EP0660990A1 - Signal access mode control method and system, in particular for image signals - Google Patents

Abstract

A signal access mode control method for wanted signals such as image signals (IMAGE), whereby a user remotely selects a wanted signal access mode. A first access mode comprises providing the user with an original wanted signal (IMAGE), and a second access mode comprises alternately and periodically transmitting to the user the wanted signal and a distortion signal (MIRE, IT) in a distorted signal. The distorted signal is still intelligible so that the user can ascertain the contents of the wanted signal and optionally select the first access mode. Different charging modes are provided for the two access modes. Physical embodiments for implementing the method are provided for switching node switching matrices (GC) in a centralised architecture, and digital distribution networks in a more futuristic distributed architecture.

Description

 Method and system for controlling mode of access to signals, especially image signals

The present invention relates generally to the field of the distribution of signals, such as image or sound signals to users, and more particularly the control of access to these signals.

Several techniques for controlling access to signals are proposed according to the prior art.

A main distinction is made between the scrambling-descrambling technique and the technique with free access limitation.

According to the first aforementioned technique, a descrambling device is made available to the user and a useful signal, such as image and / or sound signal, is transmitted or broadcast after scrambling into a scrambled signal (for example US- A-5 228 082). After descrambling the scrambled signal in the descrambling device, the useful signal is restored to the user. This technique, frequently used by operators of television channels, involves four main drawbacks. First of all, the supply to users of descrambling devices induces a high cost which is passed on to the users. Furthermore, knowing that the original signal is supplied to the user, in a form which is certainly scrambled, but in its entirety, - it is technically possible to manufacture, without the knowledge of the operator broadcasting the signal, a descrambling device , which results in a shortfall for the operator. Furthermore, these operations Scrambling and descrambling imply a degradation of the original useful signal in the signal supplied to the user after descrambling. Finally, the billing for the supply of the signal to the user for whom a descrambling device has been made available is invoiced according to a flat-rate pricing method, not necessarily corresponding to the actual duration of use of the signal by the user. 'user, which may appear prohibitive for potential users, and therefore hinder the marketing of this signal supply.

According to the second technique, called free access limitation technique, the user remotely controls the supply of the signal. The original unscrambled signal is supplied to it in response to this command for a short predetermined duration repeated a given number of times during a fixed period of time. Thus, for example, the user receives the original signal free of charge three times a day each time for a period of five minutes. These free accesses to the original signal bring the content of the signal, such as image if it is an image signal, to the knowledge of the user. As soon as the user wishes to receive the original signal durably, he remote controls the reception of the signal then the stopping of this reception and is billed according to the duration of reception. This technique is more frequently used in structures of limited size, such as hotels or hospitals. It mainly limits the number of times the user can access the content of the image signal. In the case of an image signal, this limitation prevents the user from knowing the content of the image at any time, and therefore does not encourage the latter to order a durable reception of this image signal knowing as soon as the user has ordered free access to the image signal said given number of times, no signal is then provided free of charge to the user during said fixed period.

The invention aims to remedy the aforementioned drawbacks by providing a method of controlling the access mode which differs from that according to the second aforementioned technique in that it allows the user to permanently access the image signal, not under its original form, but in a disturbed form, nonetheless intelligible, in order to allow it to know at all times the overall content of this signal.

To this end, a method of controlling the mode of access to a useful signal by a user, comprising the selective remote control by the user of several modes of access to the useful signal, one of the access modes consisting of supply of the useful signal to the user, is characterized in that another of the access modes consists in the alternative and periodic supply of the useful signal and of a disturbance signal to the user.

According to a preferred embodiment to which reference will be made in the detailed description which follows, the useful signal is an image signal. The disturbance signal can be a target signal or result from an overlay, a titration or an image overlay operated relative to said image signal.

According to another embodiment, the useful signal is a sound signal and the disturbance signal is an audio frequency signal at fixed frequency. The invention also relates to a system for implementing the aforementioned method in switching grids for switching nodes intended for the distribution of image signals to users, such as television channels for a professional application, or connected households. to cable networks and customers in hotels for a general public application. Typically, a television channel remotely controls a switching grid to establish a link between an input and an output of the grid. The gate input constitutes an external image capture, for example to which one of the image signals read from an image bank is applied, or a live reporting signal. The grid output is connected to a control room of the television channel. An agent in the control room can select, subject to access restrictions, any of the grid entries to receive the corresponding image in the control room.

The current deregulation of image distribution has given birth to new operators, such as satellite operators and private television channels. This deregulation has prompted professional customers in demand for images, such as television channels, to revise its policy towards operators of image distribution by asking each of them for more punctual and diversified services.

According to the prior art, various drawbacks go against this new request. Indeed, either each of the image signals received at respective inputs of the switching grid is distributed to all of the users, such as channels television, which allows each user to select one of the image signals and to decide how to broadcast it to viewers, in which case there is neither confidentiality nor selectivity in the distribution of the signals; or else the image signals are distributed to respective user groups according to rights allocated by the operator of the switching grid, in which case confidentiality and selectivity in the distribution of the signals are ensured, but access to signals offered to users are limited, which does not allow users to whom rights have not been allocated for certain signals to appreciate the interest of these signals and to become buyers of these signals.

Another objective of the invention is also to provide a system for controlling the mode of access to useful signals, such as image or sound signals, at the output of the switching gate directly remote controlled by the user in order to allow it to freely select permanently according to the access modes which are granted to it, or else an unaltered useful signal coming from the switching grid, for example in the case where the user wishes to use this useful signal, or else a signal which alternates the unaltered useful signal and another signal called disturbance signal in the case where the user wishes to know succinctly the information supported by the signal before deciding on its sustainable use. When the information consists of images, the disturbance signal can be a rod target signal or the useful scrambled image signal. An objective complementary to the previous one according to the invention consists in taxing differently a transmission of information according to whether they are intended for a pre-analysis, that is to say for example for an image preview or pre-listening of sound or for a use, that is to say in function of the two modes of access to the information defined above.

To this end, according to the invention, a system for controlling the mode of access to useful signals through a switching grid constituted by a matrix of switching cross points between several matrix line inputs receiving respectively useful signals and several matrix column outputs delivering useful signals, a single crossing point not being established at the same time in a matrix column between an output of said column and one of the inputs of the matrix, is characterized in that 'it comprises, associated with each gate output, a switching means having a first input connected to said each gate output and a second input receiving an image disturbance signal, and an output to be connected to one of the inputs switching means, and a control means controlling the switching means in order to selectively connect to said output switching means, or else said first input or bie n periodically, alternately, said first and second entries.

For example, the disturbance signal is a color test signal, an operator logo or any other predetermined image signal.

In addition, a means associated with said each gate output can be provided for combining the useful signal delivered by said each gate output. grid with an additional signal, such as a title to be entered, into a resulting signal. The switching means associated with said each gate output then further comprises a third input receiving the resulting signal to be selectively connected to the output of the switching means under the control of the control means.

