SIGNAL CODING FOR SECURE TRANSMISSION

Abstract: A multiplexed analogue component (MAC) television signal is encrypted for transmission and reception by authorised recipients only by dividing the signal into blocks and reordering the blocks prior to transmission. The signal within each block remains in its original order. Preferably, the signal representing one line of video is split into blocks which are reordered within the line time to cause translation or rotation of analogue components. Alternatively group of lines may be processed as a group to cause reordering of the analogue components between lines. The preferred arrangement does not split the blocks of analogue signals already present in the MAC signal. The receiver is arranged to receive information regarding the encryption technique with the television signal and to reorder the blocks constituting the received signal whereby to reconstruct the original signal.

Description: SIGNAL CODING FOR SECURE TRANSMISSION The present invention relates to apparatus for transmitting an encrypted information signal including an analog component and receiving apparatus for receiving such an encrypted signal and unscrambling it The use of earth satellites to radiate information services, including broadcast signals, may require the use of secure coding techniques which in this specification will be called encryption. An example of such a system would be subscription broadcast services, but the techniques to, be described later could be suited equally well to any other information distrubtion system where controlled access is required. The issue of licences or other purely administrative means of controlling receivers does not fully guarantee that licencees are the only possible recipients of the signal or information. It is therefore particularly important that a guarantee of privacy be supported by some technical" means of restricting access to an exclusive broadcast programme or other information source. The essence of a scrambling or encryption system is that, for the authorised recipient, the guarantee offered is of high integrity, that the hardware involved is simple and inexpensive and programme or information reception by unauthorised recipients is both difficult and time consuming. A number of different techniques exist whose complexity, convenience and cost of implementation vary. These techniques also vary in the degree of security which they afford for example whether they render broadcast programme material totally unintelligible or OMPI whether- they simply impair the programme suf iciently to render it worthless . For the particular case of broadcast signals, audio, video and text signals can be present singly or in combination. Novel modulation schemes such as the multiplex analog components (MAC) system which is disclosed in our co-pending British application No. 8202622 is one such broadcast signal which has in any one line period three distinct blocks of information namely a digital block and first and second analog blocks containing video information. While it is possible to provide an effective encryption algorithm for the digital block using digital methods it is much more difficult to provide an effective encryption method for the analog portions of the signal because re-ordering of the analog video information in a line is only permissible if done within limits since any wholesale re-ordering affects the signal and noise spectra adversely at the receiver. It is an object of the present invention to provide apparatus for encrypting an information signal including an analog component in such a way that there is effectively no impairment of the received signal due to the encryption. The present invention provides apparatus for encrypting an * in ormation signal including an analog component comprising means for separating the information signal into blocks, storing each of the blocks in a first order and reading out the stored information in a second order different to the first order. The present invention further provides receiving apparatus for receiving an encrypted information signal which receiving apparatus includes storage means for storing an incoming encrypted information signal as a plurality of blocks of information and means for reading out the stored information in such a manner as to reconstruct the original signal prior to encryption. In the case of a MAC signal where there are two distinct blocks of analog components and a separate block of digital signals, it is convenient to treat each analog block as an indivisisible whole. Further, it is advantageous to send information regarding the encryption techniques using the digital signal block. In order that the present invention be more readily understood embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 shows diagrammatically one line of a multiplexed analog component video waveform; Figure 2 shows diagrammatically a first encryption technique; Figure 3 shows a second encryption technique; Figure A. shows a block diagram of apparatus for encrypting an information signal; and Figure 5 shows a block diagram of de-scrambling apparatus used in a receiver. The main use of the invention is for encrypting the MAC video signal . However., this is but one use of the apparatus and techniques to be disclosed. Turning now to Figure 1, this shows diagrammatically a current MAC video waveform which will be seen to comprise a first portion (1) which contains data in digital form, a second portion (2) which contains analog information relating to the video signal, in this case the chrominance, and a further block (3) which contains further analog information which relates to the video signal in this case luminance. There are clear separations between each of the