Video scrambling

Abstract: A system for scrambling a video signal, wherein multiple modes of scrambling are available, including (i) line reversal, (ii) line inversion, (iii) line permutation, and (iv) block permutation. The invention changes the combination of modes used as time progresses. In addition, particular modes can be implemented, or suppressed, in response to such factors as (i) noise in the transmission channel and (ii) amount of motion within the video image.

Description: The invention concerns scrambling of video signals, such as those used in cable television, for security purposes. Background of the Invention Services exist which transmit television signals on various channels, such as satellite links and cable television networks. The services frequently scramble the signals, in order to prevent unauthorized parties from gaining access to the signals. However, experienced "hackers" can frequently crack the scrambling code, and defeat the purpose of the scrambling. More complex scrambling techniques can make cracking more difficult, but are, in general, more expensive to implement. US-A-5321748 and PROCEEDINGS FROM ELEVEN TECHNICAL SESSIONS OF THE ANNUAL CONVENTION AND EXPOSITION OF THE NATIONAL CABLE TELEVISION ASSOCIATION, SAN FRANCISCO, JUNE 6-9, 1993, no. CONVENTION 42, 6 June 1993, RUTKOWSKI K, pages 133-143, XP000410493 TROTT A ET AL: 'AN ENHANCED COST EFFECTIVE LINE SHUFFLE SCRAMBLING SYSTEM WITH SECURE CONDITIONAL ACCESS AUTHORIZATION' show scrambling systems wherein, in effect, a video image is sliced into horizontal strips, and the strips are re-arranged in order. For example, consecutive strips 1,2,3,4, and 5 are re-arranged to the order 3,2,4,1,5. A receiver recovers the original order. US-A-4716588 provides different scrambling modes, which are used at different times. For example, at one time, a false synch pulse may be introduced. That will, in effect, cause the picture to be torn in half along a horizontal line, and the top part to become switched with the bottom part. At another time, line inversion may be performed. At yet another time, the overall voltage of the information signal may be shifted, causing the receiving television to produce an all-black image, unless the shift is removed. US-A-4575754 shows a system which, in effect, "unstacks" the video lines in a picture, and arranges them into a linear sequence. The sequence is then scrambled as to order and transmitted. The receiver re-arranges the scrambled sequence into the correct order. According to this invention there is provided a method as claimed in claim 1. The present invention provides improved video scrambling which is simple to implement, but difficult to crack. Where multiple modes of video scrambling are available to a scrambler, including (i) line inversion, (ii) line reversal, (iii) line permutation, and (iv) block permutation, the invention changes the number of modes used during operation based on the amount of motion contained in the video image scrambled. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the accompanying drawings in which: Figure 1 illustrates line reversal of a video signal; Figure 2 illustrates permutation of lines in a video signal; Figure 3 illustrates inversion of lines in a video signal; Figure 4 illustrates permutation of blocks in a video signal; Figure 5 illustrates a combination of line reversal, line permutation, line inversion, and block permutation; Figure 6 illustrates a high-level architecture representing one form of the invention; Figure 7 illustrates reconstruction of a block of lines, using the LINE PERMUTATION TABLE; Figure 8 illustrates use of three types of TABLEs by the invention; Figure 9 illustrates, by four examples, the large number of combinations of scrambling possible under the invention; Figure 10 illustrates the division of a video signal into FRAMEs, which contain LINEs, which are composed of PIXELs; Figure 11 illustrates commands which are issued, to order a DECODER to change the tables being used; Figure 12 illustrates commands ordering the DECODER to change tables being used, and, in addition, commands which change data contained within tables themselves; and Figure 13 illustrates detection of NOISE SPIKEs which occur between SYNCH PULSES. DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a television screen S, showing 20 scanning LINEs. Figure 1 is, of course, a simplification, in that television images typically contain far more than twenty lines. For example, in the United States, under one convention of the NTSC (National Television Standards Committee), a typical screen contains 525 