The aforementioned system is particularly intended to be integrated into a switching node in the form of a local network to which is attached a plurality of local and / or remote stations transmitting first messages each including a crosspoint address word of grid detected by a grid control unit to establish a corresponding crossing point. Second messages transmitted by the stations each include a control word for access mode to the useful signal leaving the output of the gate connected to said crossing point when establishing said crossing point, said mode control word d access being received by a local network coupling means to be transmitted to said control means with the address of said output connected to said crossing point.

According to a characteristic of the invention, each second message includes, in addition, a user identification word which is received by a central supervision means connected to said local network to increment a respective tax counter in said central means, addressed by said user identification word, with an increment frequency dependent on said access mode control word in said message. The invention also provides for deporting the switching means and the control means to the user. When the useful signals are image signals, the control means then comprises a synchronization extraction means connected to said each gate output to detect and extract a synchronization signal in one of said image signals in order to selectively connecting said first and second inputs to said output from switching means alternately, depending on the detection and the failure to detect said synchronization signal in the image signal according to whether this useful image signal is respectively absent present at the input of the switching means.

The invention also relates to a system for implementing the method according to the invention in a digital distribution network of useful signals, such as image signals. Digital fiber optic image distribution networks will replace the centralized architecture of the current switching grid image distribution in the future. To this end, the system comprises a serial-parallel conversion means for converting serial bit data words from an incoming digital useful signal into parallel bit data words of said useful signal, first and second validation means for validating respectively said words of parallel bit data of the useful signal and parallel data words of disturbance signal, in outgoing parallel bit words, means for selectively activating either said first validation means, or alternately and periodically said first means of validation and said second validation means, in response to a user access mode control signal, and parallel-to-serial conversion means for converting said outgoing parallel bit words into serialized bit words transmitted to the user. means for selectively activating either said first validation means, or alternately and periodically said first validation means and said second validation means, in response to a user access control command signal, and means for parallel-to-serial conversion to convert said outgoing parallel bit words into serialized bit words transmitted to the user.

Other characteristics and advantages of the present invention will appear more clearly on reading the following description of several preferred embodiments of the invention with reference to the corresponding appended drawings in which: - Figure 1 is a block diagram of a system controlling mode of access to an image signal through a switching gate for a television center switching node according to the invention; - Figure 2 shows a schematic block diagram of an image switching node including an access mode control system according to the invention;

- Figure 3 shows a detailed block diagram of an electronic card grouping means for combining image signals included in the access mode control system shown in Figure 2;

- Figure 4 shows in detail a block diagram of a security module included in the card of Figure 3; - Figures 5 and 6 respectively show software architectures of a coupler on the one hand, and a control unit in the card of Figure 3 on the other hand, both belonging to the mode control system access according to the invention;

- Figure 7 is a block diagram of a remote access control module to the user; and

- Figure 8 is a block diagram of an access control system for implementing the invention in a digital image distribution network.

APPLICATION OF THE INVENTION TO A SWITCHING GRID

With reference to FIG. 1, a switching grid GC, also known as a video switching matrix, in a television center has M row line inputs E ^ to EJJ and N column column outputs ST; L to ST ^, and thus (M x N) switching crossing points divided into M rows and N columns.

In the following, it is assumed that all the signals applied to the inputs of the grid are television image signals, and that the grid is adapted to switch analog signals at videofrequency of the order of a few megahertz, or signals digital.

As this is a spatial switching technique, the grid is produced by placing N switching bars side by side with M inputs. A

- connection between an input E _m , m being between

1 and M, and an output ST _n , m being between 1 and

N, is established by passing the crossing point (E _m , ST _n ) which, in practice, may be in the form of a gate field effect transistor ordered. By design, the grid is distributive, that is to say any input can be switched to all outputs; the grid is also non-blocking since a connection is always possible between any input and any output.

In the context of the implementation of a GC switching grid in a television nodal center or in a central image distribution point such as the cable headend or a node of this network (EP-A-389 339 , Figure 1), or a cable television headend in a hotel, it is controlled either by a centralized system, or by individual desks, or by a programmed system or by a variegated set.

In the context of the invention, the switching grid GC belongs to a switching node in a cable and / or radio video distribution network intended to provide a service for distributing images to users, such as television channels, homes connected to a cable network, or hotel guests. A user controls the switching grid GC to establish a link between an input of the grid, connected for example to a live external image source or to a video recorder of recorded images to be transmitted, and an output typically connected to local or remote control of a television channel, a television set from a household connected to the cable network, or from a hotel room. The user selects, subject to access restrictions (EP-A-389,339, col. 4, 1.41 to col. 5, 1.6 and col. 7, 1.36 to col. 8, 1.6), any of the grid entries to receive the corresponding image. After selecting images, the user can use these images. Typically according to the prior art, agents of the switching node ensure, with users, the operation of remote consoles which remotely control the switching grid GC. With reference to the diagram in FIG. 1, to remedy the drawbacks according to the prior art, the invention adds a switching stage consisting of switches COM _] _ to COM ^ _j at the output of the switching grid GC. Each COM _n switch, n being an integer between 1 and N, is shown diagrammatically by an electromechanical switch with two fixed input contacts and a movable output contact, although in practice this switch is an analog transistor circuit. . The inputs of the switch are respectively connected to an output ST _n of the grid GC and to a source of image disturbance signal MIRE, such as bars pattern. Respective outputs If at S _j j N switches COM ^ to COM ^ apply combined image signals resulting from commands from the switches, directly to the users. The COM _n switch is remotely controlled by the corresponding user by means of a specialized terminal or an unmarked terminal connected to a server in a professional application, or even by means of an unmarked videotex terminal of the MINITEL type (Registered trademark) for a home connected to a cable network or an indoor telephone for a hotel room as part of a consumer application. A user can have access to several switches.

A user served by the output S _n wishing to view images applied to the input E _m at the same time selects the crossing point (E _m , ST _n ) in the grid to connect the terminals E _m and ST _n and controls the switch COM _n corresponding to the ST _n output of the GC grid. Two alternatives are offered to users for controlling the switch. According to a first alternative, the output ST _n of the grid GC is connected to the output S _n of the switch COM _n of the switching stage so that the user receives clearly, without alternation with a disturbance signal, the signal of color television image entering the entrance E _m . This first alternative is chosen for example when the user wishes to clearly visualize the images received at the input E _m .

According to a second alternative, the switch COM _n is controlled according to a predetermined alternation mode so that the output S _n alternately transmits the image signal selected at the output ST _n of the switching gate GC and the disturbance signal MIRE.

The alternation may consist of the transmission of one or more lines, respectively frames, successive of the image signal entering E _m , followed by the transmission of one or more lines, respectively frames, successive of the signal MIRE. This second alternative is for example chosen when the user wishes to preview a sequence of images. Each image in the sequence, or group of successive images, is not directly usable by the user since the image is degraded by the disturbance signal. The rhythm of the alternation is nevertheless chosen so that the images entering E _ro cannot be used while being recognizable, so that the user selects, if necessary, the first alternative.

Two alternative pricing methods are assigned respectively to the two alternatives, a higher tariff being applied for the supply to the user of an undisturbed image. The user thus manages his accesses himself and therefore his expenses.

