UnstableFilter.cpp

	struct ZeroDelay2Lp
	{
		double z1_state;
		float g;

		// directory from page 48
		// https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf
		inline float Process(float x)
		{
			// perform one sample tick of the lowpass filter
			float v = (x - z1_state) * g / (1 + g);
			double y = v + z1_state;
			z1_state = y + v;
			return y;
		}

		inline void SetG(double g)
		{
			this->g = g;
		}
	};
	
	float FilterCascadeZero::ProcessSample(float input)
	{
		float fb = totalResonance * 3.9f * (feedback); // totalResonance = [0 ... 0.999]
		float value = input - fb;

		// just for testing, prevent nan and infs
		if (value > 10)
			value = 10;
		else if (value < -10)
			value = -10;
		else if (isnan(value))
			value = 0;
		
		// 4 cascaded low pass stages
		value = lp1.Process(value);
		value = lp2.Process(value);
		value = lp3.Process(value);
		value = lp4.Process(value);
		feedback = value;
		output = feedback;
		return output;
	}
	
	
	
	// Setting the resonance and g parameters is straight forward. The problem occurs at low fc with high resonance, and also at higher fc with moderate resonance
	// I'm using a sampling frequency of 96 Khz, the input data is bandlimited to 20Khz.
	// fsinv is 1.0 / Samplerate
	// fc is 10...20000 hz
	
	float g = fc * M_PI * fsinv;
		lp1.SetG(g);
		lp2.SetG(g);
		lp3.SetG(g);
		lp4.SetG(g);
		

Updated

	float FilterCascadeZero::ProcessSample(float input)
	{
		input *= gain;
		float output = 0.0;


			float x = input;
			float k = totalResonance * 3.9f;
			float g = lp1.g;
			float g2 = g * g;
			float G = g2 * g2;
			float S = g2 * g * lp1.z1_state + g2 * lp2.z1_state + g * lp3.z1_state + lp4.z1_state;

			float value = (x - k * S) / (1 + k * G);
								
			// 4 cascaded low pass stages
			value = lp1.Process(value);
			value = lp2.Process(value);
			value = lp3.Process(value);
			value = lp4.Process(value);
			output = value;
		

		return output;
	}