From 5069cf65e3d04c78eaae036f7af6756fecc16d24 Mon Sep 17 00:00:00 2001
From: tildearrow <me@tildearrow.org>
Date: Thu, 9 Mar 2023 03:37:14 -0500
Subject: [PATCH] Amiga: more accurate emulation

https://www.youtube.com/watch?v=xyQlmsD7PAg&t=403s

thanks TakuikaNinja for the info!
---
 src/engine/platform/amiga.cpp | 20 +++++++++++++++++++-
 src/engine/platform/amiga.h   |  4 ++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/engine/platform/amiga.cpp b/src/engine/platform/amiga.cpp
index 3dbdb123..96a1c214 100644
--- a/src/engine/platform/amiga.cpp
+++ b/src/engine/platform/amiga.cpp
@@ -23,6 +23,7 @@
 #include <math.h>
 
 #define AMIGA_DIVIDER 8
+#define AMIGA_VPMASK 7
 #define CHIP_DIVIDER 16
 
 const char* regCheatSheetAmiga[]={
@@ -81,6 +82,7 @@ void DivPlatformAmiga::acquire(short** buf, size_t len) {
     outL=0;
     outR=0;
     for (int i=0; i<4; i++) {
+      chan[i].volPos=(chan[i].volPos+1)&AMIGA_VPMASK;
       if (!chan[i].active) {
         oscBuf[i]->data[oscBuf[i]->needle++]=0;
         continue;
@@ -125,7 +127,13 @@ void DivPlatformAmiga::acquire(short** buf, size_t len) {
         }
       }
       if (!isMuted[i]) {
-        output=chan[i].audDat*chan[i].outVol;
+        if (chan[i].outVol>=64) {
+          output=chan[i].audDat<<6;
+        } else if (chan[i].outVol<=0) {
+          output=0;
+        } else {
+          output=chan[i].audDat*volTable[chan[i].outVol][chan[i].volPos];
+        }
         if (i==0 || i==3) {
           outL+=(output*sep1)>>7;
           outR+=(output*sep2)>>7;
@@ -467,6 +475,16 @@ int DivPlatformAmiga::init(DivEngine* p, int channels, int sugRate, const DivCon
     oscBuf[i]=new DivDispatchOscBuffer;
     isMuted[i]=false;
   }
+
+  // Paula volume is implemented using PWM rather than a multiplication.
+  // source: https://www.youtube.com/watch?v=xyQlmsD7PAg
+  memset(volTable,0,64*64);
+  for (int i=0; i<64; i++) {
+    for (int j=0; j<64; j++) {
+      volTable[i][j/AMIGA_DIVIDER]+=(j<i)*(64/AMIGA_DIVIDER);
+    }
+  }
+
   setFlags(flags);
   reset();
   return 6;
diff --git a/src/engine/platform/amiga.h b/src/engine/platform/amiga.h
index e6048a85..921d1294 100644
--- a/src/engine/platform/amiga.h
+++ b/src/engine/platform/amiga.h
@@ -31,6 +31,7 @@ class DivPlatformAmiga: public DivDispatch {
     unsigned int audPos;
     int audSub;
     signed char audDat;
+    unsigned char volPos;
     int sample, wave;
     int busClock;
     bool useWave, setPos, useV, useP;
@@ -42,6 +43,7 @@ class DivPlatformAmiga: public DivDispatch {
       audPos(0),
       audSub(0),
       audDat(0),
+      volPos(0),
       sample(-1),
       wave(-1),
       busClock(0),
@@ -61,6 +63,8 @@ class DivPlatformAmiga: public DivDispatch {
   int filtConst;
   int filtConstOff, filtConstOn;
 
+  unsigned char volTable[64][64];
+
   int sep1, sep2;
 
   friend void putDispatchChip(void*,int);