With reference to FIG. 2, an image switching node is typically organized in the form of a local network RL consisting of a BUS bus to which is attached the switching grid GC, stations, or consoles, premises POL or remote PODs and central OS supervision computers. The remote POD stations are connected to the BUS bus through, for example, either a dedicated link LS, or a packet switching network by means of MO modems. The switching gates, local and remote stations POL and POD, and central supervision computers are attached to the BUS bus through respective IRL local network interfaces. These IRL interfaces can be simple integrated or external physical interfaces, for example for local POL stations, or gateways for example for remote POD stations connected to the BUS bus through the packet switching network. The various POD remote stations are allocated to the users, who thus remotely control the GC switching grid. Local POL stations are assigned to central office agents. The switching gate GC receives crossing point address words from the various local POL and remote stations POD through its own local network interface. These crosspoint address words are applied to a gate control unit UCG of the switching gate GC. The UCG control unit establishes, that is to say makes conductive, or breaks, that is to say makes nonconductive, a crossing point (E _m , ST _n ) in the grid whose address (m, n) is contained in a received address word. Of in this way, connections are established and broken between inputs of the grid GC receiving images, and outputs of this same grid GC transmitting these images to the users. The central computers OS provide supervision of the local area network RL, and functions such as restriction of access rights to the switching grid GC by limiting the authorizations for establishing crossing points.

To this general known infrastructure of a switching node is added the access mode control system SCM according to the invention represented in hatched area in FIG. 2. This access mode control system comprises a management coupler CO and an MCOM access control module. The CO management coupler is connected to the BUS bus through an IRL local network interface. This management coupler CO, detailed later with reference to FIG. 5, is intended to receive access mode control words transmitted by the local and remote stations POL and POD, to interpret these access mode control words and control the MCOM access control module accordingly.

A crossing point address word and an access mode control word are transmitted in respective first and second messages by a remote station POD of the user. These first and second messages respectively contain first and second control fields relating respectively to the control of the grid GC by the grid control unit UCG (grid control word = cross point address word) and to the command of the access control module MCOM (command word of access mode for switching). These first and second messages transmitted by the remote stations POD are intended respectively for the grid control unit UCG and for the management coupler CO and central supervision computer OS. These messages are received through IRL local network interfaces. The first message is interpreted by the UCG unit to establish or break a crossing point of the switching grid GC between a given input E _fl , and an output ST _n assigned to a given user. The second message is processed by the coupler CO to be transmitted to the access control module MCOM, in order to select the mode of access to the image signal produced at said output assigned to the user in response to the establishment of the crossing point which is chosen by the user by means of his POD station, the access mode being a mode disturbed by a test pattern or by overlay, or an undisturbed mode, called clear mode. This second message is received as a whole by at least one of said central supervision computers OS which triggers in the latter a tax counter associated with the user. For this, the second message also contains a user identification word field which identifies the user and which, after recognition in the computer, triggers a counting of clock pulses in the tax counter associated with this user. Depending on the chosen access mode represented by the access mode control word of the second message, either one of two clock signals having different periods is applied to the clock input of the counter when the counter is non-programmable, or a single clock signal is applied to the counter which is programmed according to one of two increment rates of this counter according to the access mode chosen by the user which is read in the access mode command word included in the second message. The ratio between the periods of the two clock signals or the two increment rates is for example between 10 and 50.

To facilitate the maintenance of the MCOM access control module in particular, it consists of L CA electronic cards; L to CA _L. Each card CA ^, 1 being an integer index between 1 and L, processes 4 successive respective outputs ST _n to S _{n + 3} of the switching grid GC. Thus the number N of grid outputs is equal to 4L. The cards CA ^ to CAL are interconnected in cascade by means of a link bus BL connected to the management coupler CO. Access mode command words in the second messages interpreted by the management module CO are transmitted to the cards CA; L to CAL • One of these cards, assigned to the output ST _n of the switching gate GC for which is requested the establishment of a crossing point (E _m , ST _n ) selected from the points (EL, ST _Π ) to (E _jj , ST _n ), controls the access mode in correspondence with the address output n included in the cross point address word. A outputs S ^ S ^ different CA cards ^CAL's "t thus produced image from the GC switching gate signals which have been processed by the MCOM access control module according to the user 's access, disturbed and undisturbed, chosen by the users.

One of the L electronic cards of the MCOM access control module is now detailed with reference to FIG. 3. In the description of the block diagram of FIG. 1, only the disturbance of the input image signal was considered. E _m by a test pattern signal from MIRE bars, for example with alternating one or more image (s). Other means of combining image signals are provided to make the incoming image signal unusable by the user, that is to say not satisfying the image clarity condition expected by the users. According to a first variant, the means for combining image signals, also called means for mixing image signals, are means for superimposing an image, called the foreground image, in another image, called the image 'background, represented by the image signal to be disturbed at the output ST _n ; in this case, the foreground image replaces one or more determined zones of the behind the scene image and can be constituted by a title, designating for example the image sequence to be disturbed and / or a logo of the operator of the switching node. According to yet another variant, the means for combining image signals are means for superimposing a foreground image, such as hatching or checkerboard, entirely or partially on a background image, in order to obtain a resulting image in full screen or containing a window in which the background image is visible with a more or less accentuated blur through the foreground image. According to other variants, inlay means and overprinting means are combined to blur background image areas. By way of example, the means for combining image signals, in addition to the means for combining by switching between image and pattern signals, are constituted by titration modules in FIG. 3. A card CA ^ of the access mode control module MCOM is intended to process 4 outputs ST _n to ST _{n + 3} of the GC switching grid. It essentially comprises a control unit UC organized around a microprocessor 10, as well as 4 titration modules MT ^ to MT ₄ , 4 safety modules MS ^ to MS ₄ and 4 basic analog or digital switching matrices M ^ to MC ₄ , all associated respectively with the four outputs ST _n to ST _{n + 3} of the switching grid GC processed by the card CA ^. For purposes of simplification, it is assumed that the card shown in FIG. 3 constitutes the first card CA ^ of all the cards CA _] _ at CAL _; and is therefore intended to receive and process the first four outputs ST _1; ST, ST ₃ and ST of the GC switching grid.

The control unit UC, the software architecture of which is presented subsequently with reference to FIG. 6, comprises the microprocessor 10, an EPROM read-only memory 11 storing the operating program of the control unit UC, a random access memory RAM 12 , a synchronization unit 13, a clock 14, a parallel interface circuit 15 and an RS-422 type interface 16. The clock rates the microprocessor 10 and the asynchronous type data synchronization unit 13. The interface 16 constitutes a physical interface between the link bus BL and the control unit UC.

