Patent GB-1380063-A (GB)
Title: Telecommunication exchange switching systems
IPC: H04Q11/04
Filed: 1971-05-26
Abstract: 1380063 Automatic exchange systems PLESSEY CO Ltd 30 June 1972 [26 May 1971] 17164/71 Heading H4K An exchange is provided with a full-availability switching network, a TDM control circuit and input and output storage devices such that signals in Walsh function format may be transferred therethrough. Briefly stated, in order to connect a line of a first group to a line of a second group, the signals used must belong to an orthogonal system of functions. These may be sine/cosine functions, when the system is said to be complete, or block pulses in which case the system is said to be incomplete. Walsh functions are a set of binary orthogonal functions which fall within the class of complete systems. Such functions when of low sequency (a concept akin to frequency in sine functions and equivalent to half the average number of zero crossings per unit time of a waveform), are similar to heavily clipped sine functions but as sequency increases such as appearance is not always maintained since the pulse width is not uniform throughout a "cycle"-this being likenable to a clipped waveform which is a product of trigonometrical functions. Fig. 3, (which is made up by integrated circuitry), depicts a low pass sequency filter which comprises an integrator A1 having a pulsed feedback circuit S1 for sampling an input speech waveform F(#) and a hold circuit A2 to which the integrated sample Fi(9) is passed by pulsed gate TG so as to produce an output pulse signal FO(#). Such a circuit may be connected to a subscribers line so as to convert his speech signal into Walsh function format or may simply form a pulse store for use during processing, e.g. tandem switching, of such a function. A multiplier (modulo 2 adder), Fig. 4, permits a Walsh function of one sequency to be "modulated" on a carrier so as to produce a third Walsh function of a sequency different from either the carrier or the modulation (effectively a modulo 2 addition of the two sequencies expressed in binary form). A plurality of such "modulated" functions are simply added together to provide a sequency multiplex in analogous manner to a frequency multiplex. Tandem exchange system (Figs. 1, 2), comprises a plurality of 32 channel incoming junctions I/CJ which are connectible via a time controlled network SN to a further plurality of 32 channel outgoing junctions O/GJ. Each junction is terminated by 32 multipliers each having a particular Walsh function multiplying input T1-T3 for demodulating, i.e. demultiplexing, the incoming channels and modulating, i.e. multiplexing, the outgoing channels. A low pass sequency filter is connected in cascade with each incoming multiplier, the requisite sampling and transfer pulse trains S1 and S2 respectively being supplied in parallel to all the filters as well as to a set of transfer gates 10TG1-10TG2 which separate cascade connected sample holding circuits located between a network outlet and the 32 outgoing multipliers. Channel switching is effected by cyclically reading store CS wherein each memory cell corresponds to a network outlet transfer gate 10TS1-10TS32 which is opened in conjunction with the reading of its cell and each cell contains the identity of an inlet, transfer gate 1ITS1-11TS<SP>1</SP>32 which must be opened contemporaneously therewith. The inlet gates are thus opened on an acyclic basis, although it is suggested that these could be operated cyclically while the outlet gates are operated acyclically. During the opening time of the gates the amplitude of the Walsh function step waveform currently stored in an input filter is transferred to the first outlet hold circuit from where it is transferred to the second hold circuit by the next S<SP>1</SP>2 pulse. Interhighway switching-which must of course occur simultaneously with transfer gate operations is vested in stores XPRC associated one with each network row and containing the identity of the particular cross point that must be operated during any time slot in order to connect its row's incoming junction to a desired one of the outgoing junctions.