Cryptography – Key management – Having particular key generator
Patent
1991-07-30
1993-01-05
Barron, Jr., Gilberto
Cryptography
Key management
Having particular key generator
380 11, H04N 7167
Patent
active
051774852
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of scrambling PAL, SECAM and NTSC composite video signals and to a decoder of signals scrambled by this method.
2. Discussions of the Background
A large number of methods are known for scrambling the video signals of television images in a pay television system. Among these methods there is known a method of scrambling by circular on-line permutation. This known method, while allowing a scrambling of the image, is not a priori shielded from any "pirating" because of certain temporal characteristics of the scrambled signal which are easy to locate.
SUMMARY OF THE INVENTION
The subject of the present invention is a method of scrambling television images which makes the received images unrecognizable and very difficult to unscramble by an unauthorized user, and which is appropriate for PAL as well as SECAM or NTSC video signals. The subject of the invention is also a decoder for signals thus scrambled which is both simple and inexpensive.
The scrambling method according to the invention consists in shifting the start of the scrambled zone of each line with respect to the start of the line.
According to one aspect of the method of the invention, the shift of the start of the scrambled zone varies in the course of time. Advantageously, this shift changes value every 40 ms.
According to yet another aspect of the method of the invention, the complete non-scrambled section of each useful line is delayed on transmission and this delay is equal to the duration of one line.
The decoder according to the invention comprises, in its digital video unscrambling circuit, as delay lines, two queues of the "FIFO" type for which there is access to the write/read counter only by their zero reset inputs, and a buffer register.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood on reading the detailed description of an embodiment, taken by way of non-limitative example and illustrated by the appended drawing in which:
FIG. 1 is a timing diagram of a nonscrambled composite video line,
FIG. 2 is a timing diagram of the video line scrambled according to the method of the invention,
FIG. 3 is a partial block diagram of a decoder according to the invention, and
FIG. 4 is a timing diagram of signals appearing in the decoder of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before scrambling, the composite video signal to be transmitted conforms to the CCIR report 624 with regard to the SECAM system.
The temporal reference of this video signal is constituted by the centre of the line synchronization pulse. The sample coinciding with this temporal reference bears the zero number. There are 896 samples per line (sampling clock frequency: 14 MHz). This reference is shown in FIG. 1. In this FIG. 1, X denotes the current point of the line chosen as the break point for the purpose of the scrambling.
The video signal is scrambled by defining two segments in the useful section of the line and by temporarily exchanging the position of these two segments (see FIG. 2). The position of the point defining these two segments is controlled by a pseudo-random generator which can only be used correctly if the access control word has itself been correctly decoded in the decoder. The line suppression interval is not affected by the scrambling.
A video line before scrambling is divided into 3 segments:
1st segment A: this is the line suppression interval which is not scrambled and the sequence of the chrominance subcarrier is in agreement with the frame colour identification sequence.
2nd The segment C corresponding to the end of the useful line, before scrambling.
3rd The segment B corresponding to the start of the useful line, before scrambling.
In FIG. 1 there has also been defined two overlap zones Z and Y whose function will appear in the rest of the description, and an end of active video signal zone K. The definitions and functions of these various segments and zones are given in detail in Table 1.
The segments B and C
REFERENCES:
patent: 2847768 (1958-08-01), Bridges
patent: 4563702 (1986-01-01), Heller et al.
patent: 4673975 (1987-06-01), Inaba et al.
patent: 4736420 (1988-04-01), Katznelson et al.
patent: 4916736 (1990-04-01), Ryan
International Conference on Secure Communication systems, Feb. 22-23, 1984, IEEE (New York, US) N. Lodge et al.: "Vision scrambling of C-MAC DBS signals", pp. 59-65.
Daniel Patrick
Guillon Jean-Claude
Barron Jr. Gilberto
Laboratoire Europeen de Recherches Electroniques Avancees
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