MT32: Update MT-32 emulator to latest Munt code

Several changes against original code were made. They were intentionally
kept to the minimum

1 files changed, 150 insertions, 0 deletions

diff --git a/audio/softsynth/mt32/LA32Ramp.cpp b/audio/softsynth/mt32/LA32Ramp.cpp
new file mode 100644
index 0000000000..9f1f01c3c2
--- /dev/null
+++ b/audio/softsynth/mt32/LA32Ramp.cpp
@@ -0,0 +1,150 @@
+/* Copyright (C) 2003, 2004, 2005, 2006, 2008, 2009 Dean Beeler, Jerome Fisher
+ * Copyright (C) 2011 Dean Beeler, Jerome Fisher, Sergey V. Mikayev
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License as published by
+ *  the Free Software Foundation, either version 2.1 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+Some notes on this class:
+
+This emulates the LA-32's implementation of "ramps". A ramp in this context is a smooth transition from one value to another, handled entirely within the LA-32.
+The LA-32 provides this feature for amplitude and filter cutoff values.
+
+The 8095 starts ramps on the LA-32 by setting two values in memory-mapped registers:
+
+(1) The target value (between 0 and 255) for the ramp to end on. This is represented by the "target" argument to startRamp().
+(2) The speed at which that value should be approached. This is represented by the "increment" argument to startRamp().
+
+Once the ramp target value has been hit, the LA-32 raises an interrupt.
+
+Note that the starting point of the ramp is whatever internal value the LA-32 had when the registers were set. This is usually the end point of a previously completed ramp.
+
+Our handling of the "target" and "increment" values is based on sample analysis and a little guesswork.
+Here's what we're pretty confident about:
+ - The most significant bit of "increment" indicates the direction that the LA32's current internal value ("current" in our emulation) should change in.
+   Set means downward, clear means upward.
+ - The lower 7 bits of "increment" indicate how quickly "current" should be changed.
+ - If "increment" is 0, no change to "current" is made and no interrupt is raised. [SEMI-CONFIRMED by sample analysis]
+ - Otherwise, if the MSb is set:
+    - If "current" already corresponds to a value <= "target", "current" is set immediately to the equivalent of "target" and an interrupt is raised.
+    - Otherwise, "current" is gradually reduced (at a rate determined by the lower 7 bits of "increment"), and once it reaches the equivalent of "target" an interrupt is raised.
+ - Otherwise (the MSb is unset):
+    - If "current" already corresponds to a value >= "target", "current" is set immediately to the equivalent of "target" and an interrupt is raised.
+    - Otherwise, "current" is gradually increased (at a rate determined by the lower 7 bits of "increment"), and once it reaches the equivalent of "target" an interrupt is raised.
+
+We haven't fully explored:
+ - Values when ramping between levels (though this is probably correct).
+ - Transition timing (may not be 100% accurate, especially for very fast ramps).
+*/
+//#include <cmath>
+
+#include "mt32emu.h"
+#include "LA32Ramp.h"
+#include "mmath.h"
+
+namespace MT32Emu {
+
+// SEMI-CONFIRMED from sample analysis.
+const int TARGET_MULT = 0x40000;
+const unsigned int MAX_CURRENT = 0xFF * TARGET_MULT;
+
+// We simulate the delay in handling "target was reached" interrupts by waiting
+// this many samples before setting interruptRaised.
+// FIXME: This should vary with the sample rate, but doesn't.
+// SEMI-CONFIRMED: Since this involves asynchronous activity between the LA32
+// and the 8095, a good value is hard to pin down.
+// This one matches observed behaviour on a few digital captures I had handy,
+// and should be double-checked. We may also need a more sophisticated delay
+// scheme eventually.
+const int INTERRUPT_TIME = 7;
+
+LA32Ramp::LA32Ramp() :
+	current(0),
+	largeTarget(0),
+	largeIncrement(0),
+	interruptCountdown(0),
+	interruptRaised(false) {
+}
+
+void LA32Ramp::startRamp(Bit8u target, Bit8u increment) {
+	// CONFIRMED: From sample analysis, this appears to be very accurate.
+	// FIXME: We could use a table for this in future
+	if (increment == 0) {
+		largeIncrement = 0;
+	} else {
+		largeIncrement = (unsigned int)(EXP2F(((increment & 0x7F) + 24) / 8.0f) + 0.125f);
+	}
+	descending = (increment & 0x80) != 0;
+	if (descending) {
+		// CONFIRMED: From sample analysis, descending increments are slightly faster
+		largeIncrement++;
+	}
+
+	largeTarget = target * TARGET_MULT;
+	interruptCountdown = 0;
+	interruptRaised = false;
+}
+
+Bit32u LA32Ramp::nextValue() {
+	if (interruptCountdown > 0) {
+		if (--interruptCountdown == 0) {
+			interruptRaised = true;
+		}
+	} else if (largeIncrement != 0) {
+		// CONFIRMED from sample analysis: When increment is 0, the LA32 does *not* change the current value at all (and of course doesn't fire an interrupt).
+		if (descending) {
+			// Lowering current value
+			if (largeIncrement > current) {
+				current = largeTarget;
+				interruptCountdown = INTERRUPT_TIME;
+			} else {
+				current -= largeIncrement;
+				if (current <= largeTarget) {
+					current = largeTarget;
+					interruptCountdown = INTERRUPT_TIME;
+				}
+			}
+		} else {
+			// Raising current value
+			if (MAX_CURRENT - current < largeIncrement) {
+				current = largeTarget;
+				interruptCountdown = INTERRUPT_TIME;
+			} else {
+				current += largeIncrement;
+				if (current >= largeTarget) {
+					current = largeTarget;
+					interruptCountdown = INTERRUPT_TIME;
+				}
+			}
+		}
+	}
+	return current;
+}
+
+bool LA32Ramp::checkInterrupt() {
+	bool wasRaised = interruptRaised;
+	interruptRaised = false;
+	return wasRaised;
+}
+
+void LA32Ramp::reset() {
+	current = 0;
+	largeTarget = 0;
+	largeIncrement = 0;
+	descending = false;
+	interruptCountdown = 0;
+	interruptRaised = false;
+}
+
+}