Decoder for a pay television system.

The present invention relates to a decoder for a pay TV system.

In a pay-TV system (pay-TV), and in general, the subscriber chooses the programs they want to watch and it pays to receive these programs are transmitted in scrambled form to be unintelligible to non-subscribers or to subscribers who have not chosen or paid for a particular program. The signals can be transmitted by cable, satellite or antenna.

WO-A-8102499 relates to a method and a system for scrambling the transmission of video information, particularly for pay television. Portions of the video signal, for example portions or line segments, lines or frames are transmitted in an order different from their natural sequence. The manner of rearranging the sequence of line segments for the transmission can be controlled by a code generator which changes the sequence at each frame or based on other suitable time base. The code, which may be encrypted, varies randomly and can be fully or partially transmitted with the scrambled video signal.

At the reception, a decoder receives the scrambled video signal and a code of descrambling, and returns the signal managed by applying an inverse function to that used on the show. In this known system, this requires the use of two successive memories, one for storing the received scrambled signal, and the other for reading / descrambling.

The invention relates to a decoder for a pay-TV system comprising a memory arranged to receive a video signal scrambled by permutation of lines in which each line is sampled and digitized in a number of samples, and means for descrambling the signal video stored in a variable descrambling code.

According to the invention, the memory of the decoder consists of a buffer of lines, for example lines 32, where the samples of the scrambled video signal are sequentially introduced into successive cells, each buffer line being constituted of as many cells as necessary for storing the number of samples of a video signal line.

In this memory, the entry of each sample into a buffer cell causes the output of the stored sample cell already present in the same cell, so that the memory is always filled with the number of lines and a line the buffer is always filled with the number of samples it may contain.

Finally, the order of output lines of the buffer stored to perform the descrambling is determined by selecting, under control of the descrambling code among the buffer lines, of those which enable the-swapping lines.

Other features of the invention are set forth in the subclaims.

The invention will be explained below with the description of an embodiment illustrated in the drawing.<ul><li>Figure 1 shows the principle of a pay television system,</li><li>2 shows the principle of the input and the output of a video signal line in a buffer of 32 lines,</li><li>3 shows a block diagram of the decoder according to the invention, </li><li>4 shows the principle of the interference and of descrambling of the transmitted video signals,</li><li>5 shows the rotation of a line of the video signal,</li><li>6 shows the inversion of a video signal line,</li><li>7 shows a mirror symmetry of a video signal line,</li><li>Figure 8 shows a block diagram of the CPTV card</li><li>9 shows the physical structure of a video signal line,</li><li>10 shows the structure of the physical data in the Didon data transmission system, and</li><li>11 shows the data structure in the system Dido.</li></ul>

In Figure 1, a microcomputer 1 is provided for generating data according to a teletext screen for projected emissions programs such as title, price, date or period of issuance, etc. as well as other information useful to the user. This data is introduced in 3 by an interface 2 in the video signal supplied by a camera 4 or any other producer of video signal element. The lines of the return of the vertical scanning of the video signal not normally transmit any information are used for transmitting the teletext data, so that they are transmitted at the same time as the video signal.

The video signal with the teletext data is then delivered to a scrambler 5 which scrambles the image by permutation of rows, optionally combined with an additional interference in a "deep interference". The scrambled signal is transmitted by air, by cable or satellite video receiver 6 of the subscriber. The receiver includes a decoder or descrambler 7 which delivers the signal to the TV managed 8.

The scrambled signal is compatible with PAL and SECAM standards (NTSC). The interference can be run in normal version or deep release. In normal version, the video lines are switched between them, only the active part of the line being swapped. In the SECAM system, a line pair is swapped with an even line and an odd line is switched with an odd line. In the PAL system (NTSC), the burst (color carrier frequency) is left unchanged. Each line is sampled and digitized to 8 bits at a sampling frequency f 3 = xf_burst or f = 4 xf_burst the synchronous burst, so that in the PAL (NTSC) system, the color phase is preserved. Each line is thus divided in principle into 3 (or 4) x 256 segments of 8 bits.

The principle of the input and the output of a video signal line in a buffer of 32 lines will be explained later with reference to Figure 2.

