From c6629015048bc9d4aa18d297c0c41d3e26293843 Mon Sep 17 00:00:00 2001
From: Pierre Bourdon <delroth@gmail.com>
Date: Sun, 20 Jan 2013 16:15:42 +0100
Subject: [PATCH] Implement low-pass voice filtering, untested because I
 couldn't find any game using it

---
 .../Core/Src/HW/DSPHLE/UCodes/UCode_AXStructs.h     |  6 +++---
 .../Core/Core/Src/HW/DSPHLE/UCodes/UCode_AX_Voice.h | 13 ++++++++++---
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AXStructs.h b/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AXStructs.h
index d82088b34f..cc24bd78b7 100644
--- a/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AXStructs.h
+++ b/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AXStructs.h
@@ -230,9 +230,9 @@ struct PBADPCMLoopInfo
 struct PBLowPassFilter
 {
 	u16 enabled;
-	u16 yn1;
-	u16 a0;
-	u16 b0;
+	s16 yn1;
+	s16 a0;
+	s16 b0;
 };
 
 struct AXPB
diff --git a/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AX_Voice.h b/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AX_Voice.h
index bcf69ddb4b..fcc1bb7999 100644
--- a/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AX_Voice.h
+++ b/Source/Core/Core/Src/HW/DSPHLE/UCodes/UCode_AX_Voice.h
@@ -340,6 +340,15 @@ void MixAdd(int* out, const s16* input, u32 count, u16* pvol, s16* dpop, bool ra
 	}
 }
 
+// Execute a low pass filter on the samples using one history value. Returns
+// the new history value.
+s16 LowPassFilter(s16* samples, u32 count, s16 yn1, s16 a0, s16 b0)
+{
+	for (u32 i = 0; i < count; ++i)
+		yn1 = samples[i] = (s32)a0 * samples[i] - (s32)b0 * yn1;
+	return yn1;
+}
+
 // Process 1ms of audio (for AX GC) or 3ms of audio (for AX Wii) from a PB and
 // mix it to the output buffers.
 void ProcessVoice(PB_TYPE& pb, const AXBuffers& buffers, AXMixControl mctrl)
@@ -362,9 +371,7 @@ void ProcessVoice(PB_TYPE& pb, const AXBuffers& buffers, AXMixControl mctrl)
 
 	// Optionally, execute a low pass filter
 	if (pb.lpf.enabled)
-	{
-		// TODO
-	}
+		pb.lpf.yn1 = LowPassFilter(samples, SAMPLES_PER_FRAME, pb.lpf.yn1, pb.lpf.a0, pb.lpf.b0);
 
 	// Mix LRS, AUXA and AUXB depending on mixer_control
 	// TODO: Handle DPL2 on AUXB.