Fm stereophonic broadcasting system utilizing simultaneous companding

Abstract

An FM stereophonic broadcasting system incorporating companding of the difference signal in which the broadcast signal includes both the usual difference signal and a compressed version of the difference signal which utilizes, in the transmitter, a circuit for simultaneously compressing the difference signal according to a desired characteristic and modulating the compressed difference signal onto a carrier and, in the receiver, essentially the same circuit for simultaneously demodulating and expanding an incoming compressed difference signal. Simultaneous compression/modulation is achieved by multiplying the difference signal, the carrier signal and a gain control signal, preferably derived from the output signal from the multiplier, for determining the compression characteristic, thereby producing at the output of the multiplier the carrier modulated with the difference signal compressed according to a desired characteristic. Simultaneous demodulation and expansion of a received signal is achieved by multiplying the received signal, a reference carrier signal and a gain control signal, also preferably derived from the output of the multiplier, for determining the expansion characteristic, thereby producing as the multiplier output the difference signal expanded according to the desired characteristic. Expansion may be, and preferably is, controlled by the received usual difference signal so as to be adaptive to any compression characteristic that might be employed at the transmitter.