GB854375A - Improvements in or relating to signalling systems - Google Patents

Abstract

854,375. Voice-frequency signalling. WESTERN ELECTRIC CO. Inc. June 17, 1959 [June 20, 1958], No. 20737/59. Class 40 (4). [Also in Group XL (c)] A V.F. signalling arrangement employs in combination two frequencies selected from different frequency bands A, B, and at the receiving end the bands are segregated and each is fed to a limiter which has the property of distinguishing the predominant frequency from the remainder of the band. As shown, four signalling frequencies are provided in each of bands A, B and one out of each band is selected by means of push-buttons 13 which also at 17 disconnect the speech circuit 11 from the transmission line 12. At the receiving end the signals are fed through a transistor emitterfollower 21 bridged by an impedance-matching resistor 22 to a pair of band-stop filters 23, 24 which eliminate the A and B bands respectively. The limiter 30 which receives the output of filler 23 comprises a PNP-type transistor amplifier 32 which produces a limiting effect by virtue of diodes 36 coupled through a transistor 43 acting as an impedance-transformer to a pair of transistors 44, 45 of PNP and NPN-type respectively. These operate in push-pull, each becoming saturated while the other is non-conducting, so that the output is a series of square pulses of which the mean fundamental frequency is that predominating in the input to the limiter. A similar limiter 31 receives the output of filter 24. The outputs of the limiters go via rheostats 49 to bandpass filters 47, 60 tuned to the individual signalling frequencies and thence to transistor amplifiers 54 of which the first stage 53 is biased to be non-conducting unless the input level exceeds a certain threshold value. The amplifiers 54 provide unidirectional current for operating relays 57, 62 which control signal lamps 64 or other devices each corresponding to a particular signal. In a modification, Fig. 5 (not shown), the possibility of interfering noise producing the effect of a valid signal is further reduced by the provision of an equalizer in front of the band-stop filters to enhance the energy level outside the signal bands. In this arrangement high-pass and low-pass filters may be used to separate the bands.