DS Audio: Fix audio sampling slightly too quickly

CHANGES

@@ -22,6 +22,7 @@ Bugfixes:
  - DS GX: Properly mask address for slot 2 4x4-texel textures
  - DS Slot-1: Emulate initial SPI command delay
  - DS: Fix exposed CPU frequencies and audio timing
+ - DS Audio: Fix audio sampling slightly too quickly
 Misc:
  - DS: Set boot complete bit in RAM on boot (fixes mgba.io/i/576, mgba.io/i/580, mgba.io/i/586)
  - DS Memory: Ensure DS9 I/O is 8-byte aligned

include/mgba/internal/ds/audio.h

@@ -80,6 +80,7 @@ struct DSAudio {
 	unsigned sampleRate;
 
 	int32_t sampleInterval;
+	unsigned sampleDrift;
 
 	bool forceDisableCh[16];
 	int bias;

src/ds/audio.c

@@ -86,6 +86,7 @@ void DSAudioReset(struct DSAudio* audio) {
 	mTimingSchedule(&audio->p->ds7.timing, &audio->sampleEvent, 0);
 	audio->sampleRate = 0x8000;
 	audio->sampleInterval = DS_ARM7TDMI_FREQUENCY / audio->sampleRate;
+	audio->sampleDrift = 0;
 
 	int ch;
 	for (ch = 0; ch < 16; ++ch) {
@@ -325,6 +326,12 @@ static void _sample(struct mTiming* timing, void* user, uint32_t cyclesLate) {
 	int16_t sampleLeft = _applyBias(audio, audio->sampleLeft);
 	int16_t sampleRight = _applyBias(audio, audio->sampleRight);
 
+	audio->sampleDrift += DS_ARM7TDMI_FREQUENCY % audio->sampleRate;
+
+	if (audio->sampleDrift >= audio->sampleRate) {
+		++audio->sampleInterval;
+	}
+
 	mCoreSyncLockAudio(audio->p->sync);
 	unsigned produced;
 	if ((size_t) blip_samples_avail(audio->left) < audio->samples) {
@@ -351,4 +358,9 @@ static void _sample(struct mTiming* timing, void* user, uint32_t cyclesLate) {
 		audio->p->stream->postAudioBuffer(audio->p->stream, audio->left, audio->right);
 	}
 	mTimingSchedule(timing, &audio->sampleEvent, audio->sampleInterval - cyclesLate);
+
+	if (audio->sampleDrift >= audio->sampleRate) {
+		--audio->sampleInterval;
+		audio->sampleDrift -= audio->sampleRate;
+	}
 }