FFT Length in Digital Filtering

When the FFT is employed for nonrecursive digital filtering, the time required to process one block of input data, N samples long, is often approximately of the form N(logr (N) + Q), where Q ≥ 0. Earlier results on determining optimum N have assued Q ≥0, becauese it has long been informally observed that typically little is to be gained by taking the value of Q > 0 into account, and because the assumption that Q = 0 permits determination of N without reference to machine dependent parameters