add Sound.Utilities.DCFilter and use it in gambatte

haven't decided yet whether i like the idea; it's mostly about playing nice with other audio mixing (from other applications)
well, it's not bad anyway...

BizHawk.Emulation/BizHawk.Emulation.csproj

@@ -350,6 +350,7 @@
     <Compile Include="Properties\svnrev.cs" />
     <Compile Include="QuickCollections.cs" />
     <Compile Include="Sound\CDAudio.cs" />
+    <Compile Include="Sound\Utilities\DCFilter.cs" />
     <Compile Include="Sound\Utilities\Equalizer.cs" />
     <Compile Include="Sound\Utilities\SpeexResampler.cs" />
     <Compile Include="Sound\VRC6.cs" />

BizHawk.Emulation/Consoles/Nintendo/Gameboy/Gambatte.cs

@@ -441,12 +441,13 @@ namespace BizHawk.Emulation.Consoles.GB
 		int soundbuffcontains = 0;
 
 		Sound.Utilities.SpeexResampler resampler;
-		Sound.MetaspuSoundProvider metaspu;
+		ISoundProvider metaspu;
 
 		void InitSound()
 		{
-			metaspu = new Sound.MetaspuSoundProvider(Sound.ESynchMethod.ESynchMethod_V);
+			var metaspu = new Sound.MetaspuSoundProvider(Sound.ESynchMethod.ESynchMethod_V);
 			resampler = new Sound.Utilities.SpeexResampler(2, 2097152, 44100, 2097152, 44100, metaspu.buffer.enqueue_samples);
+			this.metaspu = new Sound.Utilities.DCFilter(metaspu);// metaspu;
 		}
 
 		void DisposeSound()

BizHawk.Emulation/Sound/Utilities/DCFilter.cs

@@ -0,0 +1,100 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace BizHawk.Emulation.Sound.Utilities
+{
+	/// <summary>
+	/// implements a DC block filter on top of an ISoundProvider.  rather simple.
+	/// </summary>
+	public class DCFilter : ISoundProvider
+	{
+		/*
+		 * A note about accuracy:
+		 * 
+		 * DCFilter can be added to the final output of any console, and this change will be faithful to the original hardware.
+		 * Analog output hardware ALWAYS has dc blocking caps.
+		 */
+
+		ISoundProvider input;
+
+		int sumL = 0;
+		int sumR = 0;
+
+		Queue<short> buffer;
+
+		const int depth = 65536;
+
+		public DCFilter(ISoundProvider input)
+		{
+			this.input = input;
+			this.buffer = new Queue<short>(depth * 2);
+			for (int i = 0; i < depth * 2; i++)
+				buffer.Enqueue(0);
+		}
+
+		/// <summary>
+		/// returns the original sound provider (in case you lost it).
+		/// after calling, the DCFilter is no longer valid
+		/// </summary>
+		/// <returns></returns>
+		public ISoundProvider Detatch()
+		{
+			var ret = input;
+			input = null;
+			return ret;
+		}
+
+		public void GetSamples(short[] samples)
+		{
+
+			input.GetSamples(samples);
+
+			for (int i = 0; i < samples.Length; i += 2)
+			{
+				sumL -= buffer.Dequeue();
+				sumR -= buffer.Dequeue();
+				short L = samples[i];
+				short R = samples[i + 1];
+				sumL += L;
+				sumR += R;
+				buffer.Enqueue(L);
+				buffer.Enqueue(R);
+				int bigL = L - (sumL >> 16); // / depth;
+				int bigR = R - (sumR >> 16); // / depth;
+				// check for clipping
+				if (bigL > 32767)
+					samples[i] = 32767;
+				else if (bigL < -32768)
+					samples[i] = -32768;
+				else
+					samples[i] = (short)bigL;
+				if (bigR > 32767)
+					samples[i + 1] = 32767;
+				else if (bigR < -32768)
+					samples[i + 1] = -32768;
+				else
+					samples[i + 1] = (short)bigR;
+
+			}
+		}
+
+		public void DiscardSamples()
+		{
+			input.DiscardSamples();
+		}
+
+		public int MaxVolume
+		{
+			get
+			{
+				return input.MaxVolume;
+			}
+			set
+			{
+				input.MaxVolume = value;
+			}
+		}
+	}
+}