GB564041A - Frequency modulation systems - Google Patents

Abstract

564,041. Telephone transmission. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 26, 1943, No. 8432. Convention date, May 27, 1942. [Class 40 (iv)] [Also in Group XL] In a frequency modulation system the frequency deviation is kept within comparatively narrow limits by compressing the volume of the signal at the transmitter prior to modulation and correspondingly expanding the volume at the receiver after demodulation. A pilot signal indicating the degree of compression and expansion comprises a variable supersonic frequency sent as a frequency modulation of the carrier. The transmitter, Fig. 1, may comprise a microphone 1, amplifier 2, thermionic variable reactance 3, carrier wave source 4 and aerial 5. The amplifier 2 feeds a rectifier 9 which comes into action at a determined input voltage and produces an " envelope " voltage across its load resistance 10, which voltage is applied to the amplifier 2 to reduce its amplification as the volume of signal increases. The voltage across resistance 10 is also applied to a thermionic reactance 7 so as to vary a supersonic frequency generated at 6 and applied to the reactance 3 to frequency-modulate the carrier wave. At the receiver, Fig. 4, the amplifier 13, which may include a superheterodyne stage, feeds a detector 14 from which the detected signal passes through amplifiers 15, 16 to the sound reproducer. The supersonic pilot signal is selected by a filter 17 and its frequency variations are detected by a resonant transformer 21 and rectifier 18, the resulting voltages across the resistance 19 being applied to the amplifier 16 to expand the volume in accordance with the compression effected at the transmitter. On account of the reduced frequency deviation of the carrier, the amplifier 13 may pass a comparatively narrow band.