git.y1.nz

commit 8a234a25a3c34cf98581152c474ec62711861605
parent 5b883465370bc7ad5a3576d26e7cee2ec553d3a0
Author: Lior Halphon <LIJI32@gmail.com>
Date:   Mon, 18 Aug 2025 21:27:01 +0300

Add silence detection to eliminate rounding artifacts when playing nothing but frequencies above Nyquist. Fixes #394

Diffstat:
M
Core/apu.c
24
++++++++++++++++++++++++
M
Core/apu.h
2
++

2 files changed, 26 insertions(+), 0 deletions(-)
diff --git a/Core/apu.c b/Core/apu.c
@@ -103,6 +103,30 @@ static void band_limited_read(GB_band_limited_t *band_limited, GB_sample_t *outp
     
     output->left = band_limited->output.left * multiplier / GB_BAND_LIMITED_ONE;
     output->right = band_limited->output.right * multiplier / GB_BAND_LIMITED_ONE;
+    
+    /* This hueristic will mute the channel if it's only playing an amplitude of 1 or 2 units, usually
+       caused by rounding errors when the channel is playing a single frequency above Nyquist. */
+    
+    unsigned diff = abs(output->left - band_limited->last_output.left);
+    if (diff > 4) {
+        band_limited->silence_detection = 0;
+        band_limited->last_output.packed = output->packed;
+        return;
+    }
+    
+    diff = abs(output->right - band_limited->last_output.right);
+    if (diff > 4) {
+        band_limited->silence_detection = 0;
+        band_limited->last_output.packed = output->packed;
+        return;
+    }
+    
+    if (band_limited->silence_detection == 4000) {
+        output->packed = band_limited->last_output.packed;
+    }
+    else {
+        band_limited->silence_detection++;
+    }
 }
 
 static inline uint32_t sample_fraction_multiply(GB_gameboy_t *gb, unsigned multiplier)
diff --git a/Core/apu.h b/Core/apu.h
@@ -173,6 +173,8 @@ typedef struct {
     } buffer[GB_BAND_LIMITED_WIDTH * 2], output;
     uint8_t pos;
     GB_sample_t input;
+    GB_sample_t last_output;
+    unsigned silence_detection;
 } GB_band_limited_t;
 
 typedef struct {

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