Cryptography – Video cryptography – Video electric signal modification
Reexamination Certificate
1998-03-04
2001-03-06
Swann, Tod B. (Department: 2132)
Cryptography
Video cryptography
Video electric signal modification
C380S212000, C380S218000
Reexamination Certificate
active
06198822
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to scrambling of television video signals, particularly scrambling of slowly changing video signals such as textual messages like electronic mail or Web pages.
BACKGROUND OF THE INVENTION
Scrambling of television video signals is well known. It is often used in cable television systems in connection with the delivery of premium services. Those subscribers who have paid for the premium service are given authorization and key to decoding the scrambled video signal so that the original video image can be displayed on their television screens. More recently, interactive television systems contemplate using cable distribution networks to deliver services on an individualized basis. Such services may include Internet access, electronic mail, catalog shopping or other services which display still frame pictures on a television screen.
A scrambled signal is typically one in which the synchronization signals of the television video signal have been suppressed by baseband or rf sync suppression techniques. Conventional television sets look for the horizontal sync signals of each line to locate the beginning of a line and synchronize the raster display. Horizontal sync signals are provided at the beginning of each line outside the scope of the picture that will be displayed on the television screen. A horizontal sync signal is typically characterized by a negative DC voltage level below that of a voltage level representative of a black image. When displaying a scrambled signal with suppressed horizontal sync signals, the television looking for horizontal sync signals will typically settle upon a sharp transition—from white to black, for example. With a typical moving picture that has been scrambled, the television will lock into a variety of ever changing transitions formed in the actual picture image thus causing the horizontal orientation of each line to vary in space and time. The picture displayed on a television when it is playing a scrambled television video signal that it cannot decode destroys the original image so that it cannot be enjoyably viewed. When the scrambled signal is a still image such as an electronic mail message or a web page, there is a danger that the television will lock onto the beginning of the text or any other still vertical image element such as the edge of a text box or computer window or other framing graphic element. In the case of a textual message, with the transition from a light background to the blacker text remaining fixed in horizontal location over several lines and over time, the television may successfully and repeatedly lock into the same edge during each raster cycle. The danger is that the message may be quite readable even though the sync signals have been suppressed. Thus, the desired level of privacy for the scrambled signals may not be achieved.
Internet access and electronic mail may find prevalence with the advent of interactive cable systems. Aspects of such systems for providing interactive services on a demand basis are described in U.S. Pat. No. 5,550,578; U.S. Pat. No. 5,883,661; U.S. patent application Ser. No. 08/987,405 filed Dec. 9, 1997; and U.S. Provisional Patent Application Ser. No. 60/067,990 filed Dec. 9, 1997, all having the same assignee as the present application. The full disclosure of each of these patent references is hereby incorporated by reference herein.
SUMMARY OF THE INVENTION
Embodiments of the present invention are directed to including a pattern on the video signal of a slowly changing video image in order to promote and enhance scrambling of the signal such that text in the image is not legible upon playback by an unauthorized television set. In order to help clarify the description of the invention as set forth herein, it should be understood that the video signal is often described herein in terms of the video image it would display with its original horizontal synchronization signals in place. The pattern is generally described herein in terms of how it would appear if the active horizontal lines were displayed in their entirety and the lines were synchronized according to the original horizontal synchronization signals. It is recognized that in order to make a video signal with the pattern, the signal need not be displayed and thus the pattern need not be viewed. A horizontal synchronization signal while typically not being displayed has a signal duration that corresponds to a length of screen image if it were displayed. Locations on a line of a video image, as used herein, correspond with time periods on a video signal relative to the horizontal synchronization signal regardless of whether that horizontal signal is present as in an unscrambled signal or suppressed as in a scrambled signal.
The pattern may be added by conventional computing methods when the video to be displayed is in computer graphics or VGA format. Alternatively, the pattern may be directly imposed upon the video signal as it is being converted to or while it is in NTSC, PAL or other like television signal format for conventional television display. The pattern is characterized by a pair of segments on each horizontal line. The pair includes a black segment having a duration at least about as long as the horizontal sync signal and a white segment adjacent to each black segment to form a high contrast transition between the two. From one horizontal line to the next, the pairs of black and white segments are arranged so that the high contrast transition varies in horizontal location from one line to the next. The pattern may further include gray segments that follow each pair of black and white segments. In accordance with the presently preferred embodiment of the pattern, the pairs of black and white segments form a zigzag that would appear to extend vertically within a column located along the right side of a display of the video signal (assuming its sync signals are in place).
An unauthorized television receiving such a signal with the pattern will typically tend to lock onto the black and white transitions formed by the pattern. By varying the horizontal locations of these transitions from one line to the next, images such as text or indeed any image will be essentially shredded and illegible on the television screen.
Insertion of the pattern, may be performed by an apparatus including a sync stripper, DC restored video amplifier, a control logic device and a multiplexor. An optional low-pass filter may be added to the output of the multiplexor to remove multiplexor switching noise. The sync stripper detects the horizontal sync signal within the video signal, and determines the back porch of the video signal. This information is passed to the DC restored video amplifier for adjusting the DC level of the video signal and to the control logic device. The multiplexor has inputs for black, white and gray DC level signals and an input for the video signal. Based on the control signal from the control logic device, switching occurs for the various inputs, so that at the output of the multiplexor, an enhanced scrambled video signal is formed. Horizontal sync suppression can be done in conjunction with the aforementioned apparatus.
Other objects and advantages of the present invention will become apparent during the following description of the presently preferred embodiments of the present invention taken in conjunction with the drawings.
REFERENCES:
patent: 4562465 (1985-12-01), Glaab
patent: 4682360 (1987-07-01), Frederiksen
Askenas Mitchell
Doyle John F.
Hartson Ted
Snell Stephen C.
Bromberg & Sunstein LLP
Darrow Justin T.
ICTV Inc.
Swann Tod B.
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