I'm working on a realtime FFT processing algorithm in C using PortAudio. Currently I'm just trying to test if I can do OLA / OLS in (semi) real time using FFTW. I'm getting some distortion in the output when applying 2x sin and cos windows on the real signal before and after FFTW using the trig identity sin^2+cos^2=1. This should be an identity system but there is some added distortion on the output. I'm only using powers of 2 for the transform currently. Any ideas on why this distortion is occurring?