EP1346574A1

EP1346574A1 - Method for broadcasting electronic image signals in scrambled frame

Info

Publication number: EP1346574A1
Application number: EP01271013A
Authority: EP
Inventors: François Lebrat
Original assignee: Telediffusion de France ets Public de Diffusion
Current assignee: Telediffusion de France ets Public de Diffusion
Priority date: 2000-12-13
Filing date: 2001-11-13
Publication date: 2003-09-24
Also published as: FR2818075A1; FR2818075B1; WO2002049358A1

Abstract

The invention concerns a method for broadcasting electronic image signals in scrambled frame which consists in scrambling an image to be broadcast by transposing the lines of the image signals and successively broadcasting the signals of the transposed lines, by substituting at least one of the image lines (19) with another line (1), in particular another line (1) of the image or an interpolation line of several lines of the image.

Description

Method for broadcasting electronic image signals in a scrambled frame

The present invention relates to the dirrusion of scrambled image signals. It finds an application in particular in the broadcasting of pay television programs, for which authorized receivers have a secret descrambling key.

In this application, which is at the origin of the present invention but which should not be considered as limiting the scope of the present application, the dirrusion station scrambles each image by transmitting the various video signal lines of successive rows in a pseudorandom order established by a scrambling key or polynomial.

The descrambling key allows the receiver to determine the rank of each line in the original image and therefore to recompose it.

However, unauthorized third parties manage to recompose the image without using a key. Indeed, as it is an image, any two adjacent lines have very little different video content so that we can, starting from one of the received lines, of any rank n, search for the other two lines of content most similar to that of line n and then consider that these are rows of rows n-1 and n + 1 of the original image. The continuation of the process, upwards and downwards of the image, starting from the three lines n-1, n and n + 1 of the recomposed image band, makes it possible to gradually aggregate all the others to this band lines and thus recompose the original image.

The particular problem mentioned above was therefore to prevent or at least hinder a proper descrambling of the broadcast images, by analyzing their video content exposed above.

It was excluded, for economic reasons, to modify the fleet of installed receivers. We could not therefore change the principle of scrambling. The solution sought was to degrade, in a very perceptible manner, the quality of the images of unauthorized receivers, or to require considerable processing power on their part, and should not significantly disturb the authorized receivers.

This is how the applicant proposes the present invention. To this end, the invention relates to a method of diffusing electronic image signals in a scrambled frame in which an image to be scrambled is scrambled by permutation of image signal lines, and the signals of the permuted lines are successively diffused, characterized by the fact that at least one of the lines of the image is replaced by another line.

Thus, we carry out two scrambles of different natures, logical and analog, and it is then easy to insert into the image, by substitution, at least one line which resembles one of the lines of the image more than any another of the lines in this image. An unauthorized receiver, which tries to recompose the image according to its content, will add at least the two lines above, that is to say that the image is cut into two parts separated by a parasitic band at least one line inserted in the recomposed image.

This insertion has therefore separated, as a block, one of the image parts with respect to the other, by shifting the rows of lines by at least one unit, and the total image is thus vertically distorted or stretched. On the other hand, a receiver which uses a descrambling key recomposes the image according to the real ranks of the lines by replacing the substituted line (s) at their original positions, therefore without inserting a parasitic line strip (s) (s) additional (s) and therefore without lag in the image.

One can plan to substitute another line of the image, and then the modified image will comprise two identical lines, or else to substitute another line distinct from any line of the image, for example obtained by interpolation of several lines of l 'picture.

Advantageously, broadcasting television images comprising the edge lines to be masked by the screens of the receivers, one line is substituted for one of the edge lines.

An authorized receiver, which is therefore able to determine the rank of each line, thus displays no defect.

Preferably, and in particular in the above case, the substitution is carried out before or during the scrambling permutations. 02/49358

This makes it easier to choose the rank of the substituted line.

Advantageously still, the image being diffused in the form of interlaced frames, the substitution is made in only one of the frames, or in all but with a specific substitution for each.

