (!sustain) {  // dummy sustain
        ff33 = false;
    }
}

void Voice::gate(Synth &s) {
    uint16_t bc, ea = env;

    ff11 = ff10;

    // 0509
    if (!ff11) goto h0538;

    // 050e
    if (!ff33) goto h0563;

    // 0513
    if (!ff07) goto h051e;

h0517:
    ff07 = false;

h051e:
    bc = s.patchRam.env_s << 7;  // half scale

    if (ea < bc) ea = bc;

    ea -= bc;
    bc = ea;

    ea = (ea * decay_table[s.patchRam.env_d]) >> 16;
    ea += s.patchRam.env_s << 7;
    // printf("returning from decay phase\n");
    goto h0590;

h0538:
    // printf("got to 0x0538\n");
    if (!ff07) goto h054a;  // note on? if not skip ahead
    // 053c
    if (ff08) goto h0517;
    ff07 = false;
h054a:
    // printf("release phase\n");
    ff33 = false;
    ff08 = false;
    bc = ea;
    ea = (ea * decay_table[s.patchRam.env_r]) >> 16;
    // printf("returning from release phase\n");
    goto h0590;

h0563:
    // printf("attack phase\n");
    ff08 = false;
    ea += attack_table[s.patchRam.env_a];
    if (ea & 0xc000) {
        ea = 0x3fff;
        ff33 = true;
        ff08 = true;
    }

h0590:
    env = ea;
    // printf("%04x %d %d %d %d %d \n", ea, ff07, ff08, ff10, ff11, ff33);
}

void Synth::runLFO() {
    // compute a loop's worth of LFO

    uint16_t bc, ea;

    // 074e
    ea = ff4d;  // lfo value
    bc = lfoRateTable[patchRam.lfoRate];

    // bit zero is low for rising slope, high for falling
    if (!(ff4a & 0x01)) goto h078b;

    // 075b DSUBNB EA, BC  subtract BC from EA, skip next instruction if EA < BC
    // 075d JRE 079a       routine that handles flipping from down to up
    ea -= bc;
    if (ea < bc) goto h079a;

h075f:
    ff4d = ea;  // LFO output variable

    // bit one seems to be used to represent negative values of LFO
    if (!(ff4a & 0x02)) goto h07a2;  // routine that adds on 0x2000 to ea

    // 0765 LFO is negative (bit 1 is high) so invert the value of EA
    // so that we have a positive-only LFO running from 0 to 0x3fff
    bc = ea;
    ea = 0x2000;
    ea -= bc;
h076b:
    bc = ea;                        // BC now contains an LFO range from 0 to 0x3fff, always positive
    if (patchRam.switch2 & 0x01) {  // LFO Manual?
        bc = 0x3fff;                // fixed maximum value
    }

    // 0771
    bc = (bc * patchRam.pwmLfo) >> 7;  // scale by PWM pot amount

    // 077d
    bc = 0x3fff - bc;  // invert so pot = 0 gives 0x3fff

    // test if squarewave is on or off - if it's off set PW to 0
    if (!(patchRam.switch1 & 0x08)) bc = 0x0000;  // square off

    // final computed PWM value
    ff4f = bc;

    // 078a
    goto h07a9;

h078b:
    // BC contains rate, EA contains LFO value
    ea += bc;
    if (ea & 0xe000) {  // if we've exceeded 0x1fff
        ea = 0x1fff;    // clamp
        ff4a++;         // increment the flags
    }
    goto h075f;  // store in LFO output variable

h079a:
    ea = 0;     // output is close (enough) to zero, clamp
    ff4a++;     // increment the flags
    goto h075f; // store in LFO output variable

h07a2:          // LFO output is positive
    ea += 0x2000;   // add on 0x2000 to scale PWM to 0 - 0x3fff
    goto h076b;     // jump back to scale LFO amount

h07a9:
    return;
}