above blocks (1), (2) and (3)- Thus, it is convenient to consider these blocks and particularly the blocks containing analog information to be indivisisible and the encryption techniques which will be described in detail will be described in relation to maintaining at the analog- components in indivisisible blocks. Turning now to Figure 2, the MAC waveform is again ' shown in its original form in Figure 2a one method of encrypting this signal would be to split the digital block into two which can be easily done and to insert one portion of the digital block either at the end of the video line as shown in Figure 2b or in between the analog blocks as shown In Figure 2c in either case 2b or 2c, the overall line time is unaltered but there is a translation in time of at least one of the analog blocks which means that if the encrypted signal were received by an unauthorised piece of apparatus the video information would be corrupted. A further technique which is an extension of the technique shown in Figure 2 is to consider groups of lines wherein there are at least two lines in each group. The digital blocks for each of the lines of a group can be grouped together and sent sequentially and thereafter the analog components can be sent either in their original order or in some other order. In either of the above cases it is convenient if the information required by the receiver in order to unscramble the encrypted signal is contained in the digital information which is retained at the beginning of an encrypted line or group of lines. If necessary, also, a snyc signal can be contained in the digital information which is retained at the beginning of each line or group of lines to ensure proper generation of line syncs. A further technique for encrypting the signal is shown in Figure 3 where again in Figure 3a the MAC signal is shown in its un-encrypted form. By using storage techniques it is possible to put the analog block 3 in. the position where analog block 2 should be and to put analog block 2 where analog block 3 should be. This is a very simple but convenient technique and is shown in Figure 3b. Figure 3c shows a group of lines, in this case, 2 line A and B, with the luminance and chrominance of the lines in the group being shared between the lines in the group in a different order to that which exists with the lines originally. A still further method of encrypting the MAC signal is to randomly re-order the video lines but this approach requires more storage at the transmitter and receiver. Turning now to Figure , there is shown diagrammatically an arrangement for the encryption of a signal to be transmitted either as a broadcast signal or via a cable or some other communications link to a receiver which may be a recording machine, a computer, or a television receiver . The following description is given on the basis that the processing will be undertaken using digital techniques but it will be understood that identical process¬ ing can be undertaken using analog techniques although in the latter case the analog-to-digital and digital-to- analog converters will not probably be necessary. The incoming information is divided into blocks by a sampling circuit 10 and thereafter any non-digital components are fed to a digital-to-analog conversion circuit 11. The digital components, if any, can be fed directly to a buffer store 11a. The output of the digital- to-analog circuit 11 and the buffer store 11a are then fed to a plurality of stores 12 in a first sequence under the control of a gating circuit 14 controlled by a programirirg circuit 15- The programming circuit determines which of the encryption schemes is used and can be anyone of the above techniques or any other suitable technique. The contents of these stores 12 are read out in a second sequence different to the first so that the read out data is now encrypted. This encrypted data is fed through a buffer lό to a digital transmission link if desired or to a digital-to-analog circuit 17 and then to an analog transmission link. The programme data contained in the circuit 15 has, of course, to be present at the receiver for the transmission to enable de-encryption of the transmission. This can be done by transmitting the programming data together with the encrypted information signal or else it can be done by using a fixed programme and storing the fixed programme in the receiver. If the programming * data is sent with the encrypted information signal, it is convenient to use the digital information block for the programming data and to position the programming data at such a position in the data block such that it will be decoded by the receiver prior to reception of the encrypted information signal. Referring now to Figure 5, this shows a system whereby the programme data for controlling the de-encryption of the transmission is transmitted together with the encrypted data and after reception the programming control data is separated from the other data in a synchronisation system 20. One output of the syncrhonisation system is fed to an input of a programme sequence decoder circuit 21. whose other input is connected to the output of a scrambling programming controller 22. The output of a programme sequence decoder 21 is used to get data separated in the synchronisation system 20 to a plurality of stores 24 in a particular sequence equivalent to the second sequence at the transmitter and also to control the reading out of the information stored in the stores, the reading out being undertaken in a further sequence equivalent to the first sequence at the transmitter whereby to reconstruct the original form of the information signal. The reconstructed information signal is then fed to a digital-to-analog converter 25 if it is desired