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more comments in LFO code

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Gordon JC Pearce 2024-09-09 08:51:37 +01:00
parent 1d4e6ab2bd
commit 6bf1cc1da1
1 changed files with 36 additions and 22 deletions
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56
plugin/digital.cpp
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@ -131,55 +131,69 @@ h0590:
void Synth::runLFO() {
// compute a loop's worth of LFO
uint16_t a, b, c, d;
uint16_t bc, hl, ea, tos;
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;
if (!(ff4a & 0x02)) goto h07a2;
// 0765
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;
if (patchRam.switch2 & 0x01) {
bc = 0x3fff;
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
}
bc = (bc * patchRam.pwmLfo) >> 7;
bc = 0x3fff - bc;
// 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) {
ea = 0x1fff;
ff4a++;
if (ea & 0xe000) { // if we've exceeded 0x1fff
ea = 0x1fff; // clamp
ff4a++; // increment the flags
}
goto h075f;
goto h075f; // store in LFO output variable
h079a:
ea = 0;
ff4a++;
goto h075f;
ea = 0; // output is close (enough) to zero, clamp
ff4a++; // increment the flags
goto h075f; // store in LFO output variable
h07a2:
ea += 0x2000;
goto h076b;
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;