Abstract: Noise reducing circuits for electronic receiving instruments, such as telephone receivers in headsets or handsets that are used in noisy locations, provide compensation of the set's receiver unit-to-error microphone transfer function to enhance the noise reduction. Further circuits provide pre-conditioning of the incoming voice signal to make the noise cancellation more effective. The tendency of these noise cancelling circuits to oscillate is substantially lessened by added circuitry which rapidly detects onset of oscillation and momentarily reduces the noise cancellation without interrupting the incoming speech path altogether.

Abstract: The AM signal from a microphone is converted into an inaudible, FM signal which is combined with the AM signal to form a composite signal. The composite signal is amplified and converted into sound waves by a speaker. Any sound returning to the microphone from the speaker has an FM component and the FM component is demodulated to reconstruct the original AM sound. The reconstructed AM signal is subtracted from the signal from the microphone, thereby removing the echo and cancelling feedback.