APU: fix regression in envelope speed and add comments about envelope…

… and sweep edge-cases

src/nba/src/hw/apu/channel/envelope.hpp

@@ -9,6 +9,14 @@
 
 namespace nba::core {
 
+/**
+ * TODO:
+ * - Figure out how the envelope timer behaves when the envelope speed (divider) changes mid-note.
+ *   - Is the new value loaded into the counter right away or on the next reload?
+ * - Is the timer ticked when the envelope is deactivated (divider == 0)?
+ *    - If so, what value is loaded into the timer on channel restart?
+ */
+
 class Envelope {
 public:
   void Reset() {
@@ -25,7 +33,7 @@ class Envelope {
   }
 
   void Tick() {
-    if(step == 0) {
+    if(step == 1) {
       step = divider;
 
       if(active && divider != 0) {
@@ -44,7 +52,7 @@ class Envelope {
         }
       }
     } else {
-      step--;
+      step = (step - 1) & 7;
     }
   }
 

src/nba/src/hw/apu/channel/sweep.hpp

@@ -9,6 +9,11 @@
 
 namespace nba::core {
 
+/**
+ * TODO: figure out specifics of how the sweep timer works when the sweep speed (divider) is set to zero
+ * or changed in the middle of a note.
+ */
+
 class Sweep {
 public:
   void Reset() {