From c592c44c373b21dbebac76a581e822756747e2f7 Mon Sep 17 00:00:00 2001
From: tildearrow <me@tildearrow.org>
Date: Wed, 25 Jun 2025 17:13:12 -0500
Subject: [PATCH] SM8521: "improve" noise

unconfirmed

issue #2567
---
 src/engine/platform/sound/sm8521.c | 12 +++++++++++-
 src/engine/platform/sound/sm8521.h |  3 ++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/engine/platform/sound/sm8521.c b/src/engine/platform/sound/sm8521.c
index 6e509edd1..eafca0c92 100644
--- a/src/engine/platform/sound/sm8521.c
+++ b/src/engine/platform/sound/sm8521.c
@@ -72,11 +72,19 @@ void sm8521_noise_tick(struct sm8521_noise_t *noise, const int cycle)
 	noise->base.counter += cycle;
 	while (noise->base.counter >= (noise->base.t + 1))
 	{
+                // https://github.com/tildearrow/furnace/issues/2567
                 // unknown algorithm, but don't use rand()
+                //
+                // some research suggests VIC-like noise, although
+                // that remains to be confirmed
+                noise->oldLFSR = noise->lfsr & 1;
 		noise->lfsr = ( noise->lfsr>>1|(((noise->lfsr) ^ (noise->lfsr >> 5) ^ (noise->lfsr >> 8) ^ (noise->lfsr >> 13) ) & 1)<<31);
 		noise->base.counter -= (noise->base.t + 1);
+                if (noise->oldLFSR^(noise->lfsr&1)) {
+                  noise->out ^= 1;
+                }
 	}
-	noise->base.out = (((noise->lfsr & 0x1) ? 7 : -8) * noise->base.level) >> 1; // scale out to 8bit
+	noise->base.out = ((noise->out ? 7 : -8) * noise->base.level) >> 1; // scale out to 8bit
 }
 
 void sm8521_sound_tick(struct sm8521_t *sm8521, const int cycle)
@@ -130,6 +138,8 @@ void sm8521_reset(struct sm8521_t *sm8521)
 	sm8521->noise.base.out = 0;
 	sm8521->noise.base.counter = 0;
 	sm8521->noise.lfsr = 0x89abcdef;
+	sm8521->noise.oldLFSR = 1;
+	sm8521->noise.out = 0;
 	sm8521->out = 0;
 	sm8521->sgda = 0;
 	sm8521->sgc = 0;
diff --git a/src/engine/platform/sound/sm8521.h b/src/engine/platform/sound/sm8521.h
index e83c45b81..86a7e6e6b 100644
--- a/src/engine/platform/sound/sm8521.h
+++ b/src/engine/platform/sound/sm8521.h
@@ -65,7 +65,8 @@ struct sm8521_wave_t
 struct sm8521_noise_t
 {
 	struct sm8521_sg_t base;
-	unsigned int lfsr; // LFSR
+	unsigned int lfsr, oldLFSR; // LFSR
+        unsigned char out;
 };
 
 struct sm8521_t