Hi,
I am implementing a FIR with 512 taps emulating a guitar amp/cab Impulse Response from external wav file, using the DaisySP fir class . I can see the frequency response on the DAW spectrum analyzer plugin. As long as only the FIR is on, it looks and sounds fine. As I try to add distortion before, or reverb after the FIR stage, I see substantial harmonic content appearing at high frequencies (>10kHz) all the way up to the Nyquist freq (I am sampling at 48Khz). My distortion is x4 up-sampled with IIR AAF, the reverb has only delay lines, allpass filters and feedback. Both effects do not cause any noticeable aliasing. Any idea from where this aliasing is coming from?