GB226488A - Improvements in secret signalling systems - Google Patents

Abstract

226,488. Western Electric Co., Ltd., (Assignees of Fletcher, H.). Dec. 18, 1923, [Convention date]. Distortion of voice currents, effecting and neutralizing; thermionic modulating systems.- Speech and like waves are transmitted after division into sub-bands some of which are interchanged with or without inversion, while keeping the total range of the resultant wave within the same frequency limits as the original wave. As shown, speech waves from a transformer 11 are divided by filters F<1> - - F<3> into sub-bands 0 to 800, 800 to 1600, and 1600 to 3000. The outputs of filters F<1>, F<2> are passed to balanced modulating systems M<1>, M<2>. Waves from sources 13, 16 are added so that either the upper or lower side-band of the output in each case has the required frequency, and these side bands are passed to filters F<4> , F<5>, and combined with the unchanged sub-band from filter F<3> in transformer 17. The output of the latter is applied to a balanced modulator M<3>, supplying a radio transmitting antenna TA, which preferably suppresses the carrier and one side-band. Or the sub-bands from the filters F<3> - - F<5> may be transmitted directly over a telephone or other line. A receiving arrangement is described comprising a demodulating system for the carrier, the output of which is divided by filtering into subbands which are restored by modulation with an added frequency when they have been shifted and filtered and are recombined in a telephone receiver. The sub-division of the speech wave is preferably such that the sub-bands having the greater voice energy are of less frequency width than sub-bands of less voice energy, and the transposition is preferably between bands of high energy content.