Performance of some linear time-varying systems in control of acoustic feedback

Some linear time-varying (LTV) components used to control feedback in sound systems were tested experimentally in real-time simulators and rooms with and without external reverberation. Gain before instability (GBI) was measu red in single channels employing frequency shifting (FS), phase modulation (PM), and delay modulation (DM) implemented on a digital signal processor. FS performed according to the established theory. For PM GBI increased almo st monotonically with modulation index beta, except for cases with large lo op gain irregularities which displayed a reduced GBI for values of beta tha t corresponded to low carrier suppression. Also, GBI was practically indepe ndent of the modulation frequency f(m) already from 0.5 Hz even when this w as much lower than the correlation distance of the loop gain transfer funct ion. Rooms with different reverberation times gave different initial (time- invariant) GBI values but these differences decreased by the use of modulat ion. The GBI increase was larger for cases with external reverberation than for cases without clue to increased loop gain irregularity, and the GBI re sults depended on f(m). Since the possible GBI increase is determined by th e initial GBI, LTV system performance should be measured in terms of GBI an d not GBI increase alone. Robustness increased by equalizing the loop gain before employing LTV components. DM gave little protection for low frequenc ies but was efficient at high frequencies. (C) 1999 Acoustical Society of A merica. [S0001-4966(99)03407-4].