A voice-band data symbol is typically impaired by so-called phase jitter prior to symbol being received at a data modem. Modem circuitry estimates the phase angle of the jitter and an infinite-impulse-response (IIR) filter adjusts to that phase angle so that the jitter can be corrected for by a demodulator...http://www.google.com/patents/US4847864?utm_source=gb-gplus-sharePatent US4847864 - Phase jitter compensation arrangement using an adaptive IIR filter
