From 21cf4d3cd4fee5c83b52ea50afaf4f178edd301a Mon Sep 17 00:00:00 2001
From: Tony Konzel <saxxonpike@gmail.com>
Date: Thu, 9 Jun 2022 07:05:08 -0500
Subject: [PATCH] sound/k054539.cpp: Fixed DPCM table, and added cubic
 interpolation. (#9778)

Using powers of 2 rather than squares for the DPCM table fixes DC offsets for 4-bit sample playback.
---
 src/devices/sound/k054539.cpp | 59 ++++++++++++++++++++++++++++++-----
 src/devices/sound/k054539.h   |  4 +++
 2 files changed, 56 insertions(+), 7 deletions(-)

diff --git a/src/devices/sound/k054539.cpp b/src/devices/sound/k054539.cpp
index ae0ac92e310..639303cefac 100644
--- a/src/devices/sound/k054539.cpp
+++ b/src/devices/sound/k054539.cpp
@@ -104,13 +104,34 @@ void k054539_device::keyoff(int channel)
 		regs[0x22c] &= ~(1 << channel);
 }
 
+void k054539_device::advance_filter(int channel, int val)
+{
+	auto &hist = filter_hist[channel];
+	hist[0] = hist[1];
+	hist[1] = hist[2];
+	hist[2] = hist[3];
+	hist[3] = ((float) val) / 32768.0f;
+}
+
+float k054539_device::calculate_filter(int channel, float t)
+{
+	// Cubic hermite interpolation
+	// t is domain [0,1]
+
+	auto &hist = filter_hist[channel];
+	const float a = (-hist[0] / 2.0f) + (3.0f * hist[1] / 2.0f) - (3.0f * hist[2] / 2.0f) + (hist[3] / 2.0f);
+	const float b = hist[0] - (5.0f * hist[1] / 2.0f) + (2.0f * hist[2]) - (hist[3] / 2.0f);
+	const float c = (-hist[0] / 2.0f) + (hist[2] / 2.0f);
+	return (t * t * t * a) + (t * t * b) + (t * c) + hist[1];
+}
+
 void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_stream_view> const &inputs, std::vector<write_stream_view> &outputs)
 {
 #define VOL_CAP 1.80
 
 	static const int16_t dpcm[16] = {
-		0 * 0x100,     1 * 0x100,   4 * 0x100,   9 * 0x100,  16 * 0x100, 25 * 0x100, 36 * 0x100, 49 * 0x100,
-		-64 * 0x100, -49 * 0x100, -36 * 0x100, -25 * 0x100, -16 * 0x100, -9 * 0x100, -4 * 0x100, -1 * 0x100
+		0 * 0x100,   1 * 0x100,   2 * 0x100,   4 * 0x100,  8 * 0x100, 16 * 0x100, 32 * 0x100, 64 * 0x100,
+		0 * 0x100, -64 * 0x100, -32 * 0x100, -16 * 0x100, -8 * 0x100, -4 * 0x100, -2 * 0x100, -1 * 0x100
 	};
 
 
@@ -195,6 +216,8 @@ void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_
 					cur_pval = chan->pval;
 				}
 
+				float filter_frac = 0;
+
 				switch(base2[0] & 0xc) {
 				case 0x0: { // 8bit pcm
 					cur_pfrac += delta;
@@ -208,12 +231,17 @@ void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_
 							cur_pos = (base1[0x08] | (base1[0x09] << 8) | (base1[0x0a] << 16));
 							cur_val = (int16_t)(read_byte(cur_pos) << 8);
 						}
+
 						if(cur_val == (int16_t)0x8000) {
 							keyoff(ch);
 							cur_val = 0;
 							break;
 						}
+
+						advance_filter(ch, cur_val);
 					}
+
+					filter_frac = (float)(cur_pfrac & 0xffff) / 65536.0f;
 					break;
 				}
 
