Plotting RBJ Audio-EQ-Cookbook HPF/LPF

Sample Source Code

UI Callback

	ftype = LPF;
	f0 = 1000;
	Q = 5;

	filterL.setParameter(ftype, f0, Q);
	filterR.setParameter(ftype, f0, Q);

Filter Parameter Setup Function

void CFilter::setParameter(int ftype, float f0, float Q)
{
	float omega, sn, cs, alpha;
	float a0, a1, a2, b0, b1, b2;

	omega = 2.f * PI * f0 / 44100.f;
	sn = sin(omega);
	cs = cos(omega);
	alpha = sn / (2.f * Q);

	if (ftype == LPF) {
		// LPF
		b0 = (1.f - cs) / 2.f;
		b1 = 1.f - cs;
		b2 = (1.f - cs) / 2.f;
		a0 = 1.f + alpha;
		a1 = -2.f * cs;
		a2 = 1.f - alpha;
	} else {
		// HPF
		b0 = (1.f + cs) / 2.f;
		b1 = -(1.f + cs);
		b2 = (1.f + cs) / 2.f;
		a0 = 1.f + alpha;
		a1 = -2.f * cs;
		a2 = 1.f - alpha;
	}

	b0a0 = b0 /a0;
	b1a0 = b1 /a0;
	b2a0 = b2 /a0;
	a1a0 = a1 /a0;
	a2a0 = a2 /a0;

	x1 = x2 = 0;
	y1 = y2 = 0;
}

Filtering Function

float CFilter::filter(float sample)
{
	float result = b0a0 * sample + b1a0 * x1 + b2a0 * x2 - a1a0 * y1 - a2a0 * y2;

	x2 = x1;
	x1 = sample;
	y2 = y1;
	y1 = result;

	return result;
}

Signal Processing

void MyApp::processReplacing (float** inputs, float** outputs, VstInt32 sampleFrames)
{
	float* inL = inputs[0];
	float* inR = inputs[1];
	float* outL = outputs[0];
	float* outR = outputs[1];

	for (VstInt32 i = 0; i < sampleFrames; i++) {
		*outL++ = filterL.filter(*inL++);
		*outR++ = filterR.filter(*inR++);
	}
}