Identify CHANNEL_FIELD16 as CHANNEL_NOTE_DURATION_MODIFIER (#1162)

audio/engine.asm

@@ -2205,8 +2205,8 @@ SetNoteDuration:
 	ld e, [hl]
 	inc hl
 	ld d, [hl]
-	; add ??? to the next result
-	ld hl, CHANNEL_FIELD16
+	; add duration modifier to the next result
+	ld hl, CHANNEL_NOTE_DURATION_MODIFIER
 	add hl, bc
 	ld l, [hl]
 	; multiply Tempo by last result (NoteLength * LOW(delay))
@@ -2214,11 +2214,10 @@ SetNoteDuration:
 	; copy result to de
 	ld e, l
 	ld d, h
-	; store result in ???
-	ld hl, CHANNEL_FIELD16
+	; store result in NoteDuration and NoteDurationModifier
+	ld hl, CHANNEL_NOTE_DURATION_MODIFIER
 	add hl, bc
 	ld [hl], e
-	; store result in NoteDuration
 	ld hl, CHANNEL_NOTE_DURATION
 	add hl, bc
 	ld [hl], d
@@ -2283,9 +2282,9 @@ Tempo:
 	ld [hl], e
 	inc hl
 	ld [hl], d
-	; clear ???
+	; clear duration modifier
 	xor a
-	ld hl, CHANNEL_FIELD16
+	ld hl, CHANNEL_NOTE_DURATION_MODIFIER
 	add hl, bc
 	ld [hl], a
 	ret

constants/audio_constants.asm

@@ -49,7 +49,7 @@ DEF CHANNEL_PITCH                       rb
 DEF CHANNEL_OCTAVE                      rb
 DEF CHANNEL_TRANSPOSITION               rb
 DEF CHANNEL_NOTE_DURATION               rb
-DEF CHANNEL_FIELD16                     rb
+DEF CHANNEL_NOTE_DURATION_MODIFIER      rb
                                         rb_skip
 DEF CHANNEL_LOOP_COUNT                  rb
 DEF CHANNEL_TEMPO                       rw

macros/ram.asm

@@ -142,7 +142,7 @@ MACRO channel_struct
 \1Flags3::                   db ; 0:vibrato up/down 1:pitch slide direction
 \1MusicAddress::             dw
 \1LastMusicAddress::         dw
-                      dw
+                             ds 2
 \1NoteFlags::                db ; 5:rest
 \1Condition::                db ; conditional jumps
 \1DutyCycle::                db ; bits 6-7 (0:12.5% 1:25% 2:50% 3:75%)
@@ -152,7 +152,7 @@ MACRO channel_struct
 \1Octave::                   db ; 7-0 (0 is highest)
 \1Transposition::            db ; raises existing octaves (to repeat phrases)
 \1NoteDuration::             db ; frames remaining for the current note
-\1Field16::           ds 1
+\1NoteDurationModifier::     db
                              ds 1
 \1LoopCount::                db
 \1Tempo::                    dw