FIG 3 shows that the video signal from the head station (transmitter) is supplied to a buffer memory 9 of 32 lines for performing the line-swapping as shown below and a teletext data receiver 10 . teletext format data is transmitted at the same time and by the same channel as the video signal, said data being transmitted by the return lines of the vertical scan of the video signal. A microprocessor 11 is the central element of the decoder 7. It comprises dead and vivid memories for any information that may be displayed on the TV, this information which is shaped into teletext data format, is partly transmitted by the headend and stored in a RAM of the microprocessor and partly stored in a ROM of the microprocessor 11.

The microprocessor 11 is associated with said security card CPTV (Pay TV card) 12, normally stored in the decoder 7, and a keypad 13 thereof. The microprocessor 11 delivers on call using the keyboard 13, the teletext data in appropriate digital format text generator 14 controlling the TV display. The microprocessor 11, in conjunction with the CPTV card 12 controls the buffer memory 9 for dépermuter lines. The managed video signal as well as the text generator are available in 15 to be displayed on TV 8. In addition, the microprocessor 11 is also connected to a modem 16 itself connected to a telephone line 17.

4 shows the principle of interference and descrambling the transmitted video signals. On issue a random generator produces 25 real-time code words. 26 information on the identification of the issue to be transmitted is supplied with a transmission key 34 and the code word to an encryption system 27, which delivers real-time encrypted signal 28 to be transmitted. The transmission key 34 can be transmitted in encrypted form. At the reception, the transmitted signal is issued with the transmission key to deciphering system 29, for example according to the system DES which delivers the deciphered code word (provided that the current program was purchased) and information 26. The identification code word controls a pseudo random 30 which delivers turn pointers 31 for a table of 32 256 permutation codes. Each line of the video signal, the table selects among the lines of the buffer which allows the de-permutation of the lines.

It is possible to enter and exit the buffer 18 lines in any order according to the principle illustrated in Figure 2.

This 2 schematically shows a line 18 of the buffer memory comprising 3 x 256 segments 19 of 8 bits forming a line of the video signal. In this memory, the segments or samples are sequentially introduced in 20 successive positions or memory cells. Figure 2 shows that the introduction of a new segment 20 into one of the memory positions causes the output of the segment 21 that was stored in this position, so that the memory is always filled with 32 lines and each line 18 the buffer is always filled 256 segments.

As mentioned above, the table 32 selects, from the 18 of the buffer lines those that allow the-swapping the lines of the video signal, this depending on the descrambling key consists of the codeword.

At the output of the buffer memory 9, the data are converted into analog signals by a digital / analog converter of 8 bits. Thus a PAL or SECAM signal (NTSC) managed to display on the TV.

The scrambling / unscrambling as indicated above, provides excellent security against piracy for the following reasons:<ul><li>The number of possible permutations of lines is so large that it is difficult to find the "right combination", either by chance or by correlation.</li><li>The code or permutation key is transmitted in real time, so even if a hacker finds the right permutation, it is only for a moment, for example one second.</li><li>The permutation code is transmitted encrypted, for example, but not exclusively by the DES system, virtually unbreakable. </li><li>Deciphering codes swapping is done in a smart card microprocessor (CPTV card), as discussed below, providing all the necessary security. Alternatively, the microprocessor may be part of another element of the decoder, instead of being included in the CPTV card.</li><li>The CPTV card is reprogrammable, which allows to periodically change the encryption keys.</li></ul>

In the deep interference version is carried out in addition to the permutation of the lines as indicated above, the following interference:<ul><li>according to Figure 5, rotation of the active line on itself, that is to say for example that the active part of the line begins in the middle of the real line and is followed, after the end of the real line , the beginning of that line,</li><li>according to Figure 6, an inversion of the polarity of the current line from the level corresponding to 50 IRE. This allows an automatic adjustment of the brightness level to keep it constant. This removes the possibility of image recognition by changing ambient lighting.</li><li>according to Figure 7, a mirror symmetry of the video line according to which the active portion of the video line undergoes an axial symmetry axis perpendicular to the black level of the video.</li></ul>

When interference, only the active part of the line is permuted, including the return lines of the vertical scan, which allows the interference of the teletext data but in any case, the horizontal synchronization remains unchanged.

