From af59e28f906f8559fd9c4db447714e71eea3b620 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Sat, 1 Feb 2020 12:17:45 -0800
Subject: [PATCH] GB Serialize: Fix timing bug loading channel 4 timing

---
 CHANGES        | 1 +
 src/gb/audio.c | 8 ++++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/CHANGES b/CHANGES
index 3d6c21e10..22f13ac7e 100644
--- a/CHANGES
+++ b/CHANGES
@@ -2,6 +2,7 @@
 Emulation fixes:
  - ARM: Fix ALU reading PC after shifting
  - ARM: Fix STR storing PC after address calculation
+ - GB Serialize: Fix timing bug loading channel 4 timing
  - GBA DMA: Linger last DMA on bus (fixes mgba.io/i/301 and mgba.io/i/1320)
  - GBA Memory: Misaligned SRAM writes are ignored
  - GBA Memory: Improve gamepak prefetch timing
diff --git a/src/gb/audio.c b/src/gb/audio.c
index 63af6bc1a..4bc5fa351 100644
--- a/src/gb/audio.c
+++ b/src/gb/audio.c
@@ -1078,9 +1078,13 @@ void GBAudioPSGDeserialize(struct GBAudio* audio, const struct GBSerializedPSGSt
 	LOAD_32LE(audio->ch4.lastEvent, 0, &state->ch4.lastEvent);
 	LOAD_32LE(when, 0, &state->ch4.nextEvent);
 	if (audio->ch4.envelope.dead < 2 && audio->playingCh4) {
-		if (when - audio->ch4.lastEvent > (uint32_t) audio->sampleInterval) {
+		if (!audio->ch4.lastEvent) {
 			// Back-compat: fake this value
-			audio->ch4.lastEvent = when - audio->sampleInterval;
+			uint32_t currentTime = mTimingCurrentTime(audio->timing);
+			int32_t cycles = audio->ch4.ratio ? 2 * audio->ch4.ratio : 1;
+			cycles <<= audio->ch4.frequency;
+			cycles *= 8 * audio->timingFactor;
+			audio->ch4.lastEvent = currentTime + (when & (cycles - 1)) - cycles;
 		}
 		mTimingSchedule(audio->timing, &audio->ch4Event, when);
 	}