scanning lines, not twenty. Returning to the simplification, and with specific reference to Figures 1 - 5, four types of scrambling of these LINEs will be explained, namely, (1) line reversal, (2) line permutation, (3) line inversion, and (4) block permutation. Line Reversal Figure 1 depicts line reversal, which involves flipping a line, end-for-end. Right (R) becomes left (L), and left becomes right, as indicated. The dots indicate the original right side of each line, for reference in later Figures. Since the lines are represented by digital data, line reversal involves reversing the order of transmission of the data. Line Permutation Line permutation is represented in Figure 2. The lines are collected into groups, or blocks B. Each block contains five lines. (The parameter five is chosen for simplicity of explanation. The invention is not restricted to five lines per block. In one mode of operation, the invention can handle 32 lines per block.) In permutation, the ordering of the lines is changed, or permuted, within each block. For example, in Figure 2, the normal order (1, 2, 3, etc.) is permuted into the order (3, 2, 5, 1, 4). The lines are transmitted in this permuted order. Line Inversion Figure 3 shows line inversion. Each LINE is generated by a luminance signal. The higher values of the luminance signal represent BLACK levels, as indicated, and the lower values indicate WHITE levels. Intermediate levels range from light grey to dark grey. Inversion involves changing black to white, and white to black. (Conceptually, inversion resembles converting a photograph into a negative of the photograph.) Inversion can be viewed as generating a mirror-image of the luminance signal about a reference REF, as indicated in the Figure. In the digital domain, inversion can be accomplished by subtracting each point of the luminance signal from the reference value REF, and multiplying the result by a negative number. In addition, another number can be added, in order to level-shift the result. In the analog domain, inversion can be accomplished by running the luminance signal through an inverting operational amplifier. Such an amplifier has a negative gain. Digital and analog inversion are known in the art. Block Permutation Block permutation resembles line permutation. The lines are grouped into BLOCKs of five lines each, as shown in Figure 4. The BLOCKs themselves are then permuted, as indicated. In one embodiment, block permutation is not used. Recapitulation The discussion so far can be summarized, from one perspective, by Figure 5. Two BLOCKS, of five lines each, and labeled BLOCK 1 and BLOCK 2, are taken from the screen S. LINE REVERSAL is performed first (although need not be first). Then, in each BLOCK, LINE PERMUTATION is imposed. Next, in each block, LINE INVERSION is undertaken. Inverted lines are indicated by dashing. Then, BLOCK PERMUTATION is done. The two resulting BLOCKs within box 20 represent the scrambled signal which is to be transmitted. The information within box 20 is transmitted as follows. The lines in BLOCK 2 are first transmitted, from top-to-bottom (that is, in the sequence 3, 2, 5, 1, 4), with each line being transmitted from left (L) to right (R). Next, BLOCK 2 is transmitted in the same way. As a result, a receiver of this signal receives a signal which is scrambled. Reconstruction Line Permutation Table Figure 6 illustrates a SCRAMBLER which performs the operations summarized in Figure 5. The SCRAMBLER transmits the information along a COMMUNICATION CHANNEL, such as a cable television network, or satellite link, to a DECODER. The DECODER is located at a receiver (not shown), such as a customer's home, and reconstructs the original image. The DECODER uses four TABLES, shown in the Figure, to reconstruct the image. These TABLEs are stored in memory to which the DECODER has access. The LINE PERMUTATION TABLE tells how to re-order the lines. For example, in Figure 5, the lines of each BLOCK are received in the order 3, 2, 5, 1, 4. The LINE PERMUTATION TABLE in Figure 7 allows the DECODER to determine that the first line received is actually the third line on the screen, the second line received is actually the second line on the screen, and so on. The LINE PERMUTATION TABLE allows the lines of each block to be arranged in the proper order. Other Tables The DECODER contains other Tables, as indicated in Figure 6, and also in Figure 8. In Figure 8, the LINE REVERSAL TABLE indicates, for each line in a block, whether the line is reversed. A ZERO indicates no reversal ("N" in the Figure) while a ONE indicates reversal ("Y"). Similarly, a LINE