Thus, correct re-interleaving between frames is impossible due to the offset of part of one of the frames or independent offsets that the frames have.

The invention will be better understood with the aid of the following description of a preferred embodiment of the method of the invention, with reference to the appended drawing, in which:

FIGS. 1A, 1B and 1C represent a portion of the television image (FIG. 1A) formed by two frames of interlaced lines (FIG. 1B, 1C),

FIGS. 2A, 2B and 2C correspond respectively to FIGS. 1A, 1B and 1C and represent the image modified according to the invention and restored according to the rows of lines, by an authorized receiver,

FIGS. 3A, 3B and 3C correspond respectively to FIGS. 1A, 1B and 1C and represent the image modified according to the invention and restored according to the video content of the lines, by an unauthorized receiver, and

- Figure 4 is an image portion comprising a scrambled area of the type of Figure 3A.

The image in FIG. 4 is a portion of an image, here of conventional television, representing the words "going out tonight", which has been reproduced by a test model so as to illustrate, only in the area containing the word " evening ", n the image degradation suffered by an unauthorized receiver, as explained below.

FIG. 1A represents the portion of the image of FIG. 4 which comprises the upper part of the letters "se", consisting of 20 lines, of relative rows 1 to 20, of elementary points or pixels whose light intensity is defined by a video signal broadcast by a broadcasting station. In this example, the images are composed of 576 lines and the televisions concerned receive 25 frames per second.

More precisely, each image is here coded and broadcast in the form of two frames, a frame 1 of lines of odd rank (fig. 1B) and a frame 2 of lines of even rank (fig. 2B). Refreshing television screens is therefore performed by half-image, at a rate of 50 Hertz.

As this is pay, subscription or pay-per-view television, picture signals are scrambled before being broadcast. To this end, each image to be broadcast having been stored in a buffer memory in the form of a frame of lines, and preferably digitized to facilitate its storage and its processing, a scrambling device determines, in a pseudo-random manner, a reading order of the various lines, to send them in this order. The order varies with each image as a function of a logical scrambling key controlling the device. Conventionally, a clock signal advances a shift register or a counter, with parallel inputs and outputs, which is looped back on itself by logic gates receiving certain intermediate outputs of the register or counter and applying the combination result. from the door in question to an intermediate input of the register or counter, for, at each clock signal, pseudorandomly distorting the normal progression of the states of the register or counter supplying the rank of the line to be transmitted.

The processing and broadcasting equipment may for example include, in order, a typical audio and video acquisition encoder equipment.

DBI 2000 from TBS, a "Digital Video" serial / parallel adapter

Deserialize "type 3630 from Probel, digital line memories Epld 10K50E from PICO equipment, an LVDS adapter with ECL technology output, a DC7 type decoder from BTS and a SECAM TTV 7600 coder from BTS, receiving the three video components

Red, Green, Blue from decoder and controlling a radio transmitter. The scrambler reads the above line memory in the scrambled order. Substituting one line with another can be done by actually replacing the content of one line in memory with that of another. One can, as a variant, carry out a simple logical manipulation, by diverting the addressing in reading of the memory of lines, that is to say by addressing the substitution line not only when it must be read but also when he is expected to read the substituted line.

The authorized receivers receive in advance, in a protected manner, the key desired to descramble the lines, that is to say to determine the real rank of each line received in the original image and thus recompose it in buffer memory. and display it.

FIG. 2 illustrates the method of the invention.

The frame 2 of Figure 2C is here identical to that of Figure 1C.

On the other hand, the weft 1 of figure 2B differs from the weft 1 of figure 1B by the fact that one has substituted one of the lines, here the top one, of rank (arbitrary) 1, for another, here the bottom one, of rank (arbitrary) 19.

The arrows F2A and F2B illustrate this substitution, or overwriting, of one image line, 19, by another, 1, which subsists as such and is therefore duplicated.

An authorized receiver, capable of determining the actual ranks of the lines of the received permuted line frames, reproduces the image of FIG. 2A, the only defect of which corresponds to the fact that the video content of line 19 is not that of the line of the original image (fig. 1A), which has been destroyed.

