Digital lowpass filter causing aliasing

Question

I'm writing a biquad filter program in C++, using the RBJ Cookbook as the source for the formulas. https://www.w3.org/TR/audio-eq-cookbook/ The lowpass filter that I've made seems to half work. The frequencies above the cutoff are being attenuated, however, it is causing aliasing similar to what happens with frequencies above Nyquist's limit.

double output, a0, a1, a2, b0, b1, b2, w, alpha; 
double pi = 3.14159265359;
w = (2 * pi) * (cutoff / sampleRate);
alpha = (sin(w)) / 2 * q;
a0 = 1 + alpha;
a1 = -2 * cos(w);
a2 = 1 - alpha;
b0 = (1 - cos(w)) / 2;
b1 = 1 - cos(w);
b2 = (1 - cos(w)) / 2;

output = (b0 / a0) * inSamples[0] + (b1 / a0) * inSamples[1] + (b2 / a0) * inSamples[2]
  - (a1 / a0) * outSamples[0] - (a2 / a0) * outSamples[1];

I'm not sure where my error is, so any help is greatly appreciated!

Answer 1

your output is definitely a linear combination of previous in- and output samples.

That means it's a linear time-invariant operation.

That means it can't introduce aliases.

What you see is something else than an alias, and I'll venture the guess you're interpreting sidelobes as aliases.