Frequency shifting for acoustic feedback reduction

Acoustic feedback is a well-known phenomenon in any electro-acoustical real-time system. Whenever the loop gain of a system exceeds 1 for any frequencies, the system becomes unstable: Feeding the amplified sound signal back into the microphone causes a penetrating whistle. Among others, one concept to reduce acoustic feedback is frequency shifting. The implementation of a frequency shifter is easy by using analog circuits, but quite demanding for digital real-time applications like digital audio mixers. This paper describes the theoretical background and our realtime implementation on a TI TMS320C5416. We used the complex FIR filter approach for analytic signal generation in order to ensure equal amplitude response and constant group delay in the real and imaginary signal path. At a sampling frequency of fS = 24 kHz we got satisfactory results with a group delay of about 10 ms which is short enough for real-time systems.