INVERSION TABLE indicates, for each line in a block, whether the line is inverted or not. A ZERO indicates inversion, a ONE indicates inversion. Each line of a block is processed as indicated by the arrows in Figure 8. To repeat: the actual position of each line within the block (and thus on the screen) is ascertained from the LINE PERMUTATION TABLE. Whether the line must be reversed or inverted, or both, is ascertained from the LINE REVERSAL TABLE and the LINE INVERSION TABLE, respectively. The BLOCK PERMUTATION TABLE (Figure 6) indicates how the blocks should be ordered. More Complex Example The preceding discussion is a simplification, and has considered four modes of scrambling, namely, (1) line reversal, (2) line permutation, (3) line inversion, and (4) block permutation. It was assumed that each mode of scrambling was done according to a single type of TABLE shown in Figure 6. For example, it was assumed that a single type of line permutation, and thus a single LINE PERMUTATION TABLE, was used, namely, the TABLE shown in Figure 7. The TABLE used for scrambling and de-scrambling each mode may change during operation. Some examples will illustrate. Examples Figure 9 illustrates four columns, running from left to right. The first column shows four different LINE PERMUTATION TABLES, labeled TABLE 1 through TABLE 4, which are contained within the DECODER of Figure 6. The other columns show (a) four different LINE REVERSAL TABLES, (b) four different LINE INVERSION TABLES, and (c) four different BLOCK PERMUTATION TABLES. All are contained within the DECODER. At any given time, a given combination of TABLEs, one from each column, is used. The invention changes the combination of TABLES as time progresses, to attain more effective scrambling, as will now be explained. Figure 10 illustrates the data transmitted by the SCRAMBLER to the DECODER. A sequence of blocks B is transmitted. The blocks B are grouped into FRAMEs, as indicated. (The FRAMEs may be subdivided into fields, not shown, which are interlaced. This subdivision does not change the principles of the invention.) Each BLOCK B contains LINEs, as indicated. Each LINE, as indicated, contains digital data which represents the pixels contained within the line. The LINEs and BLOCKS are scrambled, according to the four modes discussed above. Between FRAMEs, as shown in Figure 11, the SCRAMBLER transmits codes which identify the TABLEs to be used for de-scrambling of subsequent frames. For example, the group of codes labeled 30 specify use of LINE PERMUTATION TABLE (LPT) 1, LINE REVERSAL TABLE (LRT) 2, LINE INVERSION TABLE (LIT) 3, and BLOCK PERMUTATION TABLE (BPT) 2. Examples of such TABLEs are shown in Figure 9. The group of codes 30 in Figure 11 specifies the type of de-scrambling for all subsequent FRAMEs. Later, another group of codes, such as group 33, can be transmitted, which specify another combination of TABLEs to use, for subsequent FRAMEs. In general, the SCRAMBLER transmits codes at intervals (which need not be periodic, but can be) which specify the TABLES to use for de-scrambling of subsequent FRAMEs. Replacement of Tables Figure 9 shows four BLOCK PERMUTATION TABLES (BPT). However, the number of possible BPTs is much greater than four. Each frame, containing 525 lines, is generally divided into two fields, by interlacing. Each field contains about 112 lines. If each field is divided into blocks of five lines each, then 21 blocks will contain these 112 lines. The number of possible permutations of 21 blocks, and thus the number of possible BPT TABLEs, equals 21-factorial, which equals about 5.1 x 10**19 (the symbols "**" mean "raised to the power"). It is not feasible to store this large number of BPTs within the DECODER. As a secondary consideration, even if all TABLES were, in fact, stored, the digital code needed to select a particular table, from this large number of tables, would need at least 66 bits, in order to express the number 5.1 x 10**19. (The binary base 2, raised to the 66 power, equals 7.4 x 10**19, which is greater than 5.1 x 10**19, thus indicating that 66 bits are sufficient to express a number in the range from zero to 21-factorial.) The other TABLES also contain large number of possibilities, but not so large as the BPT. Therefore, rather than store all possible tables within the DECODER, the invention transmits contents of the TABLEs at intervals, from the SCRAMBLER. Figure 12 illustrates this concept. Between FRAMEs, the group of data labeled 36 instructs the DECODER to replace the data in LPT 1 with data which is presently transmitted, and indicated by the phrase "(DATA)". Similarly, data in the other TABLES are replaced. Therefore, as thus far described, several important features are the following. 