GB Audio: Implement capacitor charge

CHANGES

@@ -54,6 +54,7 @@ Misc:
  - Qt: Options to mess around with layer placement
  - GBA Savedata: Remove ability to disable realistic timing
  - Qt: Add load alternate save option
+ - GB Audio: Improved audio quality
 
 0.6.3: (2017-04-14)
 Bugfixes:

include/mgba/internal/gb/audio.h

@@ -161,6 +161,8 @@ struct GBAudio {
 	struct blip_t* right;
 	int16_t lastLeft;
 	int16_t lastRight;
+	int32_t capLeft;
+	int32_t capRight;
 	int clock;
 	int32_t clockRate;
 

include/mgba/internal/gb/serialize.h

@@ -103,7 +103,8 @@ mLOG_DECLARE_CATEGORY(GB_STATE);
  *   | bit 5: Has channel 1 sweep occurred?
  *   | bit 6: Is channel 3's memory readable?
  *   | bit 7: Reserved
- * | 0x000A8 - 0x000AF: Rserved
+ * | 0x000A8 - 0x000AB: Left capacitor charge
+ * | 0x000AC - 0x000AF: Right capacitor charge
  * | 0x000B0 - 0x000B3: Next sample
  * 0x000B4 - 0x000153: Video state
  * | 0x000B4 - 0x000B5: Current x
@@ -302,7 +303,8 @@ struct GBSerializedState {
 	struct {
 		struct GBSerializedPSGState psg;
 		GBSerializedAudioFlags flags;
-		int32_t reserved[2];
+		int32_t capLeft;
+		int32_t capRight;
 		uint32_t nextSample;
 	} audio;
 

src/gb/audio.c

@@ -140,6 +140,8 @@ void GBAudioReset(struct GBAudio* audio) {
 	audio->sampleInterval = 128;
 	audio->lastLeft = 0;
 	audio->lastRight = 0;
+	audio->capLeft = 0;
+	audio->capRight = 0;
 	audio->clock = 0;
 	audio->volumeRight = 0;
 	audio->volumeLeft = 0;
@@ -640,18 +642,26 @@ static void _sample(struct mTiming* timing, void* user, uint32_t cyclesLate) {
 
 	mCoreSyncLockAudio(audio->p->sync);
 	unsigned produced;
+
+	int16_t degradedLeft = sampleLeft - (audio->capLeft >> 16);
+	int16_t degradedRight = sampleRight - (audio->capRight >> 16);
+	audio->capLeft = (sampleLeft << 16) - degradedLeft * 65184;
+	audio->capRight = (sampleRight << 16) - degradedRight * 65184;
+	sampleLeft = degradedLeft;
+	sampleRight = degradedRight;
+
 	if ((size_t) blip_samples_avail(audio->left) < audio->samples) {
 		blip_add_delta(audio->left, audio->clock, sampleLeft - audio->lastLeft);
 		blip_add_delta(audio->right, audio->clock, sampleRight - audio->lastRight);
-		audio->lastLeft = sampleLeft;
-		audio->lastRight = sampleRight;
 		audio->clock += audio->sampleInterval;
 		if (audio->clock >= CLOCKS_PER_BLIP_FRAME) {
 			blip_end_frame(audio->left, CLOCKS_PER_BLIP_FRAME);
 			blip_end_frame(audio->right, CLOCKS_PER_BLIP_FRAME);
 			audio->clock -= CLOCKS_PER_BLIP_FRAME;
 		}
 	}
+	audio->lastLeft = sampleLeft;
+	audio->lastRight = sampleRight;
 	produced = blip_samples_avail(audio->left);
 	if (audio->p->stream && audio->p->stream->postAudioFrame) {
 		audio->p->stream->postAudioFrame(audio->p->stream, sampleLeft, sampleRight);
@@ -1020,11 +1030,15 @@ void GBAudioPSGDeserialize(struct GBAudio* audio, const struct GBSerializedPSGSt
 
 void GBAudioSerialize(const struct GBAudio* audio, struct GBSerializedState* state) {
 	GBAudioPSGSerialize(audio, &state->audio.psg, &state->audio.flags);
+	STORE_32LE(audio->capLeft, 0, &state->audio.capLeft);
+	STORE_32LE(audio->capRight, 0, &state->audio.capRight);
 	STORE_32LE(audio->sampleEvent.when - mTimingCurrentTime(audio->timing), 0, &state->audio.nextSample);
 }
 
 void GBAudioDeserialize(struct GBAudio* audio, const struct GBSerializedState* state) {
 	GBAudioPSGDeserialize(audio, &state->audio.psg, &state->audio.flags);
+	LOAD_32LE(audio->capLeft, 0, &state->audio.capLeft);
+	LOAD_32LE(audio->capRight, 0, &state->audio.capRight);
 	uint32_t when;
 	LOAD_32LE(when, 0, &state->audio.nextSample);
 	mTimingSchedule(audio->timing, &audio->sampleEvent, when);