GB497198A - Improvements in secret telephone communication systems - Google Patents

Abstract

497,198. Secret transmission. STANDARD TELEPHONES & CABLES, Ltd. April 27, 1938, No. 12512. Convention date, May 15, 1937. [Class 40 (iv)] In a telephone system, pauses in speech are occupied by noise or by portions of a recorded speech, reproduced at a normal or an abnormally slow rate, which are prevented, under control of a pilot current, from reaching the subscribers. The pilot may be a complex current the frequency components of which are periodically varied under the control of a tape transmitter. These arrangements may be combined with scrambling arrangements of known kind, e.g. those described in British Specification 366,921 or with arrangements as disclosed in U.S.A. Specification 1,981,114 for transmitting message elements in jumbled order. Systems using a single-frequency pilot current. In the system shown in Fig. 4, when neither party is talking, pilot 'current of frequency f 1 is sent out from an oscillator 342 together with speech currents reproduced from a record 322, and pilot current of frequency f2 is received together with speech currents from the distant record, and the latter are recorded on a blank 373. The receiving path beyond the recording apparatus is disabled by relay 378. When subscriber 281 speaks, 293 removes a disablement from the sending path and 298 disables the receiving path. 296 cuts off the pilot current and 297 stops the reproducing gramophone. When the distant party speaks, the pilot current f2 is no longer received and 377, 378 are released to stop the recording phonograph and remove a disabling shunt from the receiving path. The transmitting path is disabled by 383. Additional privacy circuits are provided at 299, 349. In a simpler arrangement, Figs. 1 and 2 (not shown), the gramophone is used merely to provide interfering signals and accordingly is run continuously, no record being made at the other end. System with complex pilot, Fig. 3. The condition of relays 171 .. 177 is governed by a perforated tape 186 driven step-by-step by a magnet 188. Relays 231 .. 237 at the receiving end are kept in the same condition as the relays 171 .. 177 by an identical tape driven synchronously with the tape 186. When subscriber 182 is silent, a combination of frequencies corresponding to the energized relays is sent out and these frequencies are separated by filters and fed to detectors 222 .. 228, but since the corresponding relays at the receiving end are energized, no effect is produced. When the subscriber speaks however, control relays at 268, 271 cause the frequencies corresponding to the de-energized relays to be sent out, and these after rectification exercise control over wire 276 to complete the circuit to the subscriber 266.