From 4676a520e1b181e5891550331100b2629d7cbd7f Mon Sep 17 00:00:00 2001
From: Nebuleon Fumika
Date: Thu, 7 Feb 2013 22:27:44 -0500
Subject: Synchronise fractions of a hertz correctly in the audio output timing
 code to avoid buffer overruns. Buffer overruns pause the game for about half
 a second.

---
 source/nds/entry.cpp | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

(limited to 'source')

diff --git a/source/nds/entry.cpp b/source/nds/entry.cpp
index d82ed85..af832f9 100644
--- a/source/nds/entry.cpp
+++ b/source/nds/entry.cpp
@@ -981,8 +981,20 @@ void S9xGenerateSound ()
 	}
 }
 
-#define SOUND_EMISSION_INTERVAL ((unsigned int) ((((unsigned long long) DS2_BUFFER_SIZE * 1000000) / SND_SAMPLE_RATE) * 3 / 128) - 1) /* -1 is for roundoff errors */
-unsigned int LastSoundEmissionTime = 0;
+#define SOUND_EMISSION_INTERVAL ((unsigned int) ((((unsigned long long) DS2_BUFFER_SIZE * 1000000) / SND_SAMPLE_RATE) * 3 / 128))
+#define TRUE_SOUND_EMISSION_INTERVAL ((((double) DS2_BUFFER_SIZE * 1000000) / SND_SAMPLE_RATE) * 3 / 128)
+#define SOUND_EMISSION_INTERVAL_ERROR ((int) ((TRUE_SOUND_EMISSION_INTERVAL - SOUND_EMISSION_INTERVAL) * FIXED_POINT))
+static unsigned int LastSoundEmissionTime = 0;
+
+/*
+ * Accumulated error in the sound emission time. The unit is as follows:
+ * FIXED_POINT = 42.667 microseconds.
+ * As the error goes past FIXED_POINT, the new target for sound emission
+ * becomes 42.667 microseconds LATER. This helps with sound buffer overruns,
+ * correctly dealing with the fact that 42.667 microseconds does not fit
+ * an integer number of times in 1/32000 second (or whatever sampling rate).
+ */
+static unsigned int SoundEmissionTimeError = 0;
 
 void S9xProcessSound (unsigned int)
 {
@@ -1009,6 +1021,12 @@ void S9xProcessSound (unsigned int)
 		else
 		{
 			LastSoundEmissionTime += SOUND_EMISSION_INTERVAL;
+			SoundEmissionTimeError += SOUND_EMISSION_INTERVAL_ERROR;
+			if (SoundEmissionTimeError > FIXED_POINT)
+			{
+				LastSoundEmissionTime += SoundEmissionTimeError >> FIXED_POINT_SHIFT;
+				SoundEmissionTimeError &= FIXED_POINT_REMAINDER;
+			}
 		}
 		/* Number of samples to generate now */
 		int sample_count = so.buffer_size;
-- 
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