Method and apparatus for compression compatible video scrambling

[0002] This application is related to copending patent application Ser. No. 08/473,047 ('047), filed Jun. 7, 1995 entitled Method and Apparatus for Copy Protection for Various Recording Media Using a Video Finger Print, by John O. Ryan and Gregory C. Copeland, to U.S. Pat. No. 5,513,260 ('260) issued Apr. 30, 1996 entitled Method and Apparatus for Copy Protection for Various Recording Media, by John O. Ryan, and to copending application Ser. No. 08/294,983 ('983), Aug. 24, 1994 entitled A Video Finger Print Method and Apparatus, by Gregory C. Copeland. This application also is related to U.S. Pat. No. 5,574,787 ('787) issued Nov. 12, 1996 entitled Apparatus and Method for Comprehensive Copy Protection for Video Platforms and Unprotected Source Material, by John O. Ryan.

[0003] The present invention relates to the scrambling of video signals, and the like, and more particularly to an image scrambling method and apparatus which is compatible with all video recording, transmitting and processing systems, including signal compression processes.

[0004] There is a need for a secure video scrambling method having the property that the scrambled video be compatible with the various video compression systems currently in use. In particular, it should be compatible with compression systems based on for example the Discrete Cosine Transform (DCT), which may employ inter-field redundance coding. Motion picture experts group (MPEG-1 and MPEG-2) are examples of such compression systems. In anticipated applications, the video signal generally is subjected to the processes of scrambling, compression, decompression and descrambling—in that order. The scrambled video therefore will be subjected to the processes of compression and decompression prior to descrambling.

[0005] Scrambling systems in use tend to interfere with the natural redundancy in a television signal that the compression systems of previous mention depend upon to realize maximum compression. When redundancy is disturbed, the bit rate requirements for the compression system rise, possibly to an unacceptable level, for the transmission path of the compressed video signal. Therefore there is need for a scrambling system that does not reduce the natural redundancy to a level that requires excessive bit rates to provide a given picture quality.

[0006] Prior art scrambling systems using line cut and rotate or line shuffling techniques, for example, are fundamentally incompatible with DCT-based compression methods because they grossly de-correlate information within the DCT pixel matrix. As discussed above, optimal operation of DCT systems is predicated on the existence of a high degree of correlation within each such matrix to effect efficient intra-field compression.

[0007] Moreover, the pseudo-random field-by-field changes of the scrambling pattern which these scrambling systems often utilize to increase security, make it impossible to effect further compression by relying on the normal high level of inter-field redundancy of a video signal. For example, scrambling systems based on the process known by the trademark PhaseKrypt, as described in the U.S. Pat. No. 5,058,157, Reissue. 35,078, U.S. Pat. No. 5,438,620, 5,504,815, 5,608,789, 5,581,507 and 5,579,390, all assigned to the same assignee as this application and incorporated herein by reference, in principle also are unsuitable for achieving the purposes of the present invention because they also eliminate this natural inter-field redundancy.

[0008] There are various other scrambling methods such as sync suppression and video inversion, for example, which are quite compatible with DCT-based compression. However these methods cannot provide sufficient signal security and therefore also are unsuitable for achieving the purposes of the present invention.

[0009] The present invention provides a scrambling method and apparatus which overcomes the incompatibility problems of the prior art scrambling systems of previous mention, while further providing additional compatibility and meeting various desirable requirements.

[0010] More particularly, the invention provides a compatibility that lends itself to the prevention of undesirable interactions between the scrambling system and the compression system such as, more specifically;

[0011] allowing the compression system to operate on scrambled versus unscrambled video without any increase in the bit-rate needed to provide a given picture quality, and

[0012] assuring that system performance (noise, distortion, etc.) is unaffected by the inclusion of the scrambling function.

[0013] In addition, the present invention meets requirements such as the following:

[0014] The scrambling method and apparatus of the invention is applicable to either analog (NTSC, PAL, SECAM) or digital (CCIR-601) video signals.

[0015] The present scrambling method intrinsically provides a high level of resistance to hacking, so that it provides the basis of a secure scrambling system. In other words, it is very difficult to descramble the video by analysis of the scrambled video only.

[0016] Also, the present scrambling method admits of a very large number of permutations (scrambling patterns) herein termed “warp patterns”, which ensures the method against trial and error attacks by unauthorized persons. Authorized descrambling techniques and devices are provided with the correct permutation via a designated secure data encryption method.

