GB625737A - Improvements in or relating to frequency-modulated-carrier systems - Google Patents

Abstract

625,737. Frequency modulation. PHILIPS LAMPS, Ltd. July 4, 1946, No. 20007. Convention date, July 6, 1942. [Class 40 (v)] Compression and expansion of the strong and weak portions respectively of an F.M. signal are produced at an F.M. transmitter and receiver respectively, to improve signal to noise ratio, and the mean frequency of the transmitted signal varied in correspondence with the volume of the modulating voltage to produce at the receiver a voltage restoring the correct amplitudes. A D.C. control voltage may be produced across a resistance 5, Fig. 1, from the modulator-amplifier 1 by a diode 4 and used to effect volume compression in the amplifier 1 and to deviate the mean operating frequency of the oscillator 2 accordingly as the modulation depth is less than the maximum. At the receiver, comprising a discriminator 12, a corresponding D.C. voltage is produced and applied to an amplifier 14 to restore the correct volume contrast. Specification 564,041 and U.S.A. Specification 2,006,989 are referred to. The Specification as open to inspection under Sect. 91 describes also a method for producing a D.C. voltage across a resistance 5, Fig. 2 (Cancelled), corresponding to the modulation intensity to amplitude modulate an oscillator 16 and transmitting from an aerial 3 a control signal the amplitude of which increases as the modulating signal decreases. At the receiver aerial a circuit 18 tuned to the control signal applies by way of a rectifier 19 and filter 20 a D.C. control signal to the amplifier 14 restoring the volume contrast. Expansion at the receiver may also be effected by varying the frequency of the frequency changer 9. This subjectmatter does not appear in the Specification as accepted.