The CPTV card 12 is an active card comprising a microprocessor provided internally of a nonvolatile memory (RAM with battery, NVRAM or E2PROM) unreadable from outside (safety) and a clock. According to Figure 8, the CPTV card includes two electronic circuits A and B: A circuit is a safety device with CPU 40 and associated memories such as ROM 41, E2PROM 42, RAM 43, and a memory interface 44, and the circuit B is a separate E2PROM memory of 2 Kbytes. The CPTV card is presented in principle in the form of a credit card. It comprises the following four functions:<ul><li>decoding the encrypted transmitted permutation key, for example using the DES system. The card receives the encrypted key microprocessor 11 and returns when the issue was purchased, the key to unscrambling dépermuter the lines of the video signal.</li><li>Storing the list of numbers, codes or programming acquired and subscriptions to a particular genre of programming (sports, cultural shows, cinema, cooking, etc.). This storage of the list of purchased emissions allows CPTV card microprocessor will return to the descrambling code if the issue has been purchased. In addition, and for the purpose of statistical and billing, it is possible to know the emissions purchased.</li><li>credit management. CPTV the card stores information representative of the credit that it can use for purchasing emissions or subscriptions. To purchase a program, the user shows, by action on the keyboard 13, the list of scheduled programs received in advance of the headend and stored in the microprocessor 11 of the decoder 7 and which are selected by menu and displayed on TV screen. The TV shows in addition to the title of the program, number, price, date of issue and other useful information if needed. The user then selects the number of the issue he wants by pressing the corresponding key on the keypad 13. If the stored credit available in the card 12 is sufficient for the purchase in question, issue or subscription purchased is stored in the card on the list of purchased emissions and the amount is deducted from the available credit. When the show airs purchased, it is identified by the card compared to its registered number or code on the list of purchased emissions and the card returns to the microprocessor code or descrambling key.</li><li>The fourth function of the CPTV card is a measure of function of time by a clock. This is used in some programs, such interactive games between the user and the headend.</li></ul>

We see that the 12 CPTV card stores all the confidential or secret information necessary for descrambling the video signal, such as descrambling keys and credit. The CPTV manages credit card, store the emissions purchased and permits the unscrambling of the purchased emissions. It is erasable, all stored data can be modified. It is thus possible to modify the key unscrambling and to reuse the card once filled.

For all the processes, the user is guided by the decoder itself which displays text and handling instructions on the TV via text generator 14. For this purpose, some of the information might be displayed as text is stored permanently in a ROM of the microprocessor 11, which portion corresponds in principle to handling instructions, and the other part of the information is temporarily stored in a RAM of the microprocessor 11, this second information being broadcast by the station and head for particular programs to be broadcast.

9 shows the a return line of the vertical scanning of the video signal structure. It is seen that this line is formed of 32 bytes of teletext data and it further comprises synchronization information and Hamming code addresses for the detection / correction of errors. The 32 bytes of teletext data is protected by 2-byte code detection CRC-16 error.

To illustrate the organization of data in the lines of the video signal, Figure 10 shows an example of the structure of the physical data in the case of a data transmission system Didon (France). Each frame of the video signal is transmitted as a line L0, L1, L2, L3 or L4. So a group of 5 lines (L0-L4) is transmitted every 5 frames. We see in particular that the lines L2 and L3 transmit 32 bytes each of data pages. The B0 mappings BS to select a group of subscribers among 224 groups, each 240 subscribers. The mappings include the group number, the action to such credit recharge, and the list of subscribers affected by this particular action. The synchronization is ensured by a radical of the CRC-16 as indicated. In addition, each line is protected by a CRC-16 which enables error detection and each group of 5 lines is protected by an XOR that corrects an erroneous line. The advantage of organizing data according to 10 is that it enables you to find easily the information where it is in the decoder.

11 shows another example of data structure in the case of the DI-OS (operating system Dido). We see that the data is organized in pages of 32 65'536 bytes each. The first four pages (0-3) of the data structure includes a table which outputs for each channel may be passed the address of the first page of a block of pages for that channel and the block length which is identical to the number of pages of that block and the number of revisions or changes that have been made in that block. The advantage of the organization according to Figure 11 is that it leads to a very flexible structure for transmitting data and a highly flexible future expansion.