Overmodulation protection in FM transmitters

Abstract

The FM radio broadcast standard mandates the use of an audio pre-emphasis filter prior to frequency modulation, to enhance robustness against noise coloration inherent to FM reception. The standard also places a restriction on the maximum frequency deviation that can be imparted by a message signal modulating the FM carrier, to limit the occupied bandwidth of the broadcast signal. Because of the frequency dependent gain profile of the pre-emphasis filter, an audio signal, even if amplitude limited prior to pre-emphasis, is not guaranteed to remain so after pre-emphasis. Hence, there arises a need for a gain control mechanism specifically tailored to handle pre-emphasized audio in any FM broadcast system, to prevent overmodulation. In this paper, we present a novel, low-complexity, digital adaptive filter based algorithm for overmodulation protection in FM radio transmission.