M CHANGES → CHANGES

@@ -40,6 +40,8 @@ - GB: Fix savedata initialization (fixes mgba.io/i/1473, mgba.io/i/1478)
  - GB Memory: Better emulate 0xFEA0 region on DMG, MGB and AGB
  - GB Printer: Reset printer buffer index after printing
  - GB Video: Fix mode 0 window edge case (fixes mgba.io/i/1519)
+ - GBA Audio: Fix channel 4 aliasing (fixes mgba.io/i/1265)
+ - GB Audio: Improve channel 4 supersampling
 Other fixes:
  - Qt: Fix some Qt display driver race conditions
  - Core: Improved lockstep driver reliability (Le Hoang Quyen)

M src/gb/audio.c → src/gb/audio.c

@@ -37,7 +37,7 @@
 static void _updateSquareSample(struct GBAudioSquareChannel* ch);
 static int32_t _updateSquareChannel(struct GBAudioSquareChannel* ch);
 
-static int8_t _coalesceNoiseChannel(struct GBAudioNoiseChannel* ch);
+static int16_t _coalesceNoiseChannel(struct GBAudioNoiseChannel* ch);
 
 static void _updateFrame(struct mTiming* timing, void* user, uint32_t cyclesLate);
 static void _updateChannel1(struct mTiming* timing, void* user, uint32_t cyclesLate);
@@ -632,8 +632,11 @@ sampleRight += audio->ch3.sample;
 		}
 	}
 
+	sampleLeft <<= 3;
+	sampleRight <<= 3;
+
 	if (!audio->forceDisableCh[3]) {
-		int8_t sample = _coalesceNoiseChannel(&audio->ch4);
+		int16_t sample = audio->style == GB_AUDIO_GBA ? (audio->ch4.sample << 3) : _coalesceNoiseChannel(&audio->ch4);
 		if (audio->ch4Left) {
 			sampleLeft += sample;
 		}
@@ -642,9 +645,6 @@ if (audio->ch4Right) {
 			sampleRight += sample;
 		}
 	}
-
-	sampleLeft <<= 3;
-	sampleRight <<= 3;
 
 	*left = sampleLeft * (1 + audio->volumeLeft);
 	*right = sampleRight * (1 + audio->volumeRight);
@@ -768,12 +768,12 @@ return period * 4;
 	}
 }
 
-static int8_t _coalesceNoiseChannel(struct GBAudioNoiseChannel* ch) {
+static int16_t _coalesceNoiseChannel(struct GBAudioNoiseChannel* ch) {
 	if (!ch->nSamples) {
 		return ch->sample;
 	}
 	// TODO keep track of timing
-	int8_t sample = ch->samples / ch->nSamples;
+	int16_t sample = (ch->samples << 3) / ch->nSamples;
 	ch->nSamples = 0;
 	ch->samples = 0;
 	return sample;