strictly less than 1. As can be seen from the graphs, as increases, the amplitude of the maxima rises increasingly rapidly. Impulse response The feedback comb filter is a simple type of infinite impulse response filter. If stable, the response simply consists of a repeating series of impulses decreasing in amplitude over time. Pole–zero interpretation Looking again at the -domain transfer function of the feedback comb filter: This time, the numerator is zero at , giving zeros at . The denominator is equal to zero whenever . This has solutions, equally spaced around a circle in the complex plane; these are the poles of the transfer function. This leads to a pole–zero plot like the ones shown below. Continuous-time comb filters Comb filters may also be implemented in continuous time. The feedforward form may be described by the equation: where is the delay (measured in seconds). This has the following transfer function: The feedforward form consists of an infinite number of zeros spaced along the jω axis. The feedback form has the equation: and the following transfer function: The feedback form consists of an infinite number of poles spaced along the jω axis. Continuous-time implementations share all the properties of the respective discrete-time implementations. See also Dirac comb Fabry–Pérot interferometer References External links Signal processing Filter theory