SPU2: Downsample reverb input

These filter coefficients come from mednafen, I think they were
originally found by Neill Corlett.

I have not personally verified their accuracy.

pcsx2/SPU2/Reverb.cpp

@@ -46,6 +46,73 @@ void V_Core::Reverb_AdvanceBuffer()
 	}
 }
 
+
+
+static constexpr u32 NUM_TAPS = 39;
+// 39 tap filter, the 0's could be optimized out
+static constexpr std::array<s32, NUM_TAPS> filter_coefs = {
+	-1,
+	0,
+	2,
+	0,
+	-10,
+	0,
+	35,
+	0,
+	-103,
+	0,
+	266,
+	0,
+	-616,
+	0,
+	1332,
+	0,
+	-2960,
+	0,
+	10246,
+	16384,
+	10246,
+	0,
+	-2960,
+	0,
+	1332,
+	0,
+	-616,
+	0,
+	266,
+	0,
+	-103,
+	0,
+	35,
+	0,
+	-10,
+	0,
+	2,
+	0,
+	-1,
+};
+
+s32 V_Core::ReverbDownsample(bool right)
+{
+	s32 out = 0;
+
+	for (u32 i = 0; i < NUM_TAPS; i++)
+	{
+		out += RevbDownBuf[right][((RevbSampleBufPos - NUM_TAPS) + i) & 63] * filter_coefs[i];
+	}
+
+	out >>= 15;
+	Clampify(out, INT16_MIN, INT16_MAX);
+
+	return out;
+}
+
+
+s32 V_Core::ReverbUpsample()
+{
+	return 0;
+}
+
 /////////////////////////////////////////////////////////////////////////////////////////
 
 StereoOut32 V_Core::DoReverb(const StereoOut32& Input)
@@ -55,6 +122,11 @@ StereoOut32 V_Core::DoReverb(const StereoOut32& Input)
 		return StereoOut32::Empty;
 	}
 
+	RevbSampleBufPos++;
+
+	RevbDownBuf[0][RevbSampleBufPos & 63] = Input.Left;
+	RevbDownBuf[1][RevbSampleBufPos & 63] = Input.Right;
+
 	bool R = Cycles & 1;
 
 	// Calculate the read/write addresses we'll be needing for this session of reverb.
@@ -112,7 +184,7 @@ StereoOut32 V_Core::DoReverb(const StereoOut32& Input)
 	s32 in, same, diff, apf1, apf2, out;
 
 #define MUL(x, y) ((x) * (y) >> 15)
-	in = MUL(R ? Revb.IN_COEF_R : Revb.IN_COEF_L, R ? Input.Right : Input.Left);
+	in = MUL(R ? Revb.IN_COEF_R : Revb.IN_COEF_L, ReverbDownsample(R));
 
 	same = MUL(Revb.IIR_VOL, in + MUL(Revb.WALL_VOL, _spu2mem[same_src]) - _spu2mem[same_prv]) + _spu2mem[same_prv];
 	diff = MUL(Revb.IIR_VOL, in + MUL(Revb.WALL_VOL, _spu2mem[diff_src]) - _spu2mem[diff_prv]) + _spu2mem[diff_prv];

pcsx2/SPU2/defs.h

@@ -416,6 +416,10 @@ struct V_Core
 
 	V_Reverb Revb;              // Reverb Registers
 	V_ReverbBuffers RevBuffers; // buffer pointers for reverb, pre-calculated and pre-clipped.
+
+	s32 RevbDownBuf[2][64]; // Downsample buffer for reverb, one for each channel
+	s32 RevbUpBuf[2][64]; // Upsample buffer for reverb, one for each channel
+	u32 RevbSampleBufPos;
 	u32 EffectsStartA;
 	u32 EffectsEndA;
 	u32 ExtEffectsStartA;
@@ -492,6 +496,9 @@ struct V_Core
 	StereoOut32 DoReverb(const StereoOut32& Input);
 	s32 RevbGetIndexer(s32 offset);
 
+	s32 ReverbDownsample(bool right);
+	s32 ReverbUpsample();
+
 	StereoOut32 ReadInput();
 	StereoOut32 ReadInput_HiFi();
 

pcsx2/SPU2/spu2sys.cpp

@@ -222,6 +222,10 @@ void V_Core::Init(int index)
 	Regs.ENDX = 0xffffff; // PS2 confirmed
 
 	RevBuffers.NeedsUpdated = true;
+	RevbSampleBufPos = 0;
+	memset(RevbDownBuf, 0, sizeof(RevbDownBuf));
+	memset(RevbUpBuf, 0, sizeof(RevbUpBuf));
+
 	UpdateEffectsBufferSize();
 }
 

pcsx2/SaveState.h

@@ -24,7 +24,7 @@
 //  the lower 16 bit value.  IF the change is breaking of all compatibility with old
 //  states, increment the upper 16 bit value, and clear the lower 16 bits to 0.
 
-static const u32 g_SaveVersion = (0x9A1C << 16) | 0x0000;
+static const u32 g_SaveVersion = (0x9A1D << 16) | 0x0000;
 
 // this function is meant to be used in the place of GSfreeze, and provides a safe layer
 // between the GS saving function and the MTGS's needs. :)