Cryptography – Key management – Having particular key generator
Patent
1994-06-13
1996-09-10
Buczinski, Stephen C.
Cryptography
Key management
Having particular key generator
380 15, H04N 7167
Patent
active
055553052
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the secure transmission of video signals. The invention is particularly concerned with the secure transmission of television signals from terrestrial transmitters.
BACKGROUND OF THE INVENTION
In recent years the advent of Pay-TV systems has generated a lot of interest in video scrambling. To operate a pay-TV system successfully, the video signal transmitted must be scrambled to a degree sufficient to render it unwatchable by a viewer not equipped with a suitable decoder.
There are a number of scrambled services in operation at present, for example that operated by B Sky B Limited which broadcasts from the Astra satellite.
The British Broadcasting Corporation has launched a subscription service to be broadcast at night time. The service uses existing terrestrial transmitters.
GB 1503051 describes the scrambling method used by the B Sky B service. The system, known as line-cut and rotate (LCR) scrambles individual video lines by cutting the line at a point determined by a pseudo-random binary sequence generator (PRBS) and rotating the two line halves so that the second half of the line is transmitted first. To decode the signal a subscriber must have a suitable decoder supplied with the cut point sequence. In practice the sequence is regenerated in the decoder using keys stored in a smart card which subscribers pay a fee to receive.
This scrambling system works well for satellite and cable services, and although used in terrestrial systems LCR may be found to be unsatisfactory under some conditions such as severe multipath propagation, line tilt etc.
An alternative to LCR is line shuffling (LS) in which the video signal is scrambled by shuffling the line order whilst maintaining the integrity of individual lines. Examples of line shuffling systems are disclosed in EP-A-356200 of Screen Electronics Limited and GB-A-2086181 of Telease Limited.
SUMMARY OF INVENTION
Although the applicants have appreciated that a system based on line shuffling is inherently more rugged than one based on line cut and rotate, neither of the LS systems referred to show how to provide a sufficient degree of ruggedness for use with terrestrial transmitters and receivers.
An aim of the present invention is to provide an encoding and decoding system based on a line shuffling system which is sufficiently secure and rugged for use with terrestrial broadcasts.
The invention in its various aspects is defined in the independent claims to which reference should be made.
In a system embodying the invention, the colour bursts of shuffled active video lines remain in position, unshuffled. This has a number of advantages. Firstly, the transmitted signal comprises a completely standard PAL signal as a result of which digital equipment in the transmission chain is not likely to be disturbed which might be the case if the bursts were scrambled. Secondly, security is improved as The line sequence cannot be deduced from measuring burst phase on successive lines.
Preferably analog video is sampled at four times the reference colour subcarrier frequency F.sub.sc. Preferably the sampling is locked to the carrier phase and frequency. Prior art line shuffling systems adopt either 2 or 3 F.sub.sc sampling or line locked to ensure an integral number of samples per line. The use of 4F.sub.sc in the PAL system produces 1135.0064 samples per line which can be compensated for by adding two extra samples on one line per field and has a disadvantage in that higher speed circuitry and greater memory storage is required.
However, with hardware costs falling neither disadvantage is of concern and 4F.sub.sc subcarrier locked sampling has a number of advantages. In addition to the near integral number of samples per line, movement of lines within the sampling block leaves very few additional clues from the subcarrier phase as to how far a particular line has been moved; (It may allow lines to be divided into four groups according to whether displacement is 4n, 4n+1, 4n+2 or 4n+3 lines but no more).
Moreover, the
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Bower Andrew J.
Clarke Christopher K. P.
Robinson Adrian P.
British Broadcasting Corporation
Buczinski Stephen C.
Fournier Kevin J.
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