Each access mode command word received in a second message is transmitted by the coupler CO to the access control module MCOM in order to select a particular mode of access to the image simultaneously with the establishment by the unit. UCG grid control point of a crossing point in the GC grid. An output address n is included in the command word which is transmitted simultaneously to all the cards CA _] _ to CA _L. In the control unit UC of each card, this output address n is compared to a stored address word associated with the card. If the output address contained in the control word is identical to the stored address word, then the control unit UC picks up and interprets the access mode control word. Thus if the address n is between 1 and 4, the card CAi interprets the access mode control word, if this address is between 5 and 8, the card CA ₂ processes the mode control word access, and so on by recurrence to the CAL card P for ^an address n between (L x 4 - 3) and (L x 4). Wires RX- and RX + of the bus BL in FIG. 3 convey control words from the coupler CO to the cards CA ^ to which these control words are addressed according to a first direction of transmission. TX- and TX + wires of the BL bus are assigned to a second direction of transmission, opposite to the first, for transmitting data from the control units UC in the cards CA ^ to the coupler CO. These data are produced by the UC control units of the CAi cards ⁱⁿ response to interrogations by the CO coupler of these UC units, or else spontaneously emitted by these same UC units during fault detection, in order to ensure operation. correct CA ^ cards, as detailed later with reference to Figures 4 and 5.

Although the card CA ^ contains four parallel processing channels for respectively processing image signals at outputs ST to ST ₄ of the switching grid GC, only the first processing channel consisting of the titration module MT] _ of the module security MS ^ and of the elementary analog switching matrix MC ^ is now detailed. In the other three channels processing image signals at outputs ST ₂ to ST ₄ of the grid GC, the Titration modules MT ₂ to MT, safety modules MS ₂ to MS ₄ and elementary switching dies MC to MC ₄ are respectively identical to those in the first channel.

The output ST ^ of the switching gate GC is connected to an input of the titration module MTT. The latter comprises a synchronization extraction circuit 21 and a titration circuit 22 preferably programmable. The synchronization extraction circuit 21 is for example made from a TDA 2595 or LM 1881 component while the titration circuit can for example be built around the MB 88303 component manufactured by FUJITSU. A color television image signal IMAGE is present on the output S - ^ of the switching grid GC as soon as the user commands the establishment of a crossing point of the grid of the type (E _m , S ^). This IMAGE signal is applied to the input of the synchronization extraction circuit 21 which extracts a frame synchronization signal TSYNC and a line synchronization signal LSYNC from the image signal IMAGE. The synchronization signals TSYNC and LSYNC are applied to two respective inputs of the titration circuit 22 so that the latter locates predetermined lines in each of the even and odd fields in the image signal IMAGE, lines for which a predetermined title is embedded. Two other inputs of the titration circuit 22 respectively receive a data signal DATA and the image signal IMAGE. The data signal DATA corresponds to a predetermined text which is to be embedded in the image signal IMAGE and whose characters and their size are programmable in the titration circuit. The DATA signal, representative of ASCII characters, is transmitted by the microprocessor 10 to the titration circuit 22 through a data bus forming part of an address / data bus BAD connecting the microprocessor 10, the read only memory, the random access memory 12, the synchronization circuit 13 and the parallel interface circuit 15 in the control unit UC. The data bus of the control unit UC being connected to all the titration circuits 22 in the modules T; L to MT ₄ , a decoding logic associated with respective buffers for the titration modules MT ^ to MT is designed to transmit a data signal DATA only to the single titration circuit 22, in the titration module MT _] _ to MT4 which is connected to the output ST ₂ to ST ₄ of the switching grid GC for which the establishment of a crossing point was ordered by the user. For the sake of simplification in FIG. 3, the decoding logic and the buffer memories have not been shown. At the output of the titration circuit is provided an analog switch with two inputs to which the image signal IM of the output ST _lr is applied as a background image signal, and a title video signal, as the signal d 'stage, produced by the circuit 22 in order to deliver a signal resulting from combined images, called image signal titled IT. In the IT signal, the selected title is embedded in the images represented by the IMAGE signal. The switch is for example constituted by two amplifiers connected to a summator and having variable gains controlled respectively by complementary gain control signals established by the titration circuit in dependence on the outline of the title to be embedded. The switch output of circuit 22 is connected to an input of the elementary switching matrix 40 of the module MC ^

The analog or digital elementary switching matrix 40 has four inputs E] _, E, E ₃ and E ₄ respectively receiving the image signal IMAGE from the output ST ^ following the establishment of a crossing point by l in the column of points of the switching grid GC associated with the output ST ^, the image signal titled IT coming from the titration circuit 22, and two bar pattern signals MIRE 1 and MIRE 2. Inputs of EC control of the matrix 40 receive from the security module MSI a two-bit matrix control word CMA1 in order to select at the output S of the matrix one of the four signals at the inputs E- to E ₄ .

The output S of the elementary switching matrix 40 is applied to an input of an amplification stage 41 having two outputs S ^ and S '^, which each transmit a resulting image signal selected from among four through the matrix of switching 40, and corresponding to one of the modes of access to the image controlled by the corresponding user either to an unmodified image, or to an image disturbed by overlaying of title or switching with a test pattern.

The output S ' _] _ constitutes a test terminal of the MCOM access control module.

In order to control each of the four elementary switching matrices of the card CA; L after reception of a second message access mode control word by the microprocessor 10, the parallel interface circuit 15 in the control unit UC (FIG. 3) has a register which serves an output port PA and in which the microprocessor 10 writes a six-bit address word via the address / data bus BAD. A first part for two bits (CO, Cl) in the address word constitutes an elementary matrix input address which is included in the access mode control word of a second message received by the microprocessor 10 and which is applied as input of each of the four security modules MSI to MS4 by the parallel interface circuit 15. Four other bits, called address validation bits, AD1 to AD4 in the address word are a combination of three bits in the state "1" and a single bit in the state "0" and are transmitted respectively in the form of four logic signals by the circuit 15 through four wires respectively to respective storage inputs of the security modules MSI to MS4. Only the security module MSI to MS4 addressed by the address validation bit AD1 to AD4 in the "0" state stores the matrix input address (C0, C1) as a matrix control word CMA1 to CMA4, to select one of the inputs E ^ to E ₄ of the associated elementary switching matrix 40.

It should be noted that, depending on the mode of access to the image controlled by the user, several types of control of the elementary switching matrix 40 are envisaged. All the image access mode command words transmitted by the user are received, after processing in the coupler CO, by the control unit UC through the link bus BL.

If the user wishes to receive the image signal clearly from the output ST ^ of the switching grid GC, then, after reception of the access mode control word, the microprocessor 10 loads the address word of matrix input C0M = "00" in the register of circuit 15 serving the output port PA in order to set to "0" the bit ADl of this register which is associated with the security module MSI and which validates in this module the input address word (CO, C1) = CMA1 = "00" to control only the matrix 40 of the MCI module, and connect the input E] _ to the output S. In the case where the user only wishes to "consult" the content of the image signal IMAGE at the output of the grid ST _lf the validation address bit of module ADl is still in the state "0" and validates an address word input (CO, Cl) = CMA1 = "01" to select the input E2 of the switching matrix MCI receiving the image signal titled IT.

