Patent GB-1118608-A (GB)
Title: Improvements in or relating to telecommunications systems
IPC: H04J9/00
Filed: 1966-08-19
Abstract: 1,118,608. Pulse modulation systems. SIEMENS A.G. 19 Aug., 1966 [20 Aug., 1965], No. 37204/66. Heading H4L. In a telecommunications system in which two independent simultaneous information transmission connections are established on a single line, one connection uses a pulse modulation system in which a modulated pulse sent from a first terminal station is employed at a second terminal station to produce a derived pulse which is modulated and sent to the first terminal station. It is arranged that the derived pulse is received at the first station before a further pulse is sent from it to the second station. As described, two terminal stations E1, E2, Fig. 1, are connected to the ends of a line L by networks N, which decouple them from other stations using the line. A modulator M at the station El receives as inputs pulses from a generator P and, at a terminal e, modulating signals NF. Position modulated pulses from the output of the modulator M are amplified by an amplifier V, are transmitted to line, and at the station E2 pass via a controllable threshold circuit S to a pulse position modulation (PPM) to pulse duration modulation (PDM) converter K which may comprise a bi-stable trigger. Signals equivalent to those incoming at the terminal e of the station E1 are obtained at a terminal a of the station E2 by connecting a low-pass filter TP to the output of the converter K. A pulse generator G, which is synchronized by the pulses appearing at the output of the circuit S, and which may consist of a passive high-quality oscillatory circuit, is connected to a second input of the converter K. A modulator M at the station E2 receives as inputs pulses from the output of the circuit S (via a terminal y) and modulating signals NF at a terminal e. PPM signals are ampified by an amplifier V, are transmitted to line, and at the station E1 pass via a controllable threshold circuit S to a PPM to PDM converter K, and then via a low-pass filter TP to an output terminal a. The converter K has a second input of PPM signals from the output of the modulator M, and since the signals incoming from the station E2 carry modulation from the station E1 as well as the station E2, cancellation of the former may thus be achieved. The threshold circuits S are set to maximum values as pulses are transmitted, by means of connections to the outgoing sides of the stations, and decay exponentially thereafter, so providing protection against echoes whilst accepting incoming signal pulses. In an embodiment for application to a telephone system (Fig. 3, not shown), each subscriber line may have two subscribers, one being normally connected, and the other being connected by means of a pulse modulation system of the kind described. Decoupling between the two subscribers is effected at each end of the line by high-pass and low-pass filters. A common pulse generator at the exchange serves all subscribers using pulse modulation. In the circuitry for such a system, blocking oscillators Sp, Fig. 4, are interposed between the modulators M and the amplifiers V, and the high-pass filters to line are constituted by transformers Uh and capacitors Ch, said transformers converting the pulses from the outputs of the amplifiers V from unipolar to bipolar form for transmission on the line. The circuits S comprise capacitors C3 which are charged by outgoing pulses and which discharge through paralleled resistors R3. The potentials across the capacitors control the switching thresholds of transistors Tr. The oscillatory circuit G at the station E2 comprises a capacitor Co and an inductor Lo. Ringing is effected by a relay rls1 at the exchange-end station E1 which rhythmically opens a contact Sr1 and interrupts the stream of pulses outgoing to the subscriber-end station E2, causing a bell unit W t9 be'operated until disconnected by a cradle-switch contact s2. D.C. power at the station E2 may be provided by a secondary battery which is switched in by another cradle-switch contact sl, and which may be charged from the exchange through the subscriber line. Calling of the exchange is effected by operating a relay rls2 at the station E1 by output from the converter K when pulses are received from the station E2. A contact sr2 of the relay rls2 closes a calling loop Schl. At the station E1, the voice-frequency input and output are coupled by a 4-wire/2-wire terminating circuit Ga.