Patent GB-887275-A (GB)
Title: Improvements in receivers for voice-frequency electric signals
IPC: H04Q1/448
Filed: 1958-03-11
Abstract: 887,275. Automatic exchange systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. March 11, 1958 [March 14, 1957], No. 7797/58. Class 40 (4). An arrangement for receiving voice-frequency calling signals of different frequencies is provided with a circuit, responsive to the passage through zero of the signal, for measuring the duration between two zero values of the signal, the measuring circuit providing a number of discrete electrical outputs corresponding to different durations between zero values of the signal. Substation transmitting equipment, Fig. 1.- Each substation is provided with 10 pushbuttons, depression of which closes one of contacts K1-2 . . . K9-0, together with KE if the digit is even, and also opens KB, KA, the latter being the last to operate (open) on depression of the button and the first to operate (close) on release of the button. Closure of the line loop by contacts h1, h2 gives the usual D.C. preparatory and holding signal to the exchange. Depression of a button prepares an oscillatory circuit comprising condenser C2 and a tapping of inductance S, and when KA opens there is produced first a sudden drop in the line current and then an oscillation due to the release in energy in the oscillatory circuit. This oscillation is amplified and transmitted to line by the transistor TR1. If the contact KE is also closed then, during the depression of the button, condenser C1 charges over the high resistance R4. Then on release of the button, KA first closes to complete the D.C. path once more and then KB closes to release the energy stored in the oscillatory circuit C1, S so that a further oscillation is sent to line. Exchange receiving equipment. Figs. 2, 3.- Incoming signals are received at A. The pulse due to the initial reduction in D.C. operates flipflops F1, F2, F3. Limiting amplifier BV amplifies the incoming oscillations producing a square-wave output which, however, is inhibited as yet by gate P2. After a period sufficient to allow transients to be damped out, F1 restores and on the next trailing edge of the output from BV, gate P1 passes a pulse to restore F2 and open gate P2. The positive square-wave thus passed to transistor TR2 cuts the latter off, and the tuned circuit L, C6 thus resonates to produce an output via gate P3 to terminal U1. At the end of the square wave the transistor TR2 again conducts and the oscillation is heavily damped. At the same time F3 is restored by a pulse over condenser C5 and P3 is closed to prevent a further output over terminal U1. The tappings on transformer S, Fig. 1, are so chosen that the half-cycle time of the oscillations produced thereby are equal to one, two . . . five cycles of the oscillation produced by L, C6 and these are counted by the gas tube counting chain B1, B3 . . . B9. When the loop current rises due to the closure of KA, Fig. 1, the resulting positive pulse triggers flip-flop F4 and, after a period to allow transients to die down, F4 restores to operate flip-flop F5 and open gate P4. If the digit is even, a further oscillation is received and passes to terminal U2. Each of the counting stages B1, B3 . . . B9 primes a corresponding stage B2, B4 . . . B0 and if an input is received on U2 the discharge is transferred from the odd numbered stage to the next numbered even stage. Specification 887,276 is referred to. Reference has been directed by the Comptroller to Specification 858,275.