From e2c1233b12605c75d14883078a877c386b0e93b8 Mon Sep 17 00:00:00 2001
From: Laurens Holst <laurens@grauw.nl>
Date: Fri, 13 May 2022 16:34:48 +0200
Subject: [PATCH] Reset period counter when writing to period registers.

This is what happens on real SCC. See also e.g. openMSX core:
https://github.com/openMSX/openMSX/blob/master/src/sound/SCC.cc#L390

To reduce the noise introduced by this, we add extra checks to only
write the period when it has actually changed.
---
 src/engine/platform/scc.cpp           | 10 ++++++++--
 src/engine/platform/scc.h             |  3 ++-
 src/engine/platform/sound/scc/scc.cpp |  2 ++
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/engine/platform/scc.cpp b/src/engine/platform/scc.cpp
index 88132b124..5c4809078 100644
--- a/src/engine/platform/scc.cpp
+++ b/src/engine/platform/scc.cpp
@@ -161,9 +161,14 @@ void DivPlatformSCC::tick(bool sysTick) {
       chan[i].freq=parent->calcFreq(chan[i].baseFreq,chan[i].pitch,true,0,chan[i].pitch2,chipClock,CHIP_DIVIDER)-1;
       if (chan[i].freq<0) chan[i].freq=0;
       if (chan[i].freq>4095) chan[i].freq=4095;
-      rWrite(regBase+0+i*2,chan[i].freq&0xff);
-      rWrite(regBase+1+i*2,chan[i].freq>>8);
+      if (!chan[i].freqInit || regPool[regBase+0+i*2]!=(chan[i].freq&0xff)) {
+        rWrite(regBase+0+i*2,chan[i].freq&0xff);
+      }
+      if (!chan[i].freqInit || regPool[regBase+1+i*2]!=(chan[i].freq>>8)) {
+        rWrite(regBase+1+i*2,chan[i].freq>>8);
+      }
       chan[i].freqChanged=false;
+      chan[i].freqInit=!skipRegisterWrites;
     }
   }
 }
@@ -286,6 +291,7 @@ void DivPlatformSCC::forceIns() {
   for (int i=0; i<5; i++) {
     chan[i].insChanged=true;
     chan[i].freqChanged=true;
+    chan[i].freqInit=false;
     if (isPlus || i<3) {
       updateWave(i);
     }
diff --git a/src/engine/platform/scc.h b/src/engine/platform/scc.h
index 11d6521c7..df6ef84e5 100644
--- a/src/engine/platform/scc.h
+++ b/src/engine/platform/scc.h
@@ -29,7 +29,7 @@
 class DivPlatformSCC: public DivDispatch {
   struct Channel {
     int freq, baseFreq, pitch, pitch2, note, ins;
-    bool active, insChanged, freqChanged, inPorta;
+    bool active, insChanged, freqChanged, freqInit, inPorta;
     signed char vol, outVol, wave;
     signed char waveROM[32] = {0}; // 4 bit PROM per channel on bubble system
     DivMacroInt std;
@@ -48,6 +48,7 @@ class DivPlatformSCC: public DivDispatch {
       active(false),
       insChanged(true),
       freqChanged(false),
+      freqInit(false),
       inPorta(false),
       vol(15),
       outVol(15),
diff --git a/src/engine/platform/sound/scc/scc.cpp b/src/engine/platform/sound/scc/scc.cpp
index db74d3f0e..427461e62 100644
--- a/src/engine/platform/sound/scc/scc.cpp
+++ b/src/engine/platform/sound/scc/scc.cpp
@@ -454,6 +454,7 @@ void scc_core::freq_vol_enable_w(u8 address, u8 data)
 			if (m_test.resetpos) // Reset address
 				m_voice[voice_freq].addr = 0;
 			m_voice[voice_freq].pitch = (m_voice[voice_freq].pitch & ~0x0ff) | data;
+			m_voice[voice_freq].counter = m_voice[voice_freq].pitch;
 			break;
 		case 0x1: // 0x*1 Voice 0 Pitch MSB
 		case 0x3: // 0x*3 Voice 1 Pitch MSB
@@ -463,6 +464,7 @@ void scc_core::freq_vol_enable_w(u8 address, u8 data)
 			if (m_test.resetpos) // Reset address
 				m_voice[voice_freq].addr = 0;
 			m_voice[voice_freq].pitch = (m_voice[voice_freq].pitch & ~0xf00) | (u16(bitfield(data, 0, 4)) << 8);
+			m_voice[voice_freq].counter = m_voice[voice_freq].pitch;
 			break;
 		case 0xa: // 0x*a Voice 0 Volume
 		case 0xb: // 0x*b Voice 1 Volume