GB611983A - Improvements in or relating to broadcast and other radio systems - Google Patents

Abstract

611,983. Volume compression and expansion. BRANDON, P. S. May 24, 1946, No. 15881. [Class 40 (iv)] In a system in which signals are transmitted as amplitude modulation of a carrier, contrast control (i.e. volume compression and expansion) signals are transmitted as frequency or phase modulation of the same carrier. A frequency or phase modulated system may similarly have its contrast control signals transmitted as amplitude. modulations of the carrier. A transmitter, Fig. 1, for use in a system of the first type comprises a modulator 2 fed by a master oscillator 1 and a microphone 4, contrast control being effected manually by a potentiometer 5 to which is ganged a potentiometer 7 controlling a reactance valve 6 which varies the frequency of the master oscillator 1. Control of the output of the microphone and of the frequency of the oscillator 1 may instead be effected automatically. The signal may be received by the superheterodyne receiver, Fig. 2, comprising an R.F. stage 11, first detector 13 fed by local oscillator 12, I.F. stage 14, second detector 15 and A.F. amplifier 16. Automatic frequency control is obtained by feeding part of the I.F. output to the discriminator 18 whose output is limited and fed to a reactance valve 19 which controls the local oscillator 12. The output of the discriminator is also fed to a low-pass filter 20 for controlling the amplification of the stage 16, the effect of slow frequency drift in the receiver itself being eliminated by interposing between the filter 20 and the audio amplifier 16, a differentiating circuit 21, a limiter circuit 22 which passes only inputs above a predetermined level, and an integrating circuit 23. Similar circuits, Figs. 3 and 4 (not shown), are described for a frequency or phase modulated system having contrast control by amplitude modulation.