ABSTRACT
This chapter introduces the use of feedback in systems to create spectral processors. It begins by examining systems that only use feedback delay. Feed-back delay with arbitrary delay time system is called an inverted comb filter due to the shape of the frequency response. Specifically, the shape is the inverse of the feedforward comb filter. An important filter for audio engineering applications is the bi-quadratic (bi-quad) filter. The bi-quad filter has two feedforward and two feedback delay lines. In both cases, one delay line introduces one sample of delay and the other delay line introduces two samples of delay. There are independent gain coefficients for each path of the filter. The Butterworth filter design function, butter, can create low-pass filter (LPFs), high-pass filter (HPFs), band-pass filter (BPFs), and band-stop filters. Butterworth filters are designed to have a flat pass band.