1. Frames are divided into fields, which are divided into blocks of lines. Five lines per block are preferred. 2. The lines within a block are scrambled in four modes, namely, line permutation, line reversal, and line inversion. 3. The blocks themselves are scrambled, in block permutation, providing a fourth mode of scrambling. 4. The scrambling, which is done by an ENCODER, is done according to TABLES. 5. De-scrambling, which is done at a DECODER, is done according to identical TABLES. 6. The particular TABLEs used for scrambling are changed during transmission. The DECODER is informed of the changes by messages transmitted between frames. The change in TABLEs applies to all subsequently received frames. 7. The contents of the TABLEs can be changed by the ENCODER. The ENCODER sends a message to the DECODER which specifies the TABLES to be changed. The message includes data which is to replace the data previously contained within the specified TABLEs. After a change, the TABLEs can be specified as usual, by messages sent between frames. Dynamic Change of Scrambling Modes For present purposes, video images can be divided into two types: those containing little motion (such as a landscape), and those containing much motion (such as a basketball game). It has been found that images containing little motion do not scramble well using rudimentary scrambling techniques. Such techniques do not adequately disguise the image. For example, line inversion alone does not scramble a low-motion image very well: the scrambled image will still be discernible. (On the other hand, line inversion is very effective in scrambling high-motion images.) The invention addresses this problem by changing the total number of scrambling modes used at a given time, based on the amount of motion contained in the image. Figure 5 illustrates four modes of scrambling: LINE INVERSION, LINE PERMUTATION, LINE REVERSAL, and BLOCK PERMUTATION. The concept of amount of motion can be explained by an example. In a video image, consider a sample in the form of an 8 x 8 group of pixels, giving 64 pixels total. Assume that each pixel in the sample is represented by an eight-bit word, or byte. Assume also that a byte value of 255 represents full black luminance, and a byte value of 000 represents full white luminance. If the number representing each of the 64 pixels remains constant, or changes very little, over time, then a still-type image is assumed to exist. A greater number of scrambling modes is called for. On the other hand, if each pixel changes significantly over time, then a high-motion image is assumed, and a smaller number of scrambling modes is called for. Some video transmitters detect the amount of motion contained within an image, quantify the amount of motion, and generate a signal which indicates the amount of motion detected. For example, IEEE standard H.261, IMPEG 2, concerns video conferencing. Under this standard, the transmitter of the video signal (such as a cable television station) includes motion vectors within the video signal. These motion vectors indicate the amount of motion contained within the image. Generation of the motion vectors is known in the art. The invention responds to the motion vectors by altering the scrambling. When low-motion images are detected, a greater number of scrambling modes are used. As a specific example, every line is inverted, as in Figure 3, and the lines are randomly permuted (Figure 2 illustrates permutation). No other scrambling is done. When high-motion images are detected, a smaller number of modes are used. Specific Embodiment One goal of scrambling is to distort the video image. Another goal is to make the scrambling code difficult to crack, as by changing the code periodically. However, the inventor has observed that this periodic changing, if done too rapidly, performs a type of de-scrambling, and allows the underlying image to become discernible. For example, if the video image is presented at 30 frames per second, and if the type of scrambling is changed every few frames, then the change in scrambling itself tends to unscramble the image. The inventor has derived the following general rule, for prevention of this unscrambling. In a video image presented at 30 frames per second, line inversion is used, as shown in Figure 3. Line permutation is also used, as shown in Figure 2. However, the line permutation sequence is changed every two or three seconds. Suppression of Line Reversal in High-Noise Conditions