From 0c0472ce7613bc394acefd79209f021b656efade Mon Sep 17 00:00:00 2001
From: tildearrow <me@tildearrow.org>
Date: Fri, 5 May 2023 00:59:55 -0500
Subject: [PATCH] NES: add 20xx effect for changing DPCM freq

---
 papers/doc/7-systems/nes.md | 24 ++++++++++++++++++++++++
 src/engine/platform/nes.cpp | 30 +++++++++++++++++++++++++++---
 src/engine/platform/nes.h   |  1 +
 src/engine/sysDef.cpp       |  3 ++-
 4 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/papers/doc/7-systems/nes.md b/papers/doc/7-systems/nes.md
index 37d4a7037..dc8b635be 100644
--- a/papers/doc/7-systems/nes.md
+++ b/papers/doc/7-systems/nes.md
@@ -42,6 +42,30 @@ also known as Famicom. it is a five-channel sound generator: first two channels
   - `00`: PCM (software).
   - `01`: DPCM (hardware).
   - when in DPCM mode, samples will sound muffled (due to its nature), availables pitches are limited and loop point is ignored.
+- `20xx`: set DPCM frequency.
+  - only works in DPCM mode.
+  - see table below for possible values.
+
+# DPCM frequency table
+
+val | NTSC      | PAL
+----|-----------|-----------
+ 00 | 4181.7Hz  | 4177.4Hz
+ 01 | 4709.9Hz  | 4696.6Hz
+ 02 | 5264.0Hz  | 5261.4Hz
+ 03 | 5593.0Hz  | 5579.2Hz
+ 04 | 6257.9Hz  | 6023.9Hz
+ 05 | 7046.3Hz  | 7044.9Hz
+ 06 | 7919.3Hz  | 7917.2Hz
+ 07 | 8363.4Hz  | 8397.0Hz
+ 08 | 9419.9Hz  | 9446.6Hz
+ 09 | 11186.1Hz | 11233.8Hz
+ 0A | 12604.0Hz | 12595.5Hz
+ 0B | 13982.6Hz | 14089.9Hz
+ 0C | 16884.6Hz | 16965.4Hz
+ 0D | 21306.8Hz | 21315.5Hz
+ 0E | 24858.0Hz | 25191.0Hz
+ 0F | 33143.9Hz | 33252.1Hz
 
 # length counter table
 
diff --git a/src/engine/platform/nes.cpp b/src/engine/platform/nes.cpp
index d6b64ecff..974e71d51 100644
--- a/src/engine/platform/nes.cpp
+++ b/src/engine/platform/nes.cpp
@@ -337,14 +337,22 @@ void DivPlatformNES::tick(bool sysTick) {
           goingToLoop=parent->getSample(dacSample)->isLoopable();
           // write DPCM
           rWrite(0x4015,15);
-          rWrite(0x4010,calcDPCMRate(dacRate)|(goingToLoop?0x40:0));
+          if (nextDPCMFreq>=0) {
+            rWrite(0x4010,nextDPCMFreq|(goingToLoop?0x40:0));
+            nextDPCMFreq=-1;
+          } else {
+            rWrite(0x4010,calcDPCMRate(dacRate)|(goingToLoop?0x40:0));
+          }
           rWrite(0x4012,(dpcmAddr>>6)&0xff);
           rWrite(0x4013,dpcmLen&0xff);
           rWrite(0x4015,31);
           dpcmBank=dpcmAddr>>14;
         }
       } else {
-        if (dpcmMode) {
+        if (nextDPCMFreq>=0) {
+          rWrite(0x4010,nextDPCMFreq|(goingToLoop?0x40:0));
+          nextDPCMFreq=-1;
+        } else {
           rWrite(0x4010,calcDPCMRate(dacRate)|(goingToLoop?0x40:0));
         }
       }
@@ -353,6 +361,8 @@ void DivPlatformNES::tick(bool sysTick) {
     if (chan[4].keyOn) chan[4].keyOn=false;
     chan[4].freqChanged=false;
   }
+
+  nextDPCMFreq=-1;
 }
 
 int DivPlatformNES::dispatch(DivCommand c) {
@@ -406,7 +416,12 @@ int DivPlatformNES::dispatch(DivCommand c) {
             goingToLoop=parent->getSample(dacSample)->isLoopable();
             // write DPCM
             rWrite(0x4015,15);
-            rWrite(0x4010,calcDPCMRate(dacRate)|(goingToLoop?0x40:0));
+            if (nextDPCMFreq>=0) {
+              rWrite(0x4010,nextDPCMFreq|(goingToLoop?0x40:0));
+              nextDPCMFreq=-1;
+            } else {
+              rWrite(0x4010,calcDPCMRate(dacRate)|(goingToLoop?0x40:0));
+            }
             rWrite(0x4012,(dpcmAddr>>6)&0xff);
             rWrite(0x4013,dpcmLen&0xff);
             rWrite(0x4015,31);
@@ -555,6 +570,14 @@ int DivPlatformNES::dispatch(DivCommand c) {
       rWrite(0x4013,0);
       rWrite(0x4015,31);
       break;
+    case DIV_CMD_SAMPLE_FREQ: {
+      bool goingToLoop=parent->getSample(dacSample)->isLoopable();
+      if (dpcmMode) {
+        nextDPCMFreq=c.value&15;
+        rWrite(0x4010,(c.value&15)|(goingToLoop?0x40:0));
+      }
+      break;
+    }
     case DIV_CMD_SAMPLE_BANK:
       sampleBank=c.value;
       if (sampleBank>(parent->song.sample.size()/12)) {
@@ -658,6 +681,7 @@ void DivPlatformNES::reset() {
   dpcmMode=dpcmModeDefault;
   goingToLoop=false;
   countMode=false;
+  nextDPCMFreq=-1;
 
   if (useNP) {
     nes1_NP->Reset();
diff --git a/src/engine/platform/nes.h b/src/engine/platform/nes.h
index cbc08e2a1..2ab7ede20 100644
--- a/src/engine/platform/nes.h
+++ b/src/engine/platform/nes.h
@@ -52,6 +52,7 @@ class DivPlatformNES: public DivDispatch {
   unsigned char sampleBank;
   unsigned char writeOscBuf;
   unsigned char apuType;
+  signed char nextDPCMFreq;
   bool dpcmMode;
   bool dpcmModeDefault;
   bool dacAntiClickOn;
diff --git a/src/engine/sysDef.cpp b/src/engine/sysDef.cpp
index 6afbf4188..4e45df018 100644
--- a/src/engine/sysDef.cpp
+++ b/src/engine/sysDef.cpp
@@ -712,7 +712,8 @@ void DivEngine::registerSystems() {
       {0x15, {DIV_CMD_NES_ENV_MODE, "15xx: Set envelope mode (0: envelope, 1: length, 2: looping, 3: constant)"}},
       {0x16, {DIV_CMD_NES_LENGTH, "16xx: Set length counter (refer to manual for a list of values)"}},
       {0x17, {DIV_CMD_NES_COUNT_MODE, "17xx: Set frame counter mode (0: 4-step, 1: 5-step)"}},
-      {0x18, {DIV_CMD_SAMPLE_MODE, "18xx: Select PCM/DPCM mode (0: PCM; 1: DPCM)"}}
+      {0x18, {DIV_CMD_SAMPLE_MODE, "18xx: Select PCM/DPCM mode (0: PCM; 1: DPCM)"}},
+      {0x20, {DIV_CMD_SAMPLE_FREQ, "20xx: Set DPCM frequency (0 to F)"}}
     }
   );