Finally, in the case where the user wishes to "consult" the image signal IMAGE at the output of the gate ST1 in a form disturbed by an alternation with a bar pattern signal MIRE 1 or MIRE 2, respectively with the signal image titled IT, the control unit UC switches, after reception of the access mode control word transmitted by the user and received through the bus BL, alternately the inputs E _] _ and E ₃ or E ₄ , respectively E ^ and E, of the switching matrix 40 respectively receiving the image signal IMAGE and the signal MIRE 1 or MIRE 2, respectively the image signal IMAGE and the image signal titled IT. For this, at a frequency determining the desired disturbance, the microprocessor 10 writes alternately in the register of the parallel interface circuit associated with the output port PA, the input address words (CO, C1) = "00" and (CO, C1) = "10" or "11", respectively "00" and "01", to connect alternately the input E ^ and the input E ₃ or E ₄ , respectively the input E ^ and the input E ₂ at output S of the switching matrix 40. Simultaneously with these entries in the register of circuit 15, the microprocessor 10 maintains the module address bit AD1 associated with the security module MSI at "0". With reference to FIG. 4, the security module MSI mainly comprises three AND gates with two inputs 30a, 30b and 30c, a security switch 31, two monostable flip-flops 32 and 33, an inverter 34, two OR gates with two inputs 35 and 36, three D type flip-flops 37a, 37b and 37c, a bias resistor 38 and a pilot light in the form of a light-emitting diode LED 39. The first respective inputs of the AND gates 30a and 30b receive the bits CO and Cl composing the matrix entry address word. A first input of the AND gate 30c receives a respective one of the four address validation bits AD1 to AD4, ie here AD1 for the module MSI. Second inputs of the AND gates are connected to a supply terminal + V _C c in the high logic state "1". Between this supply terminal and ground are connected in series the switch 31 and a directly polarized short-circuit protection diode 31a. The outputs of AND gates 30a and 30b are respectively applied to data inputs D of flip-flops 37a and 37b having clock inputs receiving from the output of gate 30c, through the inverter 34, the validation signal of address AD1 carried by a respective one of the four module address wires connected respectively to the modules MSI to MS4. The outputs Q of flip-flops 37a and 37b are connected to the control inputs EC of the matrix 40 of the first processing channel.

During the general initialization of the card CAi by means of a push button (not shown), the control unit UC produces a RESET signal initializing, in addition, the microprocessor 10. The monostable flip-flop 33, active on a falling edge, is triggered by the RESET signal applied to its input and has a stable state at the high state "1". The output of this monostable flip-flop 33 is connected to an input of the monostable flip-flop 32 and to the supply terminal + ^V _{DC *} ^The monostable flip-flop 32, active on a falling edge, has a stable state in the low state "0 "and has an output connected to a first input of the OR gate 35, a second input of which receives the module address validation signal AD1 through the AND gate 30c. The output of the OR gate 35 is applied to a clock input of the D flip-flop 37c. The data input D of this flip-flop 37c is connected to the output of the OR gate 36 whose inputs are connected to the outputs Q of flip-flops 37a and 37b. The inverted output Q of the flip-flop D 37c is applied to ground M through the resistor 38 and the indicator LED diode 39 connected in cascade. The terminal common to the resistor 38 and to the inverted output Q of the flip-flop D 37c is looped back to one of four inputs of an input port PB of the parallel interface circuit 15 connected respectively to the security modules MSI to MS4 of the CA card ^. By this loopback, the microprocessor 10 of the control unit UC periodically reads a register associated with the input port PB to verify that the last bits of input address (C0, C1) transmitted by the unit UC to the module MSI security and stored in RAM 12 correspond well to the matrix control word CMA1 stored at the output of flip-flops D 37a and 37b, and therefore that the control of the switching matrix 40 is correctly carried out.

The operation of the MSI security module is as follows. During normal operation of all the circuits of the card CA _] _, the switch 31 is in the open position, which keeps the AND gates 30a, 30b and 30c open, including the second inputs are in the high state "1" or in the low state "0". The two matrix input address bits CO and Cl are thus transmitted by the open doors 30a and 30b to be memorized at the output of flip-flops 37a and 37b, in the matrix control word CMA1 in response to a falling edge of the ADl module address validation signal. After memorization of the bits CO and Cl, the rising edge of the signal AD1 causes, through the OR gate 35, the memorization of the state (co + Cl) at output Q of the flip-flop 37c. The state (co + Cl) at output Q of flip-flop 37c is at "1" if and only if the switching matrix 40 is commanded to produce at output S the clear image signal IMAGE, nor disturbed by a signal of bars pattern, nor titrated. This state (co + Cl) at the output of flip-flop 37c is retransmitted in the form of a scan signal SCRUT to the input port PB of the parallel interface circuit 15, so that the microprocessor 10 detects a failure of the card. IT! when SCRUT is in state "0" while the user has requested to receive the IMAGE signal in clear.

During the general initialization of the card

CA] _, the RESET reset signal from the microprocessor 10 is applied to the input of the monostable flip-flop 33 which produces, in response to the falling edge of the RESET signal, a low state pulse "0" of duration τ ₂ . This low state pulse closes the AND gates 30a, 30b and 30c. A state "0" is provided at the outputs of the two AND gates 30a and 30b. Knowing that the validation signal ADl is in the high state "1", the setting in the low state "0" of the second input of the AND gate 30c for the duration τ induces an initialization pulse in the state high "1" of duration τ ₂ at the output of the inverter 34. The rising edge of this initialization pulse valid in output Q of flip-flops D 37a and 37b, the outputs of AND gates 30a and 30b in a command word of matrix CMA1 taking the value CMA1 = ("0", "0") selecting at output of the switching matrix 40 the signal useful image IMAGE clear, neither disturbed by the MIRE signal, nor titled.

In the case of detection of a failure of the card CA; L by the microprocessor 10, by means of the signal SCRUT, as specified above, the switch 31 is closed for example under the control of a local station POL of an agent of the switching node via the local area network RL, or else under the action of a push button (not shown) on the card CA ^ The second inputs of AND gates 30a and 30b are thus set to the state bottom "0" through the diode 31a. The output of the AND gate 30c previously in state "1", is forced to state "0". A pulse in the high state "1" is thus supplied at the output of the inverter 34. The rising edge of this pulse validates the states "0" and "0" at the respective outputs of flip-flops 37a and 37b in the control word of matrix CMA1, which selects the input E _] _ of the elementary switching matrix 40 for transmitting the image signal IMAGE by the output S of said matrix. The closing of the switch 31, following a failure of the CA _lf card thus guarantees the user an optimal access mode to the image, neither disturbed nor titled; the user benefits from an optimal image access service as long as there is no discrepancy between the states "0" and "0" of the input address bits (C0, C1) and the state (co + Cl) at "1" stored at the output of the flip-flop 37c and transmitted to the microprocessor 10 as the signal SCRUT. On the other hand, as soon as the state (co + Cl) is at "0", signifying a non-optimal access to the image not desired by the user while CO and Cl are positioned at "0", the incrementation of the tax meter associated with the user is stopped.

The monostable flip-flop 33 is used, for each initialization or detection of failure, to produce a clock pulse applied to the clock inputs of flip-flops 37a and 37b in order to store at the output of said flip-flops the states "0" and "0" for the matrix control word CMA1 in order to select an image access mode without disturbance or titration.