[0017] The present scrambling method primarily is intended for entertainment applications such as video movies, television (TV) shows, and the like. Consequently, it is not essential that the scrambling method effect total concealment of the image. However, sufficient image concealment or distortion is provided so that the entertainment value of the video effectively is eliminated.

[0018] In addition to the foregoing requirements and attendant advantages, for maximum operational flexibility, the invention ensures that the scrambled video signal appears as a normal video signal to most if not all video processing methods and devices designed to operate with unscrambled video signals, and the like.

[0019] In particular, the video signal scrambled in accordance with the invention is compatible with all video recording, transmitting and processing devices likely to exist in a production or editing environment. Passage of the scrambled video through these devices does not cause any signal degradation (after descrambling) in excess of that caused by passage of unscrambled video through these same devices.

[0020] Further, the present scrambling method, or at least a particular embodiment of it. is compatible with consumer video cassette recorders (VCRs). That is, the invention makes it possible to record the scrambled video signal on a consumer grade VCR, and replay it later for descrambling without incurring significant extra signal degradation.

[0021] More particularly, the invention provides a library of warp patterns, each of which is capable of imparting a selected degree of local spatial distortion to a video image. A pattern select code determines which stored warp pattern is supplied to a warp engine. The warp engine in turn applies the selected warp pattern to the video image to correspondingly locally spatially distort the image. In a typical application, the thusly scrambled video image and the pattern select code (suitably encrypted) which identifies the pattern, is transmitted via suitable networks such as cable, satellite, etc., to, for example, subscribing users.

[0022] The invention includes compatible descrambling of the scrambled signal to authorized users. To this end, the pattern select code is detected in the transmitted scrambled video image and is used to select a descrambling warp pattern, from a plurality of stored warp patterns, which is a complement to the warp pattern used to scramble the original video image. A warp engine applies the complementing warp pattern to the scrambled image to restore it to the original unscrambled video image, thereby allowing the authorized users access to the image.

[0023] FIG. 1 is a photograph of a scene without scrambling being applied by the invention;

[0024] FIG. 2 is a photograph of a first simulation of a scrambled picture in accordance ith the invention;

[0025] FIG. 3 is a photograph of a second simulation of a scrambled picture in accordance ith the invention;

[0026] FIG. 4 is a photograph of a third simulation of a scrambled picture in accordance th the invention;

[0027] FIG. 5 is a photograph of a fourth simulation of a scrambled picture in accordance th the invention;

[0028] FIG. 6 is a block diagram illustrating an embodiment of a scrambling apparatus in accordance with the invention; and

[0029] FIG. 7 is a block diagram illustrating an embodiment of a descrambling apparatus in accordance with the invention.

[0030] A compression system such as, for example, MPEG 1 and 2 of previous mention, relies on the condition that good correlation exists in the image. That is, that any point in an image is very similar to the points nearby. If a scrambling system shuffles the points of the image around within the image, then the image information is scattered and it no longer is true that at any given point on the image the adjacent points are very similar. That is, the scene has been de-correlated, and a substantial amount of high frequency detail which was not present before, has been added. Such a scrambling system greatly increases the bandwidth requirements of, and therefore is not compatible with, many compression systems.

[0031] The present invention circumvents the problem of incompatibility since it provides a scrambling technique which does not de-correlate the image. Al pixels that were near other pixels prior to applying the scrambling process have the same relationship after the scrambling process. There is no re-arrangement of the pixels within the image, although the distances therebetween may have been altered somewhat. Thus, in some areas the frequency content may be increased and in others it may be reduced. It follows that a small amount of high frequency information may be lost due to low pass filtering in the associated video processing apparatus, which however is an acceptable compromise in view of the advantage of complete compatibility of the present invention with all conceivable compression systems.

[0032] The explanation of the invention method and apparatus is facilitated by the following analogy. Consider a scene viewed through a sheet of colorless obscuring glass, e.g. transparent glass with a bumpy surface. By specifying the “bumpiness” parameters, a sheet of glass may be designed which would prohibit recognition of faces and important image details in a scene, although the general features of the scene might still be discernible. In effect, the scene and thus the video signal generally no longer is of commercial value since the entertainment value has been removed.

[0033] However, although it obscures the scene, the present scrambling technique is compatible with all conceivable forms of compression systems because the application of the scrambling technique has not caused any de-correlation of the image. That is, pixels which are adjacent to each other prior to the application of the scrambling process, remain correspondingly adjacent after the image is scrambled. The various pixels may be slightly closer or further from each other in any direction, but they have not been displaced from their original order relative to each other.