to have the information in analog form or directly to digital equipment of a combination thereof. In the case of MAC, it is necessary to reconstruct both digital and analog signals for use by the receiver. The above description of Figures 4 and 5 , described how only the analogue blocks contained in a MAC signal are processed but the MAC signal also has a digital block present in each line. The different techniques described in Figures 2 and 3 can -be complimented in a number od fiferent ways. For example, the digital block need not be stored at all and can be simply latched from the input to the output of Figure 4 for combination with encrypted analogue information. However, as shown in Figure 4 of the drawings, the digital block of the MAC signal is shown as being split and portions stored in respective additional stores 12a under the control of the circuit 14- After reading out, the information in the stores 12a is combined with the programme sequence data from the programming circuit 15 and then sent with the encrypted analogue blocks. Figure 5 which shows the de-encrypting circuit strips the digital information from the received signal and decodes the programme sequence data therefrom. The digital information read out from the stores 24 includes the digital blocks which may include the sound channel. The above description has been given on the basis that the analogue blocks are no sub-divided but it is conceiable in certain circumstances that the analogue blocks may themselves be divided and their order within a line altered. All the above measures are designed as deterants. It must be recognised that the "key" which solves the encryption is available publicly in the form of a legit¬ imate receiver. Thus, manufacture of these receivers must be secure. O PI

Claims: CLAIMS : 1. Apparatus for encrypting a television signal having in each raster scan line a digital sectionof information and first and second sections of analogue information, comprising a plurality of stores whose total capacity is sufficeint to store information representing at least one-- line, means for ; dividing the line into blocks of information and for loading blocks of information into said stores, means for reading out said stores, and means for controlling the reading and writing to be undertaken in different orders, the information within a block when read out being in the same order as when written into a store. 2. Apparatus according to claim 1, wherein the capacity of the plurality of stores is sufficient to store inform¬ ation representing at least two lines and the control means is arranged to controlling the reading and writing means such that a succession of lines A B C D are reordered. 3- Apparatus according to claim 1, wherein one block contains the information of one of the analogue sections. 4- Apparatus according to claim 1, wherein a plurality of blocks contain the information of one of the analogue sections. 5- Apparatus according to claim 3 or 4j wherein the con¬ trol means is arranged to alter the order of at least the analogue blocks within a line. 6. Apparatus according to claim 1, 2 , 4 or 5 wherein the control means is arranged to write and read the digital information into and from the store in such a manner as to split it into a plurality of sections, at least one of which is placed after one of the analogue blocks after reading out. 7- Apparatus according to claim 6, wherein the. control means is arranged to place a section of digital information after both analogue blocks of a line. 8. Apparatus according to claim 1 or 5j wherein the capacity of the stores is sufficient to store information representing at least two lines and the control means is arranged to control the writing and reading means so as to group the digital blocks of each line together to be followed by the analogue blocks. 9. Apparatus according to any one of the preceding claims and including an. analogue to digital converter for converting the analogue information into digital signals prior to storage. 10. Apparatus according to claim 9 , wherein all the stores are digital stores. 11. Apparatus according to any one of claims 1 to 8 wherein the stores for receiving analogue information are analogue stores. 12. Receiving apparatus for receiving an encrypted signal produced by apparatus according to claims 1 to 11 and for reconstructing the original signal, comprising means for receiving the encrypted signal, means for separating the signal into blocks of information, means for writing the blocks of information into store, for reading blocks of information out of the store,and control means for controlling the reading and writing of information so as to construct the order of the information in the original signal, each block being unaltered throughout. O ?I_ "" IPO 13. Receiving apparatus according to claim 12, wherein the encrypted signal contains decoding information relating to the encryption technique for assisting in decoding the encrypted signal and wherein there are means responsive to the decoding information for conditioning the controlling means. 14. Receiving apparatus acςording to claim 13, and comprising means or receiving .additional information for assisting In the decoding of the encrypted signal. 15- Receiving apparatus according to claim 12 or 13 and comprising a digital to analogue converter for converting at least some of the decoded information into an analogue signal.

Inventors: DRURY GORDON MARTIN, FLANNIGAN BARRY ARTHUR, LODGE NICOLAS KIM, MASON ARTHUR GORDON, DRURY, GORDON, MARTIN, FLANNIGAN, BARRY, ARTHUR, LODGE, NICOLAS, KIM, MASON, ARTHUR, GORDON
Applicants: INDEP BROADCASTING AUTHORITY, INDEPENDENT BROADCASTING AUTHORITY
Application year: 1983
Main classification: H04N7
IPC codes: H04N7