@@ -231,12 +259,17 @@ void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_
 							cur_pos = (base1[0x08] | (base1[0x09] << 8) | (base1[0x0a] << 16));
 							cur_val = (int16_t)(read_byte(cur_pos) | read_byte(cur_pos+1)<<8);
 						}
+
 						if(cur_val == (int16_t)0x8000) {
 							keyoff(ch);
 							cur_val = 0;
 							break;
 						}
+
+						advance_filter(ch, cur_val);
 					}
+
+					filter_frac = (float)(cur_pfrac & 0xffff) / 65536.0f;
 					break;
 				}
 
@@ -273,8 +306,12 @@ void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_
 							cur_val = -32768;
 						else if(cur_val > 32767)
 							cur_val = 32767;
+
+						advance_filter(ch, cur_val);
 					}
 
+					filter_frac = (float)(cur_pfrac & 0xffff) / 65536.0f;
+
 					cur_pfrac >>= 1;
 					if(cur_pos & 1)
 						cur_pfrac |= 0x8000;
@@ -285,9 +322,12 @@ void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_
 					LOG("Unknown sample type %x for channel %d\n", base2[0] & 0xc, ch);
 					break;
 				}
-				lval += cur_val * lvol;
-				rval += cur_val * rvol;
-				rbase[(rdelta + reverb_pos) & 0x1fff] += int16_t(cur_val*rbvol);
+
+				const float filter_val = calculate_filter(ch, filter_frac);
+
+				lval += filter_val * lvol;
+				rval += filter_val * rvol;
+				rbase[(rdelta + reverb_pos) & 0x1fff] += int16_t(filter_val*rbvol);
 
 				chan->pos = cur_pos;
 				chan->pfrac = cur_pfrac;
@@ -299,10 +339,13 @@ void k054539_device::sound_stream_update(sound_stream &stream, std::vector<read_
 					base1[0x0d] = cur_pos>> 8 & 0xff;
 					base1[0x0e] = cur_pos>>16 & 0xff;
 				}
+			} else {
+				// Fill the interpolation vectors with silence when channel is disabled
+				advance_filter(ch, 0);
 			}
 		reverb_pos = (reverb_pos + 1) & 0x1fff;
-		outputs[0].put_int(sample, lval, 32768);
-		outputs[1].put_int(sample, rval, 32768);
+		outputs[0].put(sample, lval);
+		outputs[1].put(sample, rval);
 	}
 }
 
@@ -317,6 +360,7 @@ void k054539_device::init_chip()
 {
 	memset(regs, 0, sizeof(regs));
 	memset(posreg_latch, 0, sizeof(posreg_latch)); //*
+	memset(filter_hist, 0, sizeof(filter_hist));
 	flags |= UPDATE_AT_KEYON; //* make it default until proven otherwise
 
 	ram = std::make_unique<uint8_t []>(0x4000);
@@ -339,6 +383,7 @@ void k054539_device::init_chip()
 	save_item(NAME(cur_ptr));
 	save_item(NAME(cur_limit));
 	save_item(NAME(rom_addr));
+	save_item(NAME(filter_hist));
 
 	save_item(NAME(m_timer_state));
 }
diff --git a/src/devices/sound/k054539.h b/src/devices/sound/k054539.h
index 406836af65a..8581957318e 100644
--- a/src/devices/sound/k054539.h
+++ b/src/devices/sound/k054539.h
@@ -88,6 +88,8 @@ private:
 	uint8_t posreg_latch[8][3];
 	int flags;
 
+	float filter_hist[8][4];
+
 	unsigned char regs[0x230];
 	std::unique_ptr<uint8_t []> ram;
 	int reverb_pos;
@@ -108,6 +110,8 @@ private:
 	void keyon(int channel);
 	void keyoff(int channel);
 	void init_chip();
+	void advance_filter(int channel, int val);
+	float calculate_filter(int channel, float t);
 };
 
 DECLARE_DEVICE_TYPE(K054539, k054539_device)