With reference to FIGS. 5 and 6, it is now detailed in the form of a state diagram, the software architectures of the management coupler CO and the control unit UC.

FIG. 5 relates to the software architecture of the CO coupler.

The second control messages each comprising an access mode control word transmitted by a user are received through the bus of the local area network RL by a CO LOW CO _Q module which manages the physical layer of access to the BUS bus, and of the link layer relating to the flow regulation and the shaping of the frames constituting the MESSAGE command messages. MESSAGE messages other than access mode messages are also received by the CO coupler, in particular for the purpose of configuring the latter. A CO ^ filtering module classifies the messages according to their category, according to whether they are intended for the CO coupler, whether they are control messages intended for the MCOM control module, or whether they are erroneous messages. For an erroneous message, the filter module Cθ _! transmits it to an alarm management module C0 ₄ , which forms an ALARM alarm message which is returned to the user who transmitted the MESSAGE message to inform him of the impossibility of processing his message. In the case of a MESSAGE message intended for the CO coupler, this is transmitted by the filtering module to a command interpreter C0 in the CO coupler. The interpreter C0 translates the message in the form of elementary commands which are transmitted to a command establishment module CO5 which reads beforehand a static configuration database B2 relating to the declared hardware structure (chosen access modes, address on local bus, number of cards, text of the titration module, etc.) and a database of state variables relating to the availability of the cards declared in B2 and of the switching matrices 40. Typically, a message intended for the coupler is by example a LEC read request message of an entry address word of an elementary switching matrix CMA stored in the base of state variables B4. It is recalled that the microprocessor 10 of the card CA ^ periodically reads, via the parallel interface circuit 15, inter alia, the states constituting the words of address of entry of matrix CMA for the four processing channels in the card CA ^. These input address word states are stored in the database B ₄ as state variables.

In the case of a message intended to control the MCOM module, in particular in order to control a given access mode to the image, this is transmitted by the filtering module CO ^ to a CO ₃ analysis automaton. The automaton reads two databases Bl and B3 respectively constituting a sequence descriptor and a grid state table TEG, for, on the one hand, identifying the sequence SEQ to be established and, on the other hand, for checking in the updated table B3, in response to each received command message. access, that a crossing point establishment was requested in the grid column serving the output ST _n corresponding to the controlled access mode.

The sequence SEQ to be established is transmitted to the command establishment module C0 ₅ which supplies, via a shaping module CO, a control frame CDE in the bus BL to all the cards CA ^ to CAL ^ u module MCOM control unit. This CDE control frame further contains information representative of the input address bits of matrix CO and Cl for controlling an elementary switching matrix 40, and the address of the card CAi recipient of the address bits accompanied of the address AD1, that is to say in practice the address of the gate output ST _n .

With reference to FIG. 6, the software architecture of a control unit UC in a card CAi ^is as follows.

A management module UC ₀ takes among the CDE control frames transmitted in the link bus BL, the CDE control frames which are intended for the card CAi to which the control unit UC belongs. In the UC _Q management module, each picked control frame is deprived of the information relating to the link layer concerning the transport of the frames, such as the address of the card, to constitute a command [CDE]. This command [CDE] is applied to a UCi command interpretation module. The latter activates in particular a programmable timing module UC ₄ which determines a switching period equal to the sum of a first duration Tl during which the output S of the matrix 40 is connected to the first input E _i and of a second duration T3 during which the output S is connected to one of the test pattern inputs E ₃ and E ₄ or E ₂ , when the user commands an access mode of the type disturbed by the alternation between the useful image signal IMAGE and one of the test pattern signals MIRE, or the signal image titled IT. The UC _i module also provides a search for specific personalized text relating to the at least partial marking of the image by combination of images, such as titration of the image in the MTi titration module ”When the [CDE] command cannot be interpreted , that is to say erroneous, it is transmitted to an alarm module UC ₇ responsible for forming an alarm message which is sent back to the coupler CO through the management module UC _Q which completes it beforehand in a frame . The UC _Q management module manages the permanent dialogue with the CO coupler. If this dialogue is interrupted by causing a silence greater than a predetermined duration, the UC _Q module is responsible for forming a command [CDE] transmitted to the UC module and intended to guarantee the user an optimal access mode to the signal. image IMAGE, neither disturbed nor titled alternately.

After activation of the timing and text search functions, the command interpretation module UCi delivers a two-bit address word (C0, C1) to a command establishment module UC ₃ . This module UC ₃ controls the elementary switching matrix 40 in order to select one of the four inputs Ei to E ₄ of this matrix 40 to be connected to the output S. In the case of an access mode of the type disturbed by test pattern controlled by the user, the timer module UC4 periodically activates the module UC ₃ , so that the latter controls the grid switching 40 by alternately selecting, at each delay period, the input Ei during said first duration T1 and then the other matrix input E3 or E ₄ , respectively El and E2, during the second duration T3. As already said, the matrix is controlled by means of the parallel interface circuit 15, by loading the register associated with the corresponding processing channel and serving the output port PA of said circuit 15. As soon as the module UC ₃ controls the switching matrix 40, simultaneously the corresponding two-bit word (C0, C1) is stored in a state storage table UC ₅ .

During periodic readings of the matrix control words CMA1 by the microprocessor 10 through the port PB of the interface circuit 15, the matrix control words CMA1 and the address words (CO, Cl) which are stored in the table UC ₅ , are compared in a monitoring module UCg in order to detect a malfunction of the CA card _Φ Returning to FIG. 4, during such a malfunction of the card, the switch 31 is closed in order to guarantee the user an optimal mode of access to the image.

The invention is also applicable to video signals other than analog video signals in color, or in black and white, of a SECAM, PAL or NTSC television standard. The incoming IMAGE signals in the switching grid can be primary red R, green V and blue B signals, or MAC signals (analog component multiplexing) based on compression and time multiplexing of the analog luminance and chrominance components, or digital signals. For MAC signals, the combination of images such as titration circuits are modified so that the titled signal is of the MAC type and the capturing of the title is carried out, at each of predetermined lines of an image frame, in predetermined intervals of the signal compressed luminance and compressed chrominance signal. For digital signals, each IMAGE signal processing channel in CAi to AC cards ^is fully implemented as digital circuits when the incoming IMAGE signals are digital.

The application which has just been described for implementing the method of the invention in a switching grid is not limited to an image signal. It can be extended to a useful sound signal which accompanies for example the image signal, in which case the two image signals and sound form an audiovisual program. The alternation perturbation then applying to the useful sound signal, by analogy with the MIRE signal in the image signal, is then for example an audio frequency signal at a fixed frequency of sinusoidal nature. The image disturbance signal and the fixed frequency disturbance signal can be transmitted during the same half-wave, or respectively with the useful sound signal during one half-wave and with the useful image signal IMAGE during the other half-wave.

According to a variant of the implementation of the method according to the invention in a switching grid for analog television signals, the main disturbance function performed in the access control module MCOM is deported to an access control module installed with a user. The user orders only the establishment of a crossing point in the switching grid.

