Reduction of acoustic feedback oscillations by use of spectrum shifting

The problem of whistling noise or self-oscillation in public address systems operating in closed halls or rooms can be very disturbing to the listeners. This noise is sustained when the positive acoustic feedback, of the public address-room system, meets the oscillation criteria. This feedback system results from the acoustic sound signal reflected off the room walls and any other obstacles in the room, originating from the public address speakers and then re-entering the microphone. When all of the audio signal components entering the public address system are shifted by a frequency increment Δf of 6 Hz, an increase of 5 dB in the useable signal level was achieved and whistling noise reduction is attained. This thesis describes an apparatus for frequency shifting by small increments in steps of 1 Hz. The system is of simple implementation and effectively reduces the whistling noise and increases the value of achievable gain without introducing any speech distortions. While the idea turned out to be not original, since it was described in the early sixties, the time for it may be now, because integrated circuits made it feasible in terms of cost, size and portability. Due to this, we feel that reintroducing the idea at this time may be fortuitous