GB570488A - Improvements in systems for the secret electrical transmission of speech - Google Patents

Abstract

570,488. Secret transmission. PATELHOLD PATENTVERWERTUNGS- & ELEKTRO-HOLDING AKT.-GES. March 16, 1943, No. 4286. Convention date, Dec. 20, 1941. [Class 40 (iv)] Secrecy is obtained by attenuating at least a substantial part of the speech band, the attenuation being dependent on frequency and variable with time. The speech band may be divided into sub-bands of equal or different widths, the attenuation over a sub-band being constant or varying with frequency ; some or all of the sub-bands may be displaced in frequency, with or without inversion. The speech frequencies may be equalized initially. In the arrangements shown, original speech Zo is divided by filters into 5 sub-bands which are displaced, by modulation and selection of lower side bands, to a common range 3000-3500 #. The outputs of the modulators M are connected by a permutation switch R to selected attenuators W which may be greater in number than the number of sub-bands, the selection being changed from time to time. The outputs of the attenuators are connected over a further permutation switch Q to devices C providing different delays and connected by a third permutation switch P to modulators N which displace the sub-bands from the common range so that they are distributed over the whole of the speech range. The delay stage may precede the attenuation stage or the delay stage may be omitted. The receiver, which performs corresponding restoring operations, has its permutation switches synchronized with those at the transmitter. The permutations preferably occur at a frequency greater than syllable frequency. The attenuators may comprise valves or other elements controlled by variable voltages which may be differently phased voltages derived from a common source.