Patent GB-632375-A (GB)
Title: Improvements in or relating to switching systems
IPC: H04Q11/04
Filed: 1946-10-04
Abstract: 632,375. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd., RANSOM, D. H., ROBERTS, W. L., and TAPLIN, R. H. Oct. 4, 1946, No. 29653. [Class 40 (iv)] An exchange comprises a number of transmission paths and a lesser number of channels for the transmission of pulses of electrical energy, electronic means for connecting individual transmission paths to transmission channels, and electronic means for ensuring that two or more transmission channels cannot simultaneously be connected to a given transmission path. The subscriber's lines terminate on the electrodes or dynodes of a cathode beam distributer which provides a number of communication channels in the form of different positions in a recurrent cycle of pulses. When a call is made, a pulse linefinder is seized and opens a communication gate for the pulses of the calling line, suppression pulses being generated to prevent seizure of any other linefinder by the call, and a delay trigger circuit in the seized linefinder maintaining that linefinder insensitive to other calls. Linefinder. The beam of a cathode-ray tube distributer sweeps a circular path to scan the dynodes connected to the subscribers, initiation of a call applying a negative potential to the related dynode to cause secondary emission to a common anode, which feeds several' linefinders. A search pulse unit, Fig. 1, produces equal on-off pulses, (line a, Fig. 4), at a frequency slightly different from the scanning frequency, the pulses being fed into a search pulse shaper, amplifier and phase-shifter in No. 1 linefinder, where the pulse duration is reduced as in line e, the pulses being passed on to successive linefinders with successive phase displacement, as shown in lines e, f. The amplified search pulse passes through an open search pulse suppression gate to a signal gate and delay trigger circuit and when, because of the difference in frequencies, a search pulse comes into phase with a caller's signal from the common anode of the distributor, the combined signals are strong enough to trigger a delay circuit to on, as shown in lines g, h of Figs. 4, 4a, the phase-displacement of the search pulses preventing two linefinders responding to one call. The delay circuit is insensitive to further pulses during the delay period, which is slightly less than the distributer scanning cycle, after which the delay gate opens, as shown in Fig. 4b, at the signal pulse after the first coincident search pulse. The delay gate then (1) sends a suppression pulse to the search pulse suppression gates of all linefinders at the caller's time position, cutting out further search pulses from all linefinders at this time, as shown shaded in lines e, f, g, (2) sends a gate pulse, line i, to its own signal gate and delay trigger circuit to pass the signal pulse of the calling subscriber to the delay circuit and retrigger it at the required time intervals, (3) opens itself to pass the caller's signal direct from the distributer to the line selector, Fig. 2. The signal gate and delay trigger circuit, the delay circuit and delay gate then run continuously throughout the call synchronized with the caller's signal pulses. Line selection. Each dialling impulse reduces the amplitude of the caller's pulse for the duration of such impulse (line 3 of Fig. 7). The modulated pulses pass from the linefinder to a demodulator D, Fig. 2, the output of which consists of designation or other signals substantially in the same form as received by the distributer from the subscriber's instrument. The end of a designation signal sent to a filter and differentiator H triggers a delay circuit I (line 9, Fig. 7), which sends a pulse slightly longer than the scanning cycle period (line 10) to a coincidence mixer J. For the duration of the call, a connection from the linefinder to an integrator rectifier K causes the application of a steady D.C. potential to a gate circuit and mixer L, which also receives synchronizing pulses from a circuit Q and line-timing pulses from a source R, the line-timing pulses being blocked until a call causes the application of the potential from K, when the line timing and synchronizing pulses pass to a gate circuit M, normally open, and so to a delay circuit N. When the equipment is not in use, the delay circuit N receives only the synchronizing pulses which trigger N to on for a period slightly less than the scanning period, to produce gating pulses (lines " 11, 17, 18 ") bracketing subsequent synchronizing pulses, the gating pulses being fed continuously to the coincidence mixer J. At the coincidence in J of the long pulse from I (line 10) and a gating pulse from N (line J), a blanking pulse (line 16) is sent to the gate M to suppress the synchronizing pulse occurring at that time, which delays triggering of the circuit N one step in the scanning cycle. Each subsequent dialling impulse, by blanking out a line-timing pulse, shifts the gate pulse one step in the scanning cycle, after which the gate pulse from N is locked to the next succeeding line timing pulse. At the termination of designation signals, the phase-shifted gate pulse is fed along connection 17 to pass through a called gate E connected at that time in the cycle through the associated distributer to the called subscriber. The phase-shifted gate pulses also pass (1) to the search pulse gates, Fig. 1, of all linefinders where, superimposed on suppression pulses from the linefinder delay gate, they prevent the selection of the called subscriber's line by a free linefinder when he answers the call, and (2) to a mixer gate O, Fig. 2, which opens in response to pulses from the delay gate C and delay circuit N at the times corresponding to the calling and called subscribers in each cycle for six microseconds, bracketing the related pulses on the two distributer anodes (line " 4, 18 and 25 ", Fig. 9). The selected line timing pulses pass through the mixer gate O and are modulated at P by communication signals of the two parties from an amplifier mixer G, which receives the signals from the calling and called parties through delay gates C, E and demodulated D, F, respectively. The modulated pulses pass from P to the grid of the distributer to vary the intensity of the beam at the time positions of the two parties to cause fluctuating secondary emission from the dynodes. The resultant signals are demodulated to actuate the receiving apparatus (line A, Fig. 9), each party receiving his own signals as well as those of the other party. Modifications and details. Instead of having one distributer handling both incoming and outgoing calls, two distributers may be used, one for each function, in which case the speaker's signals would not be repeated back to him. The delay circuits may be of the type described in Specification 632,376. The common anode of the cathode beam distributer feeds a coupling circuit (27, Fig. 21, not shown) which maintains the pulse level irrespective of the number of calls. The dynode used for the production of the synchronizing pulses may be used to give an extra channel if precautions are taken to ensure that the energy taken to provide the synchronizing pulse does not impair the transmission of intelligence. The tube 34, Fig. 26, is normally inert but when coincidence between a search pulse and a calling pulse occurs, a negative pulse on line 40 fires the priming gap 38, 39. Line timing pulses 42 fire succeeding sequence gaps 35-36 until the firing of the last gap 35-37 generates a gate pulse 41 which extinguishes the sequence gaps either directly or through a device ED. Succeeding line timing pulses again fire the sequence gaps in order, and as the number of sequence gaps is made equal to the number of time positions in the distributer cycle, the gate pulse occurs at the same time position in each cycle. The caller's first pulse changes a cleardown circuit 55 (Fig. 27, not shown), to offnormal condition. When the call ends, the circuit 55 reverts to normal condition and applies a negative clear-down pulse 43 to the anode 38 to extinguish the priming discharge of the tube 34, which then becomes inoperative.