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

Abstract

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 measured 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 almost monotonically with modulation index β, except for cases with large loop gain irregularities which displayed a reduced GBI for values of β that corresponded to low carrier suppression. Also, GBI was practically independent of the modulation frequency fm already from 0.5 Hz even when this was much lower than the correlation distance of the loop gain transfer function. Rooms with different reverberation times gave different initial (time-invariant) GBI values but these differences decreased by the use of modulation. The GBI increase was larger for cases with external reverberation than for cases without due to increased loop gain irregularity, and the GBI results depended on fm. Since the possible GBI increase is determined by the initial GBI, LTV system performance should be measured in terms of GBI and not GBI increase alone. Robustness increased by equalizing the loop gain before employing LTV components. DM gave little protection for low frequencies but was efficient at high frequencies.