GB890797A - Improvements in or relating to alternating current signal receiving circuits - Google Patents

Abstract

890,797. Voice immunity circuits. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. July 2, 1959 [July 3, 1958], No.21275/58. Class 40 (4). A series connection of a slow-to-operate and fast release circuit with a fast operating but slow-to-release circuit acts as a band pass filter which cuts off when the input includes noise or speech. As shown in Fig. 2, a sinusoidal input of 150 c.p.s., or buzzer tone with a fundamental of 150 c.p.s., is applied to normally conducting transistor TX6 which produces a square wave output. By adjusting the level at which TX6 cuts off the harmonic content of the input may be eliminated. Normally, non-conducting transistors TX7, TX8 comprise a slow-to-operate circuit with TX7 following the square wave input but TX8 conducting only after capacitor C2 has charged sufficiently. Thus TX8 conducts only for inputs with a frequency less than that which provides a period of twice its switching-on time. Normally conducting transistors TX9, TX10, cut off immediately in response to input to TX9. At the end of an input TX9 conducts at once but TX10 will conduct only after the delay imposed by capacitor C3. A frequency exists below that which just operates TX8 which pulses TX10 just often enough to keep it cut off. A band of frequencies is therefore defined above which TX8 does not conduct and below which the output of TX10 is constantly interrupted. For signals in the band the output of TX10 is continuous. Transistors TX11, TX12 constitute a slow to operate circuit which, with a time constant of one second guards against spurious or transient signal inputs. With a persistent in-band signal transistor TX13 cuts off normal conduction and drives TX14 to operate relay RA. A narrow signal frequency band is obtained by the use of two slow-operating and slow release circuits connected in parallel as shown in Fig. 3 where corresponding transistors are given the same numbers as in Fig. 2. The upper transistor chain produces an output for signals below a frequency F1 while the lower transistor chain produce an output for signals below a frequency F2, less than F1. The output of the upper transistor chain operates a relay RB only so long as there is no output from the lower transistor chain to make transistor TX24 conduct and clamp transistor TX25. Specification 888,843 is referred to.