Computer Simulations of Periodically Time-Varying Filters for Acoustic Feedback Control
Peter U. Svensson · Journal of the Audio Engineering Society · 1995
Several modern systems for reverberation enhancement use time-varying electronic filters to increase their gain before instability (GBI). A computer simulation study is presented of systems with linear periodically time-varying (LPTV) filters based on an iterative frequency-domain method. Results are given from simulations and measurements of a single feedback loop with phase and delay modulators and feedback via an electronic reverberation unit. Comparisons of measured and predicted values of GBI with a sinusoidal phase modulator showed deviations typically around 0.1 dB and at most 0.3 dB for modulation frequencies between 0.5 and 4 Hz and modulation indexes of up to 4.2 radians. Phase modulators displayed GBI improvements increasing almost monotonically with a modulation index up to 4.2 radians. Delay modulators gave slight improvements when used broad band, but when high-pass filtered at 600 Hz substantial improvements resulted with variations as low as ±0.5 ms. The detrimental effects of modulation in the form of audible amplitude and frequency fluctuations were studied as well.