A signal substitution system is employed in audio processing circuitry to compensate for impulse noise and signal dropouts. Stored or delayed audio signal is substituted in the IF portion of the receiver circuitry ahead of the IF filters thereby precluding the noise impulses from exciting the filters...http://www.google.co.uk/patents/US4462048?utm_source=gb-gplus-sharePatent US4462048 - Noise reduction circuitry for audio signals