According to this variant, a preferred embodiment of which is illustrated in FIG. 7, the access control module 5 mainly comprises a switching means 51 of which first and second inputs ei and e ₂ respectively receive a useful image signal in response the establishment of a crossing point and a titration signal supplied by a titration module 52 or any other image disturbance module. This module 52 comprises a circuit 521 with programmable read-only memory EPROM and counter, scanned by line and frame synchronization signals HS and VS supplied by a time base 522 piloted by quartz. In the access control module 5, a line synchronization extraction circuit 53 extracts from the useful image signal a line synchronization signal which, through a low-pass filter 54, is applied in the form of a continuous signal to a direct input of a comparator 55. The continuous signal is compared with a predetermined voltage threshold VS in the comparator 55 which delivers a logic signal to a control input e _c of the switching means 51. Depending on the presence or absence of a synchronization signal in the useful signal or as a function of the presence or absence of the useful signal itself, thanks to the opening or closing of the crossing point in the grid GC, which is received at the first entry ei of the switching means on 51 and at the input of the extraction circuit 53, the switching means is controlled to connect the first input ei or the second input e to its output S _C0M . Thus, at the output of the switching means 51, the useful image signal and the titration signal are supplied alternately when a crossing point is established in the switching gate and an image signal is received by the remote access control module. In practice, the user ends this alternation, appearing by default, by an appropriate command transmitted to the server of the switching node which stabilizes the crossing point in the grid GC, or which authorizes the supply of the useful signal with its synchronization signals. , which therefore triggers separate pricing.

APPLICATION OF THE INVENTION TO DIGITAL DISTRIBUTION NETWORKS

The invention is not limited to the implementation of the method in a switching grid, but also provides for its implementation in a digital distribution network by optical fiber.

By analogy with the evolution of the Telephone Network, the distribution of images, at least at the professional level, is carried out today in a similar way to the distribution of voice signals in the old Switched Telephone Network, where the user had to contact an operator, in this case the operator of the switching grid, to obtain the establishment of a call. Image distribution will evolve in a similar way to the Telephone Network since the main technical obstacles to this evolution will soon be overcome thanks to the techniques of speed compression and high speed data transport over optical fiber. We can thus envisage, in the near future, the birth of national digital transport networks, possibly switched, supporting image signals for example between a national video library and users, by analogy with the databases accessible today by terminals. The centralized architecture of the distribution of images by switching grid will therefore disappear in favor of a distributed architecture in which the user will have autonomy of access to the various services offered.

In the context of these future image distribution networks, the invention provides two different access modes to an image signal which are a disturbed access mode and an optimal or clear access mode. Depending on the disturbed access mode, the useful IMAGE image signal is supplied alternately with a disturbance image signal. This access mode is selected by the user to consult the content of the image signal, for example to know the subject of a film or a documentary magazine. As soon as the user wishes to receive the image signal clearly, undisturbed by the disturbance signal, he selects the optimal access mode.

FIG. 8 shows an access control system for implementing the method according to the invention in a digital image distribution network.

The system comprises a time insertion device 6, a serial-parallel conversion deserializer 7a, a parallel-series conversion serializer 7b, and a safety circuit 8. This system is for example inserted between a switching node (not shown ) of the digital image distribution network and the user. It is assumed that the IMAGE image signal produced at the output of the switching node is of the 4.2.2 type according to Recommendation 601 of the CCIR (International Radiocommunication Advisory Committee) and comprises 625 lines per image at the rate of 50 interlaced frames per second . This image signal consists of a series of 10-bit multiplex binary words relating either to the content of the image, or to time marks for synchronization and suppression of line and frame. Each of the words relating to the content of the image consists, on the one hand, of an 8-bit word resulting from sampling and quantification over 256 levels of a luminance signal at a frequency of 13, 5 MHz or a chrominance signal at a frequency of 6.75 MHz for each of the blue and red components, and on the other hand, two bits introduced for synchronization purposes for the serial transmission of the image signal . The line frequency is 15 625 Hz. Each digital image line comprises 1728 words of which 288 are assigned to digital horizontal suppression and 1440 of which are assigned to the active part. Among the 1440 words, 720 luminance words are alternated with the 2 x 360 words of both chrominance components.

The deserializer 7a is designed for example around the SBX 1602 circuit of the company THOMSON and receives the image signal IMAGE formed from 10-bit binary words in series. The deserializer further comprises clock recovery means, decoding means and series-parallel conversion means for reproducing on 8 parallel outputs, at the frequency of 10-bit words of the incoming IMAGE signal, words to 8 parallel bits of useful signal which are contained respectively in the 10-bit words and which have been subtracted from the two bits introduced for synchronization purposes for the serial transmission of the image signal. The deserializer 7a also produces, at two respective clock outputs SY and CL, a frame synchronization signal SYT in response to frame time references included in the first and last lines of the even and odd frames, and a parallel clock signal HP synchronous with the 8-bit parallel words produced at the outputs of the deserializer 7a.

The eight bits of each parallel word produced at the respective outputs of the deserializer 7a are applied to respective inputs of eight AND gates with two inputs 600a to 607a in a useful signal validation circuit 60a. The outputs of AND gates 600a to 607a are applied respectively to the first inputs of eight gates OR to two inputs 600c to 607c. Second inputs of the eight OR gates 600c to 607c are respectively connected to outputs of eight AND gates with two inputs 600b to 607b in a disturbance signal validation circuit 60b.

First inputs of the AND gates 600b to 607b receive, at the frequency of the parallel clock signal HP, 8 parallel bits of disturbance signal words stored in a PROM memory 63, through the data bus BD of this memory 63. These eight-bit parallel disturbance signal words are supplied by the memory 63 in a periodic sequence of (1728 x 625/2) or (1728 x 625) eight-bit words constituting one or two image frames. by means of a counter 62 whose outputs address said memory 63. This counter 62 is incremented by the parallel clock signal HP coming from the deserializer 7a and is reset to zero (RESET) by the frame synchronization signal SYT.

In order to provide the user with a disturbed access mode by alternating the useful image signal and the disturbance signal, image signal word frames and signal signal word frames disturbances are periodically multiplexed at a frequency multiple of the duration of a frame. For this, a modulo-2 ° - 61 counter is incremented by the frame synchronization signal SYT, and has a qth output Sg_ι applied to a first input of an AND gate with two inputs 611, the index q being chosen arbitrarily between 1 and 1 integer Q which can be of the order of 3 to 8 for example. This output Sg_ι is alternately and periodically at a state "0" during 2 ^< ï ^-1 successive pulses of the frame synchronization signal SYT and at a high state "1" during (2 ^ - 2 ^< 3 ~ ¹ ) successive successive pulses of the SYT signal.

The output of the AND gate 611 applies a validation signal through an inverter 64 to second inputs of the AND gates 600a to 607a and directly to respective second inputs of the AND gates 600b to 607b of the circuit 60b respectively. When the AND gate 611 is open, the gates 600a to 607a and 600b to 607b are alternately opened respectively during (2 ^< 3 ~ ¹ ) successive pulses of the signal SYT and (2 ^Q - 2 ⁽ 3 ~ ¹ ) successive successive pulses of this SYT signal At the outputs of circuits 60a and 60b are thus validated alternately and periodically 8-bit parallel word sequences of the image signal and the disturbance signal, which are applied to eight parallel inputs of the serializer 7b through the OR gates 600c to 607c.

