From 9310d2cac849077a946b332ad3b3b70cf0ac6573 Mon Sep 17 00:00:00 2001
From: cam900 <dbtlrchl@naver.com>
Date: Sat, 30 May 2020 23:28:04 +0900
Subject: [PATCH] nes_apu.cpp: Correct noise algorithm

---
 src/devices/sound/nes_apu.cpp | 39 +++++------------------------------
 src/devices/sound/nes_apu.h   |  1 -
 src/devices/sound/nes_defs.h  |  6 +-----
 3 files changed, 6 insertions(+), 40 deletions(-)

diff --git a/src/devices/sound/nes_apu.cpp b/src/devices/sound/nes_apu.cpp
index 27ef8977f72..3f5c7787a93 100644
--- a/src/devices/sound/nes_apu.cpp
+++ b/src/devices/sound/nes_apu.cpp
@@ -22,7 +22,7 @@
 
  *****************************************************************************
 
-   NES_APU.C
+   NES_APU.CPP
 
    Actual NES APU interface.
 
@@ -81,23 +81,6 @@ void nesapu_device::create_syncs(unsigned long sps)
 	}
 }
 
-/* INITIALIZE NOISE LOOKUP TABLE */
-static void create_noise(u8 *buf, const int bits, int size)
-{
-	int m = 0x0011;
-	int xor_val, i;
-
-	for (i = 0; i < size; i++)
-	{
-		xor_val = m & 1;
-		m >>= 1;
-		xor_val ^= (m & 1);
-		m |= xor_val << (bits - 1);
-
-		buf[i] = m;
-	}
-}
-
 DEFINE_DEVICE_TYPE(NES_APU, nesapu_device, "nesapu", "N2A03 APU")
 
 nesapu_device::nesapu_device(const machine_config &mconfig, const char *tag, device_t *owner, u32 clock)
@@ -109,11 +92,6 @@ nesapu_device::nesapu_device(const machine_config &mconfig, const char *tag, dev
 	, m_irq_handler(*this)
 	, m_mem_read_cb(*this)
 {
-	for (auto & elem : m_noise_lut)
-	{
-		elem = 0;
-	}
-
 	for (auto & elem : m_vbl_times)
 	{
 		elem = 0;
@@ -169,8 +147,6 @@ void nesapu_device::device_start()
 	m_irq_handler.resolve_safe();
 	m_mem_read_cb.resolve_safe(0x00);
 
-	create_noise(m_noise_lut, 13, apu_t::NOISE_LONG);
-
 	calculate_rates();
 
 	/* register for save */
@@ -199,7 +175,7 @@ void nesapu_device::device_start()
 	save_item(NAME(m_APU.tri.enabled));
 
 	save_item(NAME(m_APU.noi.regs));
-	save_item(NAME(m_APU.noi.cur_pos));
+	save_item(NAME(m_APU.noi.seed));
 	save_item(NAME(m_APU.noi.vbl_length));
 	save_item(NAME(m_APU.noi.phaseacc));
 	save_item(NAME(m_APU.noi.output_vol));
@@ -409,12 +385,7 @@ s8 nesapu_device::apu_noise(apu_t::noise_t *chan)
 	while (chan->phaseacc < 0)
 	{
 		chan->phaseacc += freq;
-
-		chan->cur_pos++;
-		if (apu_t::NOISE_SHORT == chan->cur_pos && (chan->regs[2] & 0x80))
-			chan->cur_pos = 0;
-		else if (apu_t::NOISE_LONG == chan->cur_pos)
-			chan->cur_pos = 0;
+		chan->seed = (chan->seed >> 1) | ((BIT(chan->seed, 0) ^ BIT(chan->seed, (chan->regs[2] & 0x80) ? 6 : 1)) << 14);
 	}
 
 	if (chan->regs[0] & 0x10) /* fixed volume */
@@ -422,11 +393,11 @@ s8 nesapu_device::apu_noise(apu_t::noise_t *chan)
 	else
 		outvol = 0x0F - chan->env_vol;
 
-	output = m_noise_lut[chan->cur_pos];
+	output = chan->seed & 0xff;
 	if (output > outvol)
 		output = outvol;
 
-	if (m_noise_lut[chan->cur_pos] & 0x80) /* make it negative */
+	if (chan->seed & 0x80) /* make it negative */
 		output = -output;
 
 	return (s8) output;
diff --git a/src/devices/sound/nes_apu.h b/src/devices/sound/nes_apu.h
index 93aef1cad8c..731bf565940 100644
--- a/src/devices/sound/nes_apu.h
+++ b/src/devices/sound/nes_apu.h
@@ -71,7 +71,6 @@ private:
 	apu_t   m_APU;                   /* Actual APUs */
 	u32     m_samps_per_sync;        /* Number of samples per vsync */
 	u32     m_buffer_size;           /* Actual buffer size in bytes */
-	u8      m_noise_lut[apu_t::NOISE_LONG]; /* Noise sample lookup table */
 	u32     m_vbl_times[0x20];       /* VBL durations in samples */
 	u32     m_sync_times1[SYNCS_MAX1]; /* Samples per sync table */
 	u32     m_sync_times2[SYNCS_MAX2]; /* Samples per sync table */
diff --git a/src/devices/sound/nes_defs.h b/src/devices/sound/nes_defs.h
index 0abe5b4666e..5651dc85a88 100644
--- a/src/devices/sound/nes_defs.h
+++ b/src/devices/sound/nes_defs.h
@@ -94,7 +94,7 @@ struct apu_t
 		}
 
 		uint8 regs[4]; /* regs[1] unused */
-		int cur_pos = 0;
+		u32 seed = 1;
 		int vbl_length = 0;
 		float phaseacc = 0.0;
 		float output_vol = 0.0;
@@ -147,10 +147,6 @@ struct apu_t
 	static constexpr unsigned SMASK   = 0x15;
 	static constexpr unsigned IRQCTRL = 0x17;
 
-	static constexpr unsigned NOISE_LONG     = 0x4000;
-	static constexpr unsigned NOISE_SHORT    = 93;
-
-
 	apu_t()
 	{
 		memset(regs, 0, sizeof(regs));