Figures 3A, 3B, 3C correspond. respectively in FIGS. 2A, 2B, 2C, but illustrate the case of descrambling on the basis of the image information contents of the various lines. As explained at the beginning, a third party who does not have a descrambling key can try to compare the video signal, analog or digitized, of a line, of any rank n of each frame 1 and 2 to search, in the other lines , the two lines whose video content is most similar to that of the line of rank n considered.

These two lines are then considered to be of respective ranks n-1 and n + 1 and are therefore arranged on either side of the reference line of rank n.

The aggregation process continues by searching, in the remaining lines of the frame considered, the two lines of content similar to lines n-1 and n + 1 respectively, to which respective ranks n-2 and n + 2 are assigned.

The reconstituted useful image line strip thus expands towards the up and down by aggregating new lines until the whole frame is recomposed.

In the present case, when one of the two strictly identical rows of rows 1 and 19, is aggregated during a row search cycle almost identical to the above strip, the row with strictly identical content 19, 1 is aggregated during the next cycle, therefore "glued" to the previous one 1, 19.

In FIG. 3B, the useful image strip above would be placed just above the drawing, lines 1 and 19 being aggregated below, followed in order by the odd lines 3 to 17. We therefore see that line 19 in Figure 3B occupies the space provided for line 3 (see fig. 2B) and that the odd lines 3 to 17 are therefore offset as a whole by one position (passage from an odd row to an odd row neighbor) down.

The explanation would obviously remain valid with an aggregation upwards, returning Figure 3.

The overall image of a complete frame 1 therefore represents the original frame but which would have been cut at a certain height into two parts to insert an additional line.

The image of an unauthorized receiver is therefore locally distorted, by vertical elongation.

To accentuate this distortion, provision may be made for several lines of the frame 1 to undergo the substitution applied to the line 19. This has the effect of widening the band of spurious lines 1, 19.

Provision may also be made to substitute a first plurality of lines, such as line 1, for a second plurality of other lines, such as line 19, at least equal to the first plurality.

We therefore apply several times, in a frame, the substitution or substitutions explained above. The reconstructed frame 1 therefore comprises a corresponding plurality of parasitic bands, of mutually independent sizes. On reading the above description, in which the frame 2 does not intervene directly, it is therefore understood that the invention already provides the desired result even when there is no interlacing of frames here provided , that is to say when the image is constituted by a single frame similar to the frame 1, the frame 2 being used here to increase the vertical definition.

However in the present case, the weft 2 has the advantageous effect of blurring the image of the weft 1 when it is interlaced therewith to obtain the image of FIG. 3A.

Indeed, if the relative order of the lines of even and odd rank is effectively respected for the lines 1 and 2 (first line of the frame 2) and for the image part not drawn above these, the line 19 parasitic is on the other hand row between lines 2 and 4. Line 3, row between lines 4 and 6, thus occupies a position shifted downwards, as well as the block of odd lines 5 to 17 following. There follows a blurring effect, visible in FIG. 3A, all the more accentuated the higher the number of duplicated lines.

Thus, the word "evening" in Figure 4 corresponds to Figure 3A, but with increased scrambling by the fact that two lines (19) have been substituted.

To eliminate any defect in the image restored (fig. 2 A) by authorized receivers, provision may be made for the line or lines 19 undergoing substitution to be lines with a top or bottom edge of the image intended to be masked by the edges of the TV screen.

As a variant, in order to limit the defect due to duplication, provision may be made to change, preferably pseudo-randomly, from one image to the next, the row or rows of rows 19 whose original content has been lost due to substitution. The position in height of the parasitic line which they represent (bottom of fig. 2A) thus evolves permanently in the authorized receiver and represents a parasite at 25 Hz, subliminal. Preferably, the line substitution is carried out before the line scrambling permutation, in order to easily choose the position of the substituted line 19.

