Patent GB-549002-A (GB)
Title: Improvements in and relating to electrical communication systems
IPC: H04K1/00
Filed: 1941-01-30
Abstract: 549,002. Secret transmission. TELEPHONE MANUFACTURING CO., Ltd., and PADDLE, L. H. Jan. 30, 1941, No. 33107/39. [Class 40 (iv)] In a secret transmission system the subscribers are able to select one of a number of predetermined secrecy plans, each plan involving periodic changes in the method of transmission (e.g. clear to scrambled or one scrambling scheme to another) in accordance with a repeated cycle. Means are provided to enable the secrecy apparatus to be cut out. In the arrangement described, synchronism is always controlled from the same station which is called the master station. Call by master-station, Fig. 1.- On the removal of the handset, relays A, B, JA pull up and 50-cycle calling current is sent over the line for a period determined by a thermal relay HE which energizes a relay T after, say, two seconds. T releases JA to terminate the signal, locks itself and energizes ZA. At the receiving end, Fig. 2, R responds to the signal and energizes RR which pulls up sluggishly and energizes RA which locks up and rings the subscriber. When the latter replies, relays A, B pull up to open the ringing circuit. The parties can now converse to arrange the secrecy plan. Secret transmission.-During secret transmission, keys SCK are operated at both stations. As a result, relays SCR switch the talking instruments and line to the secrecy apparatus and a group of valves VA-VC at the master station generates ¢ second impulses which are repeated by JB over the line to a relay R and locally to a relay JR which operates after an.adjustable delay corresponding to the transmission time of the line. Relays JR, R thus step switches SWC in synchronism at the two stations throughout the conversation to cause periodical changes in the manner of scrambling, the duration of the various periods in a cycle being determined by the manner in which the terminals of SWC are multipled and connected to the wipers A, B, C of code-determining switches SWA, SWB and the actual manner of scrambling during any given period being determined by the positions of these switches which hunt for positions marked by manually-operated code selecting switches CSA, CSB under control of relays P, Q whenever B is operated. The arrangement shown provides six periods and four manners of scrambling which are brought into use by the operation respectively of relays TA..TD. These control relays X..Z which determine the manner in which the speech currents are led to balanced modulators MA, MB for scrambling at MC, MD for restoring intelligibility and whether 1.5 Kc. or 3 Kc. current is to be fed to these modulators. The speech band is divided into two sub-bands which are always transposed and may or may not be inverted according to the manner of scrambling in use. Clearing.-When the master station hangs up, A, B and T at that station fall and, since ZA remains locked up, E is operated and applies a 500 x 20 pulse to the line during the time taken for HE to operate T. ZA then falls to complete release at the master station, SWC being sent home. At the subsidiary station, M responds to the clearing signal unlocking RA and operating MR which locks. When the subsidiary subscriber hangs up, release is completed but SWC is not restored until a new calling signal is received. Call by subsidiary subscriber.-When the receivers removed, relays A, B, S pull up and a pulse of 500 x 20 current is sent out. At the master station, N responds, energizing NR which locks and rings the master subscriber. Synchronizing.-If the switches SWC fall out of synchronism, the key SYNK is thrown to release T whereupon the pulse-generator stops, and SWC at the master station goes home. JA sends a 50-cycle pulse and this operates R, RR at the subsidiary station to send the switch SWC there home also. Shortly afterwards, HE energizes T which locks as at the beginning of the call, stops the 50-cycle signal and re-starts the pulse generator. Specification 539,940 is referred to.