From 6bf1cc1da1d46ff4ad2682901d51969981b1704c Mon Sep 17 00:00:00 2001
From: Gordon JC Pearce <gordonjcp@gmail.com>
Date: Mon, 9 Sep 2024 08:51:37 +0100
Subject: [PATCH] more comments in LFO code

---
 plugin/digital.cpp | 58 ++++++++++++++++++++++++++++------------------
 1 file changed, 36 insertions(+), 22 deletions(-)

diff --git a/plugin/digital.cpp b/plugin/digital.cpp
index a26677e..8fb5486 100644
--- a/plugin/digital.cpp
+++ b/plugin/digital.cpp
@@ -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;
-    if (!(patchRam.switch1 & 0x08)) bc = 0x0000;   // square off
+    // 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;