Cryptography – Communication system using cryptography – Frequency shift or inversion
Patent
1984-01-13
1989-01-31
Cangialosi, Salvatore
Cryptography
Communication system using cryptography
Frequency shift or inversion
380 38, H04K 104, H04K 108
Patent
active
048022195
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method of distorting a speech signal by rearranging frequency bands so that the speech becomes incomprehensible, the invention also relating to an apparatus for carrying out the method.
BACKGROUND ART
Different methods are known for making unwarrented listening-in difficult in wireless telecommunication. One method involves so-called time scrambling, where a time interval of the signal is divided into blocks of a given length, which are then rearranged and transmitted. The character of human speech and the desire to obtain low apprehensibility in the distorted signal results in that the blocks cannot be made too short. Longer blocks would, on the other hand, result in troublesome delay. Trials have shown that the delay which is introduced by a scrambler with tolerably low apprehensibility of the distorted signal is just over one second, which is very irritating in duplex connections.
Another known method of distorting speech is frequency scrambling, where the signal frequencies are first divided into a number of bands. The bands are then rearranged, with some of them being reversed, to provide an unintelligible signal.
Frequency scrambling is a simple method in principle, but it necessitates expensive and voluminous filter equipment to transpose up a frequency band by modulation, carry out a band limitation and finally a down transposition.
DISCLOSURE OF INVENTION
An object of the invention is to provide a method which, using less expensive means than those already known, e.g. integrated technique, affects the comprehensibility so that a suitable degree of secrecy is obtained.
The basic idea behind the invention is that the periodic repetition of the signal frequency spectrum in sampling, which is known in the art and described, inter alia, in chapter 3 of the publication Digital Signal Processing, Uppenheim & Schafer, Prentice Hall, is utilized for both displacing and reversing the respective frequency bands.
The invention is characterized by the disclosures in the claims.
BRIEF DESCRIPTION OF DRAWINGS
The invention is described in detail below with the aid of an embodiment and with reference to the accompanying drawings, on which
FIG. 1 illustrates the principle of the known frequency scrambling method,
FIGS. 2a, 2b explain parts of the theoretical ground on which the invention stands,
FIGS 3a-d are diagrams illustrating the different steps in the method,
FIGS. 4a, b are diagrams explaining the reversal of the frequency band, and
FIG. 5 illustrates an apparatus for carrying out the method in accordance with the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1a illustrates the frequency spectrum of a speech signal. Satisfactory scrambling is obtained if the band width is divided into four sub-bands, for example, and selected sub-bands are moved to the frequency position of other sub-bands and/or are reversed. As will be seen from FIG. 1a, the sub-band 1 has been moved to the position of the sub-band 2, the sub-band 2 has been reversed and moved to the position of the sub-band 4, sub-band 3 has only been reversed and sub-band 4 has been reversed and moved to the position of sub-band 1. The disadvantage with this method is that it necessitates modulation of a band, e.g. a 4000 Hz band, by a frequency f.sub.1, which is substantially higher than the cut off frequency of the band, in order to transpose the band up, as well as filtering out the desired sub-band, e.g. of a width of 1000 Hz, and finally modulation with a frequency f.sub.2 to transpose down the filtered-out sub-band to the intended position. These steps necessitate expensive and voluminous filter equipment, and an object of the invention is to achieve the same purpose provided by the apparatus in the art, more simply and less expensively.
FIGS. 2a and 2b explain the theoretical basis of the invention. FIG. 2a illustrates a signal with the theoretical band width B, which is sampled at a sampling frequency f.sub.s, periodical repetition of the signal frequency spectrum thus occurring
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"Analog Voice Privacy with a Microprocessor" by Sergei Udalov; May 14-16, 1980 pp. 27-37.
Cangialosi Salvatore
Telefonaktiebolaget L M Ericsson
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