From f3c346024eb6a25a070017189fa119e23a930f33 Mon Sep 17 00:00:00 2001
From: Persune <gumball2415@gmail.com>
Date: Mon, 8 Apr 2024 15:05:35 +0800
Subject: [PATCH] Implement nonlinear FDS DAC from plgDavid

---
 xgm/devices/Sound/nes_fds.cpp | 28 ++++++-------
 xgm/devices/Sound/nes_fds.h   | 74 ++++++++++++++++++++++++++++++++++-
 2 files changed, 85 insertions(+), 17 deletions(-)

diff --git a/xgm/devices/Sound/nes_fds.cpp b/xgm/devices/Sound/nes_fds.cpp
index 9d74c62f..627bf376 100644
--- a/xgm/devices/Sound/nes_fds.cpp
+++ b/xgm/devices/Sound/nes_fds.cpp
@@ -37,8 +37,8 @@ void NES_FDS::SetStereoMix(int trk, INT16 mixl, INT16 mixr)
 ITrackInfo *NES_FDS::GetTrackInfo(int trk)
 {
     trkinfo.max_volume = 32;
-    trkinfo.volume = last_vol;
-    trkinfo.key = last_vol > 0;
+    trkinfo.volume = vout;
+    trkinfo.key = vout > 0;
     trkinfo._freq = last_freq;
     trkinfo.freq = (double(last_freq) * clock) / (65536.0 * 64.0);
     trkinfo.tone = env_out[EMOD];
@@ -79,7 +79,7 @@ void NES_FDS::Reset ()
     master_io = true;
     master_vol = 0;
     last_freq = 0;
-    last_vol = 0;
+    vout = 0;
 
     rc_accum = 0;
 
@@ -225,21 +225,19 @@ void NES_FDS::Tick (UINT32 clocks)
     }
 
     // output volume caps at 32
-    INT32 vol_out = env_out[EVOL];
-    if (vol_out > 32) vol_out = 32;
+    // store for trackinfo and for later DAC calculation
+    vout = env_out[EVOL];
+    if (vout > 32) vout = 32;
 
     // final output
     if (!wav_write)
-        fout = wave[TWAV][(phase[TWAV]>>16)&0x3F] * vol_out;
+        wout = wave[TWAV][(phase[TWAV]>>16)&0x3F];
 
     // NOTE: during wav_halt, the unit still outputs (at phase 0)
     // and volume can affect it if the first sample is nonzero.
     // haven't worked out 100% of the conditions for volume to
     // effect (vol envelope does not seem to run, but am unsure)
     // but this implementation is very close to correct
-
-    // store for trackinfo
-    last_vol = vol_out;
 }
 
 UINT32 NES_FDS::Render (INT32 b[2])
@@ -247,13 +245,13 @@ UINT32 NES_FDS::Render (INT32 b[2])
     // 8 bit approximation of master volume
     const double MASTER_VOL = 2.4 * 1223.0; // max FDS vol vs max APU square (arbitrarily 1223)
     const double MAX_OUT = 32.0f * 63.0f; // value that should map to master vol
-    const INT32 MASTER[4] = {
-        int((MASTER_VOL / MAX_OUT) * 256.0 * 2.0f / 2.0f),
-        int((MASTER_VOL / MAX_OUT) * 256.0 * 2.0f / 3.0f),
-        int((MASTER_VOL / MAX_OUT) * 256.0 * 2.0f / 4.0f),
-        int((MASTER_VOL / MAX_OUT) * 256.0 * 2.0f / 5.0f) };
+    const double MASTER[4] = {
+        ((MASTER_VOL / MAX_OUT) * 2.0f / 2.0f),
+        ((MASTER_VOL / MAX_OUT) * 2.0f / 3.0f),
+        ((MASTER_VOL / MAX_OUT) * 2.0f / 4.0f),
+        ((MASTER_VOL / MAX_OUT) * 2.0f / 5.0f) };
 
-    INT32 v = fout * MASTER[master_vol] >> 8;
+    INT32 v = INT32((FDS_LUT_norm[wout] * double(63 * vout) * MASTER[master_vol]) + 0.5);
 
     // lowpass RC filter
     INT32 rc_out = ((rc_accum * rc_k) + (v * rc_l)) >> RC_BITS;
diff --git a/xgm/devices/Sound/nes_fds.h b/xgm/devices/Sound/nes_fds.h
index 212e472b..0e969c3e 100644
--- a/xgm/devices/Sound/nes_fds.h
+++ b/xgm/devices/Sound/nes_fds.h
@@ -26,14 +26,14 @@ class NES_FDS : public ISoundChip
     double rate, clock;
     int mask;
     INT32 sm[2]; // stereo mix
-    INT32 fout; // current output
+    UINT32 wout; // current output
+    UINT32 vout; // current volume
     TrackInfoFDS trkinfo;
     int option[OPT_END];
 
     bool master_io;
     UINT32 master_vol;
     UINT32 last_freq; // for trackinfo
-    UINT32 last_vol;  // for trackinfo
 
     // two wavetables
     enum { TMOD=0, TWAV=1 };
@@ -61,6 +61,76 @@ class NES_FDS : public ISoundChip
     INT32 rc_k;
     INT32 rc_l;
 
+    // nonlinear DAC LUT
+    // data from plgDavid, normalized
+    // data capture shared from the NESDev server
+    const float FDS_LUT_norm[64] = {
+        0.0,
+        0.011465572752058506,
+        0.0233255997300148,
+        0.03682375326752663,
+        0.04673049971461296,
+        0.061058409512043,
+        0.07386837154626846,
+        0.09098339825868607,
+        0.09331251680850983,
+        0.10951442271471024,
+        0.12239760905504227,
+        0.1415075808763504,
+        0.1483096331357956,
+        0.16800136864185333,
+        0.18295270204544067,
+        0.20666064321994781,
+        0.18730713427066803,
+        0.20707780122756958,
+        0.2199448049068451,
+        0.2434738427400589,
+        0.24558208882808685,
+        0.26910510659217834,
+        0.28437408804893494,
+        0.312602698802948,
+        0.29783013463020325,
+        0.32291364669799805,
+        0.33768296241760254,
+        0.3677976429462433,
+        0.36768317222595215,
+        0.3981393277645111,
+        0.4163309335708618,
+        0.4529191851615906,
+        0.3774969279766083,
+        0.40523025393486023,
+        0.41825342178344727,
+        0.45081785321235657,
+        0.44415655732154846,
+        0.4759543240070343,
+        0.4921776056289673,
+        0.5303648114204407,
+        0.4969009757041931,
+        0.5296915769577026,
+        0.5450936555862427,
+        0.5845786333084106,
+        0.576058030128479,
+        0.6141541004180908,
+        0.6341322660446167,
+        0.6813235282897949,
+        0.6059473752975464,
+        0.6424090266227722,
+        0.6577944159507751,
+        0.7020274996757507,
+        0.6884633898735046,
+        0.7309963703155518,
+        0.7510766983032227,
+        0.802958607673645,
+        0.7523477077484131,
+        0.7961556911468506,
+        0.8153874278068542,
+        0.869225800037384,
+        0.8552623987197876,
+        0.9073793292045593,
+        0.9342948198318481,
+        1.0
+    };
+
 public:
     NES_FDS ();
     virtual ~ NES_FDS ();