[0034] Thus, the technique of the invention is based upon a “local spatial distortion” of the active video signal. More particularly, the invention comprises a particular kind of optical image scrambling which when modeled in the video domain becomes the basis of a video scrambling system having all of the desirable properties and requirements, and thus the attendant advantages, of previous mention.

[0035] To this end, a video processing means for providing the scrambling method and apparatus of the invention, embodies a local spatial image distortion of a video signal similar to that caused to a scene which is viewed through obscured glass. Such a device is referred to herein as a spatial image distortion (SID) scrambler having the following properties:

[0036] displacement of each pixel of the original image by a certain amount (which may be zero) horizontally and/or vertically.

[0037] maintaining a given pixel in the scrambled image with the same set of contiguous pixels that it had in the original image.

[0038] pixels lying along the borders of the image preferably are not displaced to locations outside those borders, to avoid loss of portions of the image.

[0039] An array of number pairs, one pair for each pixel, is needed to describe the scrambling pattern for each field. The field scrambling pattern, herein termed a “warp” pattern for technical accuracy and ease of description, may be arranged to remain fixed for relativity long periods, to change slowly with time, or to change from frame-to-frame. By appropriate choice of warp patterns it is possible to effect a scrambling system having all of he properties and requirements specified above.

[0040] It is to be understood that the term “warp pattern” as employed herein refers to selected permutations or local scrambling patterns commensurate with the non-decorrelating scrambling technique of the invention, illustrated by the bumpy obscuring glass ample of previous discussion. There is an enormous number of possible warp patterns available which in effect designate the amount by which each pixel has been displaced in a right, left, up or down direction. The warp pattern can be fixed or stable for an entire movie, or the pattern can be morphed from one pattern to another preferably gradually over a selected time interval such as a number of frames to a longer time interval of a plurality of seconds. The purpose for changing warp patterns is to increase the level of security, that is, to make it even more difficult for unauthorized persons to discover the identify of the warp pattern and thus override the scrambling process.

[0041] FIG. 6 illustrates an embodiment of a scrambling apparatus of the invention herein referred to as a scrambler 10. A digital video signal or the like is supplied to a field/frame 10 store 14 via an input lead 12. The field/frame store 14 provides means to store a field or frame of video and to supply the video to a first input of a warp engine 16. A warp pattern select code generator 18 generates a warp pattern select code which, in turn, selects a desired warp pattern from a warp pattern library 20 which may include for example a look up table. The selected warp pattern is supplied to a second input of the warp engine 16. The warp pattern is applied by the warp engine to distort accordingly the pixel positions in the appropriate lines of the field or frame of video from the field/frame store 14.

[0042] FIGS. 1-5 illustrate an undistorted original picture or image and four possible warp patterns of distortion, respectively. As may be seen, each warp pattern causes a different degree of distortion on the image.

[0043] The warp pattern select code from code generator 18 also is coupled to a warp pattern select code inserter 22. The warp pattern select code is added to the scrambled video signal from the warp engine 16 to provide information for the descrambling process. The warp pattern code may be inserted in a number of ways and will usually be encrypted. The output of warp pattern select code inserter 22 consists of the warp pattern select code suitably embedded in the scrambled video signal, and is coupled via an output lead 24 to for example, a transmission network or a compression engine.

[0044] An embodiment of a descrambler 30 is illustrated in FIG. 7 wherein similar elements of the FIGS. 6, 7 are similarly numbered. A scrambled digital video signal with the warp pattern select code, corresponding for example to the signal on output lead 24 of FIG. 6, is supplied to a warp pattern select code detector 32. The warp pattern select code supplied via the code generator 18 of FIG. 6 is detected and supplied to a warp pattern library 38. The warp pattern library 38 contains the complements to the warp patterns in the warp pattern library 20 in the scrambler 10 of FIG. 6. The scrambled digital video signal is supplied to a field/frame store 34 which provides a field or frame of scrambled digital video to a warp engine 36. The field/frame store 34 and the warp engine 36 are generally similar to the store 14 and engine 16 of FIG. 6. The warp pattern select code signal from detector 32 instructs the warp pattern library 38 to send the correct complementary warp pattern to a second input of the warp engine 36. Since the warp pattern coupled to the warp engine is the complement of the warp pattern used to scramble the signal, the warp engine 36 restores the pixels distorted in the scrambling to their proper original positions. Thus, the output of the warp engine 36 comprises a descrambled video signal supplied on an output lead 40.

[0045] Although the invention has been described herein relative to specific embodiments, various additional features and advantages will be apparent from the description and drawings, and thus the scope of the invention is defined by the following claims and their equivalents.