From 1c41e1e051803344f1e854f4da23c9cfa470f60b Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 19 Jun 2023 22:32:27 -0700
Subject: [PATCH] GBA Audio: Fix sample timing drifting when changing sample
 interval

---
 CHANGES         | 1 +
 src/gba/audio.c | 9 +++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/CHANGES b/CHANGES
index 3dd20b6c3..189ca1c3b 100644
--- a/CHANGES
+++ b/CHANGES
@@ -11,6 +11,7 @@ Emulation fixes:
  - GB Serialize: Add missing Pocket Cam state to savestates
  - GB SIO: Disabling SIO should cancel pending transfers (fixes mgba.io/i/2537)
  - GB Video: Implement DMG-style sprite ordering
+ - GBA Audio: Fix sample timing drifting when changing sample interval
  - GBA BIOS: Fix clobbering registers with word-sized CpuSet
  - GBA Video: Disable BG target 1 blending when OBJ blending (fixes mgba.io/i/2722)
 Other fixes:
diff --git a/src/gba/audio.c b/src/gba/audio.c
index 9cbf1dc24..ed4ea8474 100644
--- a/src/gba/audio.c
+++ b/src/gba/audio.c
@@ -253,11 +253,16 @@ void GBAAudioWriteSOUNDCNT_X(struct GBAAudio* audio, uint16_t value) {
 }
 
 void GBAAudioWriteSOUNDBIAS(struct GBAAudio* audio, uint16_t value) {
+	GBAAudioSample(audio, mTimingCurrentTime(&audio->p->timing));
 	audio->soundbias = value;
 	int32_t oldSampleInterval = audio->sampleInterval;
 	audio->sampleInterval = 0x200 >> GBARegisterSOUNDBIASGetResolution(value);
-	if (oldSampleInterval != audio->sampleInterval && audio->p->stream && audio->p->stream->audioRateChanged) {
-		audio->p->stream->audioRateChanged(audio->p->stream, GBA_ARM7TDMI_FREQUENCY / audio->sampleInterval);
+	if (oldSampleInterval != audio->sampleInterval) {
+		audio->lastSample += oldSampleInterval * audio->sampleIndex;
+		audio->sampleIndex = 0;
+		if (audio->p->stream && audio->p->stream->audioRateChanged) {
+			audio->p->stream->audioRateChanged(audio->p->stream, GBA_ARM7TDMI_FREQUENCY / audio->sampleInterval);
+		}
 	}
 }