GB309063A - Improvements in secret electric signalling systems - Google Patents

Abstract

309,063. Standard Telephones & Cables, Ltd., (Bell Telephone Laboratories, Inc.). Jan. 3, 1928. Telophone currents, inverting and transposing frequencies of.--For obtaining secrecy in telephone and like transmission systems, the original band of waves is fed in parallel to a number of modulators so as to shift the band to different but overiapping frequency positions, and, the shifted bands are then passed through filters having identical cut-off frequencies. Fig. 2 shows diagrammatically the four bands A - - D, each of speech frequency range, and each shifted to a different extent, so that the overlapping portions, each representing a different section of the original band, may be selected by identical filters having cut-offs represented by the horizontal lines E, F. The electrical diagram, Fig. 1, shows the line L1 branched to four filters F1 each passing the whole speech band, and leading to four modulators MA - - MD, by which the frequencies are raised to different extents, as indicated in Fig. 2. The shifted bands are then passed through identical but separate filters Fe, each adapted to pass the band EF. Fig. 2. These sub-bands are then interchanged and manipulated in the secrecy apparatus S and are passed to demodulators DA - - DD, each of which can have its carrier set at either of two frequencies, all as disclosed in Specification 299,915. The subbands are thus brought back within the speechfrequency range, and pass through filters F2, arranged to pass the whole speech range, and through filters FA - - FD, arranged to pass different sections of the speech frequency range, to a line L2 where they are re-combined to form a scrambled mixture of the sub-bands which is not intelligible in an ordinary receiver. The scrambled mixture may be re-constituted into intelligible speech currents by passing it through an apparatus similar to that shown in Fig. 1. The filters F1, F2 serve to prevent the carrier currents from the respective sources OA - - OD, OA<1> - - OD<1>, from reaching the other channels. The secrecy system may be combined with a two-way repeater as described in Specification 299,915. Resistances R at the input end of the secrecy apparatus are arranged to match the impedance of the channel circuits with that of the line L1. A compensating filter CF is arranged at the output end, for improving the efficiency of the band filters FA - - FD near their cut-off limits.