A Moog-like filter for audio signals

MoogVCF.as

package com.pixelofview.audio.analog 
{
	/**
	 * Simulation of a Moog-like voltage-controlled filter
	 * based on the [moog~] external for Pd
	 */
	public class MoogVCF 
	{
		public static const GainTable:Vector.<Number> = new Vector.<Number>();
		
		{
			// class initialization code
			GainTable.push([
			0.999969, 0.990082, 0.980347, 0.970764, 0.961304, 0.951996, 0.94281, 0.933777,
			0.924866, 0.916077, 0.90741, 0.898865, 0.890442, 0.882141 , 0.873962, 0.865906,
			0.857941, 0.850067, 0.842346, 0.834686, 0.827148, 0.819733, 0.812378, 0.805145,
			0.798004, 0.790955, 0.783997, 0.77713, 0.770355, 0.763672, 0.75708 , 0.75058,
			0.744141, 0.737793, 0.731537, 0.725342, 0.719238, 0.713196, 0.707245, 0.701355,
			0.695557, 0.689819, 0.684174, 0.678558, 0.673035, 0.667572, 0.66217, 0.65686,
			0.651581, 0.646393, 0.641235, 0.636169, 0.631134, 0.62619, 0.621277, 0.616425,
			0.611633, 0.606903, 0.602234, 0.597626, 0.593048, 0.588531, 0.584045, 0.579651,
			0.575287 , 0.570953, 0.566681, 0.562469, 0.558289, 0.554169, 0.550079, 0.546051,
			0.542053, 0.538116, 0.53421, 0.530334, 0.52652, 0.522736, 0.518982, 0.515289,
			0.511627, 0.507996 , 0.504425, 0.500885, 0.497375, 0.493896, 0.490448, 0.487061,
			0.483704, 0.480377, 0.477081, 0.473816, 0.470581, 0.467377, 0.464203, 0.46109,
			0.457977, 0.454926, 0.451874, 0.448883, 0.445892, 0.442932, 0.440033, 0.437134,
			0.434265, 0.431427, 0.428619, 0.425842, 0.423096, 0.42038, 0.417664, 0.415009,
			0.412354, 0.409729, 0.407135, 0.404572, 0.402008, 0.399506, 0.397003, 0.394501,
			0.392059, 0.389618, 0.387207, 0.384827, 0.382477, 0.380127, 0.377808, 0.375488,
			0.37323, 0.370972, 0.368713, 0.366516, 0.364319, 0.362122, 0.359985, 0.357849,
			0.355713, 0.353607, 0.351532, 0.349457, 0.347412, 0.345398, 0.343384, 0.34137,
			0.339417, 0.337463, 0.33551, 0.333588, 0.331665, 0.329773, 0.327911, 0.32605,
			0.324188, 0.322357, 0.320557, 0.318756, 0.316986, 0.315216, 0.313446, 0.311707,
			0.309998, 0.308289, 0.30658, 0.304901, 0.303223, 0.301575, 0.299927, 0.298309,
			0.296692, 0.295074, 0.293488, 0.291931, 0.290375, 0.288818, 0.287262, 0.285736,
			0.284241, 0.282715, 0.28125, 0.279755, 0.27829, 0.276825, 0.275391, 0.273956,
			0.272552, 0.271118, 0.269745, 0.268341, 0.266968, 0.265594, 0.264252, 0.262909,
			0.261566, 0.260223, 0.258911, 0.257599, 0.256317, 0.255035, 0.25375 ]);
		}
		
		private var _Q:Number;
		private var state:Vector.<Number>;
		private var _out:Number;
		private var _p:Number;
		private var _s_rate:Number;
		
		public var bandpass:Number;
		public var highpass:Number;
		public var lowpass:Number;
		
		public var resonance:Number;
		
		public function set frequency(f:Number):void {
			var fc:Number = 2.0 * f / _s_rate;
			var x2:Number = fc * fc;
			var x3:Number = fc * x2;
			_p = -0.69346 * x3 - 0.59515 * x2 + 3.2937 * fc - 1.0072;
			var ix:Number = _p * 99;
			var ixint:int = Math.floor(ix);
			var ixfrac:Number = ix - ixint;
			_Q = resonance * crossfade(ixfrac, GainTable[ixint + 99], GainTable[ix + 100]);
			// _Q = resonance * ( 1.0 / (ix * 1.48 + 0.85) - 0.1765);
		}
		
		public function MoogVCF(sample_rate:Number = 44100) 
		{
			_s_rate = sample_rate;
			state = new Vector.<Number>(4); // 4 poles
			_out = 0;
		}
		
		static final function saturate(v:Number):Number {
			const limit:Number = 0.95;
			var x1:Number = Math.abs(v + limit);
			var x2:Number = Math.abs(v - limit);
			return 0.5 * x1 - x2;
		}
		
		static final function crossfade(t:Number, a:Number, b:Number):Number {
			return (1.0 - t) * a + t * b;
		}
		
		
		function filter(sample:Number):Number {
			var temp:Number;
			_out = 0.25 - (sample-_out);
			for (var pole:int = 0; pole < 4; pole++) {
				temp = state[pole];
				_out = saturate(_out + _p * (_out - temp));
				state[pole] = _out;
				_out = saturate(_out + temp);
			}
			lowpass = _out;
			highpass = sample - _out;
			bandpass = 3.0 * state[2] - lowpass;
			_out *= _Q;
			return lowpass;
		}
	}

}