A Study of Periodically Time-Variant Electroacoustic Reverberation Enhancement and Public Address Systems
Several modern systems for reverberation enhancement employ time variance in the electronic filters to increase the possible gain before instability. This paper presents some results from a computer simulation of such systems. The simulation is based on an iterating frequency domain method. A single feedback loop is studied where a linear, periodically time-variant filter consists of a single FIR tap plus modulation. Feedback is employed via a perfectly exponential room impulse response of 1.0 s reverberation time simulating a case where feedback via the reverberant field is dominating. The frequency range has been limited to 500 Hz for some cases and 250 Hz for other cases. As long as improvements in comparison with classical, time-invariant systems of the same bandwidth are made, differences as compared to a full frequency range should be rather small. Phase modulation, delay time modulation, and frequency shifting have been studied, both with sinusoidal and pseudo-random modulation signals. Results confirm the high theoretical improvement of the gain before instability for frequency-shifting systems. A difference of 2 dB from classical predictions arises. The other investigated modulation methods can give up to 2.5 dB of improvement. For the type of delay-time modulation simulated, a modulation span as low as ±1 ms is effective above approximately 250 Hz, even with a modulation frequency of 0.5 Hz. Low modulation frequencies decrease the negative audible effects of the modulation. The method presented is novel and facilitates studies of rather complex modulation schemes even though very basic modulation types were studied here. Effects of source-to-microphone, and loudspeaker-to-receiver transfer functions can also be incorporated as well as multiple channels.
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