As a variant, it is possible to associate in transmission, to the substituted line 19, a substitution identifier, or specific code word indicating for example the real rank 19 of this line. On reception, the identifier controls the authorized receiver so that it inhibits the reproduction of the duplicated video continuous image signals from line 19 having undergone substitution. The authorized receiver replaces it for example with a gray line, not very visible. We can, as a variant, replace the substituted line 19 by duplicating one of the two adjacent lines, of rows 18 and 20 in this example, or even define and display an estimated initial content of the substituted line 19, by performing an interpolation. smoothing from the two (or more) rows of rows 18 and 20. The substituted line 19 is thus restored from the content of at least one row close to it.

Thus, in general, the substitution line 19 can be a line (1) of the image or else a line distinct from any other line of the image, for example obtained by interpolation of several lines of the image.

Furthermore, to avoid the detection of duplication of line 1 by a sophisticated unauthorized receiver, which would then eliminate online copying 19, provision may be made to modify a detail of line 19 so that it is not strictly identical to line 1, while remaining very similar to this one for aggregating in unauthorized receivers.

In particular, provision may be made to define, by interpolation between adjacent or otherwise close lines, video content of a fictitious line resembling, more than any other line, the substitution line 1. It is thus possible, for example, to interpolate between the content of line 1 and that of one of the two adjacent lines, such as line 2, or even between these same two lines, separated by line 1.

In authorized receivers, line 1 which is duplicated intervenes solely by its content, which produces the parasitic line 19, the original rank 1 of this duplicated line being without effect. On the other hand, it is this original rank 1 which determines the height position of the parasitic band of unauthorized receivers. We can therefore plan to choose a line to duplicate 1 in the zone substantially halfway up the original image, to cut the pirate images 3 A substantially into two equal parts and thus make them difficult to use. We can also change the rank 1 of the line to duplicate, so that the images Successive 3A hackers present a parasitic horizontal band progressively scanning the screen of unauthorized receivers.

As explained at the beginning, the invention is of obvious interest for the application of transmission, and in particular broadcasting, of television signals described here, but it is not however limited to this. One can in particular think of the transmission of fixed image signals, such as faxes.

Claims

1 - Method for broadcasting electronic image signals in a scrambled frame in which:

an image to be scrambled is scrambled by permutation of image signal lines, and

- The signals from the permuted lines are successively broadcast, characterized in that at least one of the lines of the image (19) is replaced by another line (1).

2 - Process according to claim 1, in which another line (1) of the image is replaced.

3 - Method according to one of claims 1 and 2, wherein one substitutes another line distinct from any line of the image.

4 - Method according to one of claims 1 to 3, wherein the substitution line (19) is an interpolation line of several lines of the image.

5 - Method according to one of claims 1 to 4, wherein one substitutes a first plurality of lines for a second plurality of other lines at least equal to the first plurality.

6 - Method according to one of claims 1 to 5, wherein, broadcasting television images having edge lines to be masked by the screens of the receivers, substituting a line (1) for one of the edge lines .

7 - Method according to one of claims 1 to 6, wherein the substitution is carried out before the scrambling permutations.

8 - Method according to one of claims 1 to 6, wherein the substitution is carried out during the scrambling permutations.

9 - Method according to one of claims 1 to 6, wherein the substitution is carried out after the scrambling permutations.

10 - Method according to one of claims 1 to 9, wherein the image being broadcast in the form of interlaced frames, substitution is carried out in only one of the frames.

1 1 - Method according to one of claims 1 to 9, wherein the image being broadcast in the form of interlaced frames, a specific substitution is made in each frame.

12.- Method according to one of claims 1 to 11, wherein there is associated, with a substituted line (19), a substitution identifier to be able to control, on reception, the inhibition of the reproduction of the image signals of said line (19).

13.- Method according to one of claims 1 to 12, wherein one associates, with a substituted line (19), a substitution identifier in order to be able to control, in reception, a restitution of the substituted line (19) from the content of at least one line close to the substituted line (19).

14.- Method according to claim 13, wherein the substituted line is restored (19) by interpolation between two lines located on either side thereof.