Pulse or digital communications – Repeaters – Testing
Patent
1994-07-07
1995-09-12
Cain, David C.
Pulse or digital communications
Repeaters
Testing
380 10, H04N 7167, H04B 1713
Patent
active
054504546
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the coding and decoding of a modulated radio frequency signal.
More particularly, the invention relates to a method for coding and decoding a radio frequency signal, modulated with a TV program or other message and transmitted from a transmitting center to a plurality of receiver locations, wherein coding is effected by shifting, at short intervals, the frequency of each frequency component, contained in the radio frequency signal transmitted from the transmitting center, and decoding is effected by co-ordinately shifting, at authorized receiver locations, the frequency of each frequency component, contained in the radio frequency signal received at said locations.
Such methods for coding and decoding a radio frequency signal, modualted with a TV program, are previously known through U.S. Pat. No. 3,789,131 and 3,885,089. According to these known methods, coding is performed by bringing the signal transmitted from the transmitting center to switch between two or more fixed TV channels, while decoding is performed by bringing the signal, received at one moment on one of these channels and at the next moment on another channel, to become switched from the respective channels to a channel, on which it is then transmitted to a TV set, provided at the receiver location in question, for use in said set in the same manner as an ordinary program signal, transferred from the transmitting center to the receiver location on a single channel.
As a consequence of the above described switching of the transmitted radio frequency signal between two or possibly more fixed channels, the known methods offer av very poor safety against an unauthorized utilization of the signal coded through frequency shifting. The fact is that it is rather easy to produce a so-called pirate decoder, by which the received signal may be decoded also at unauthorized receiver locations.
The invention has for its purpose to provide an improved coding and decoding method of the kind initially specified, which offers a far better safety against an unauthorized utilization of the transmitted coded signal than does any of the above described known methods. In this connection, it should be emphasized that the invention also aims to provide a coding and decoding method, which may be used not only for coding and decoding a radio frequency signal modulated with a TV program, but also for coding and decoding a radio frequency signal modulated with another type of message, for instance a sound radio program.
The method, in accordance with the invention proposed for the above purpose, is mainly characterized in that the frequency shifts in the transmitted signal are obtained by subjecting a first uncoded radio frequency signal, modulated with the message in question, to mixing with a first unmodulated radio frequency signal, the frequency of which is shifted in accordance with a selected first pattern, and to subsequent filtering in order to form a coded radio frequency signal, modulated with said message, and in that the received radio frequency signal is frequency shifted by being subjected to mixing with a second unmodulated radio frequency signal, the frequency of which is shifted according to a second pattern corresponding to said first pattern, and to subsequent filtering in order to form a second uncoded radio frequency signal, modulated with said message and corresponding to said first uncoded radio frequency signal.
The proposed use of a mixing with an unmodulated radio frequency signal having a variable frequency, both for coding and decoding the signal transmitted from the transmitting center to the receiver locations and modulated with a TV program or other message, makes it possible to execute the frequency shifting of each frequency component contained in said signal in a large number of different steps, having an arbitrary magnitude selected totally independent of any division of the utilized radio frequency band into different channels. This means that coding may be performed according to a very large number of patter
REFERENCES:
patent: 3789131 (1974-01-01), Harney
patent: 3885089 (1975-05-01), Callais et al.
patent: 4380743 (1983-04-01), Underhill et al.
patent: 4843638 (1989-06-01), Walters
patent: 4931749 (1990-06-01), Walters
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