SPU: DC filter output

Some games leave paused voices hanging with volume turned on which
results in dc offset. Filter it out.

pcsx2/SPU2/Mixer.cpp

@@ -603,6 +603,19 @@ StereoOut32 V_Core::Mix(const VoiceMixSet& inVoices, const StereoOut32& Input, c
 	return TD + ApplyVolume(RV, FxVol);
 }
 
+static StereoOut32 DCFilter(StereoOut32 input) {
+	// A simple DC blocking high-pass filter
+	// Implementation from http://peabody.sapp.org/class/dmp2/lab/dcblock/
+	// The magic number 0x7f5c is ceil(INT16_MAX * 0.995)
+	StereoOut32 output;
+	output.Left = (input.Left - DCFilterIn.Left + clamp_mix((0x7f5c * DCFilterOut.Left) >> 15));
+	output.Right = (input.Right - DCFilterIn.Right + clamp_mix((0x7f5c * DCFilterOut.Right) >> 15));
+
+	DCFilterIn = input;
+	DCFilterOut = output;
+	return output;
+}
+
 // used to throttle the output rate of cache stat reports
 static int p_cachestat_counter = 0;
 
@@ -679,7 +692,7 @@ __forceinline
 	// output by design.
 	// Good thing though that this code gets the volume exactly right, as per tests :)
 	Out = clamp_mix(Out);
-
+	Out = DCFilter(Out);
 	SndBuffer::Write(StereoOut16(Out));
 
 	// Update AutoDMA output positioning

pcsx2/SPU2/defs.h

@@ -513,6 +513,8 @@ extern u16 OutPos;
 extern u16 InputPos;
 // SPU Mixing Cycles ("Ticks mixed" counter)
 extern u32 Cycles;
+// DC Filter state
+extern StereoOut16 DCFilterIn, DCFilterOut;
 
 extern s16 spu2regs[0x010000 / sizeof(s16)];
 extern s16 _spu2mem[0x200000 / sizeof(s16)];

pcsx2/SPU2/spu2sys.cpp

@@ -37,6 +37,7 @@ V_CoreDebug DebugCores[2];
 V_Core Cores[2];
 V_SPDIF Spdif;
 
+StereoOut16 DCFilterIn, DCFilterOut;
 u16 OutPos;
 u16 InputPos;
 u32 Cycles;
@@ -149,6 +150,8 @@ void V_Core::Init(int index)
 	DMAPtr = nullptr;
 	KeyOn = 0;
 	OutPos = 0;
+	DCFilterIn = {};
+	DCFilterOut = {};
 
 	psxmode = false;
 	psxSoundDataTransferControl = 0;