Sometimes, the channel carrying the video signal is contaminated by high electrical noise. During periods of such noise, decoding of line-reversed scrambling (line reversal is illustrated in Figure 1) becomes difficult. To combat this problem, the channel is monitored for noise, and scrambling in the line-reversal mode is avoided, when noise exceeds a threshold. Noise is inferred from the presence of pulses which (a) exceed the synch pulses and (b) occur between actual synch pulses. Figure 13 illustrates the detection of noise. When a SYNCH PULSE is detected, a DETECTION WINDOW is opened, of suitable extent. For example, the DETECTION WINDOW can extend between adjacent SYNCH PULSES. NOISE SPIKEs, which are contained within the DETECTION WINDOW, are counted. These SPIKEs are defined as signals which exceed a threshold T, which preferably equals the SYNCH PULSE strength. If the number of NOISE SPIKEs counted exceeds a limit, during a prescribed time period, such as 60 frames (which, in this example, represents 60 DETECTION WINDOWS), then excess noise is determined to be present. When excess noise is present, line-reversal is performed. Typically, a cable television operator, as opposed to the consumer, performs the test for excess noise. The operator informs the DECODER in Figure 6, by an appropriate signal, when line reversal is being done. The DECODER responds by decoding line reversal, or not doing so, as appropriate. Synchronization of Audio The video signal and the audio signal undergo different types of processing within the DECODER. Further, the two signals are carried by different carriers, which follow different signal paths. Consequently, the video signal and the audio signal will not arrive at their respective destinations at the same time. Restated, the finally processed video signal, ready for display on a television monitor, will not be synchronous with the finally processed audio signal, ready for broadcast by a speaker. A time difference will exist. It is necessary to determine this time difference, in order to synchronize the video with the audio. One approach to determining the time difference is to first generate two synchronous clocks, one in the SCRAMBLER, and one in the DECODER of Figure 6. Then, the SCRAMBLER transmits a sample audio signal, and records the time of transmission. Assume this time to be T1. Next, the DECODER receives the sample audio signal, and processes it, and records the time, T2, when processing has completed. The DECODER transmits the time T2 to the SCRAMBLER. The SCRAMBLER determines the time difference, T2 -T1. Since the clocks which produced T1 and T2 are synchronous, the difference T2 - T1 indicates the time difference between the video and audio signals. A similar procedure can be used to ascertain the video delay time. Based on the two delay times (audio and video), the time by which the earlier signal must be delayed is computed, in order to synchronize it with the later-arriving signal. Therefore, as to synchronization, the delay is ascertained between the audio and video signal. One approach has been given above. Other approaches are known in the art. Then, the earlier-arriving signal is buffered, or otherwise delayed, while awaiting arrival of the later signal. Then, the signals are delivered, in synchrony, to a receiver, such as a television set. Numerous substitutions and modifications can be undertaken without departing from the scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.

Claims: A method of scrambling a video image in a system wherein a scrambler has available multiple modes of scrambling the video image, including the steps of: a) ascertaining amount of motion occurring in the video image; and b) changing the number of scrambling modes used, in response to changes in the amount of motion. A method according to claim 1, in which one of the scrambling modes used includes the step of implementing line inversion when the amount of motion exceeds a threshold. A method according to claim 1, in which the modes of scrambling include two or more of the following: (i) line permutation, (ii) line inversion, (iii) line reversal, and (iv) permutation of blocks of lines. A method according to claim 1 including the following steps: c) inverting video lines; d) permuting video lines according to a sequence; and e) occasionally changing said sequence. A method according to claim 4, in which said sequence is changed about every two seconds.

Inventors: HOBBS GARY LAMONT, HOBBS, GARY LAMONT
Applicants: AT & T CORP, AT&T CORP.
Application year: 1996
Main classification: H04N7
IPC codes: H04H1, H04N7, H04K1, H04L9