The serializer 7b is designed for example around the circuit SX 1601 of the Company THOMSON. It also includes clock recovery means, parallel-to-serial conversion means and coding means, and receives at two other parallel inputs (not shown) a low state signal "0" to form with the eight bits from the OR gates 600c to 607c, 10-bit words. The means of coding the serializer 5b modify the state "0" of the two bits applied to the other two parallel inputs to form a synchronization digit in the 10-bit words supplied in series at an output of the serializer 7b.

A CDE access mode control signal delivered by the user is applied to a second input of the AND gate 611. In a high state "1", the CDE signal opens the gate 611 to provide at the output of the serializer 7b a image signal IMAGE 'corresponding to the disturbed access mode and resulting from an alternation of the useful image signal IMAGE and the disturbance signal. At a low state "0" for an optimal access mode to the image, the CDE signal closes the door 611 and the serializer 7b delivers to the user an image signal IMAGE 'identical to the original image signal IMAGE . The second input of the AND gate 611 is also connected to a terminal of a safety circuit 8 comprising a voltage source + Vcc at a high state "1", a switch 80 and a protection diode 81 identical and arranged to the same way as the elements Vcc, 31 and 31a in FIG. 4. This safety circuit 8 is used to force, by closing the switch 80, the state of the control signal CDE applied to the second input of the door AND 61a in the low state "0" upon detection of a malfunction, in order to guarantee the user an optimal mode of access to the image. In order to align the internal parallel clocks of the deserializer 7a and serializer 7b, the output CL of the deserializer 7a applies the parallel clock signal HP to delay compensation means connected to a clock input CL of the serializer 7b intended to receive another parallel synchronization clock signal HP '. Knowing that following a rising edge of the parallel clock signal HP produced by the deserializer 7a, an 8-bit word of disturbance signal does not appear in the data bus BD of the PROM memory 63 until after a delay due to reading the memory 63, it is necessary to delay the parallel clock signal HP produced by the deserializer 7a appropriately by the parallel clock signal HP 'applied to the serializer 7b. This delay function is provided by the delay compensation means comprising a shift register 65, an inverter 66, two AND gates with two inputs 67a and 67b and an OR gate with two inputs 68. First inputs of the AND gates 67a and 67b receive the validation signal from the AND gate 611 respectively directly and through the inverter 66. Second inputs from the gates 67a and 67b receive the HP signal respectively through the register 65 and directly. The two outputs of AND gates 67a and 67b are applied through the OR gate to the clock input CL of the serializer 7b.

When the validation signal from the AND gate 611 is in a high state "1" to validate the parallel words of disturbance signal at the outputs of the AND gates 600b to 607b, the gate 67a is open, and the clock signal of parallel synchronization HP is delayed by the register 65 by a duration equal to the delay in reading a word in the memory 63. When the validation signal from the AND gate 611 is at a low state "0" to validate the words image signal useful at the outputs of the AND gates 600a to 607a, the gate 67b is open, and the parallel synchronization signal HP is reproduced without modification into the signal HP 'at the output of the OR gate 68.

Claims

1 - Method for controlling the mode of access to a useful signal (IMAGE) by a user, comprising - the selective remote control (CDE) by the user of several modes of access to the useful signal (IMAGE), one of the access modes consisting in supplying said useful signal to the user, characterized in that another of the access modes consists in the alternative and periodic supply of said useful signal and a disturbance signal to the user.

2 - Method according to claim 1, characterized in that the useful signal is an image signal, and the disturbance signal is a target signal or results from an inlay, a titration or a superimposition d image operated relative to said image signal.

3 - Method according to claim 1, characterized in that the useful signal is a sound signal and the disturbance signal is an audio frequency signal at a fixed frequency.

4 - Access mode control system (SCM) to useful signals (IMAGE) for the implementation of the method according to any one of claims 1 to 3, through a switching grid (GC) constituted by a matrix of switching cross points ((E STi) to (E _jj , STJJ)) between several matrix line inputs (Ei to EJJ) receiving respectively- useful signals and several column outputs (STi to ST ^) of matrix delivering useful signals (IMAGE), only one crossing point being established at a time in a matrix column between the output (ST _n ) of said column and one (E _m ) of the inputs of the matrix, characterized in that it comprises, associated with each gate output (ST _n ), a switching means (COM _n ; 40; 51) having a first input (E; ei) connected to said each gate output (ST _n ) and a second input (E ₃ or E ₄ , * e ₂ ) receiving a disturbance signal from image (MIRE 1) and an output (S; S _C QM) to be connected to one of the inputs of the switching means, and a control means (UC; 53,54,55) controlling the switching means in order to connect selectively at said output (S; SÇ; OM) ^ ^eΞ switching means, either said first input (Eι; eι) or periodically, alternately, said first and second inputs (Eι, E ₃ or E ₄ .e ₁ .e2).

5 - System according to claim 4, characterized in that the control means (UC) comprises a preferably programmable timing means (UC4) for addressing and cyclically connecting (Tl + T3) the first input (Ei) to the output (S) of said switching means (40) for a first predetermined duration (Tl), then the second input (E ₃ or E ₄ ) at the output (S) of the switching means (40) for a second predetermined duration (T3 ).

6 - System according to claim 4 or 5, characterized in that it comprises means (MT _n ) associated with said each gate output (ST _n ) for combining the useful signal (IMAGE) delivered by said each gate output with an additional signal (TITLE) as a resultant signal (IT), and in that the switching means (40) associated with said each gate output further comprises a third input (E) receiving the resulting signal (IT) to be selectively connected to the output (S) of the switching means under the control of the control means (UC).

7 - System according to claim 6, characterized in that the means for combining comprises an image inlay means and / or an image titration means (MT _n ), and / or an overlay means picture.

8 - System according to claim 6, characterized in that the useful signals are image signals (IMAGE) and the means for combining (MT _n ) comprises means (21) for extracting frame synchronization signals and line (TSYNC, LSYNC) of the useful image signal (IMAGE) delivered by said each output (ST _n ), and an image overlay means (22) for superimposing in synchronism with said signals

frame and line synchronization (TSYNC, LSYNC) an additional image (DATA) in the image represented by said image signal delivered (IMAGE).

9 - System according to any one of claims 4 to 8, characterized in that it comprises validation means (30a, 30b) for validating each of input address words (C0, C1) transmitted by said control means (UC) for selecting one of the inputs (Ei to E4) of said switching means, storage means (37a, 37b) for storing said each of input address words

(C0, C1) in a stored switching command word (CMAn) selecting one of the inputs of said switching means in response to an output address validation signal (ADn) transmitted by said control means (UC) and relating to said each output (ST _n ), and means (36,37c) for taking a value (("0", "0"), ("1", "0"), ("0", "1") or ("1", "1") ) of the memorized switching control word (CMAn), said control means (UC) comprising means for comparing said value taken with the input address word (C0, C1), so that, when said value and word compared are different, said control means deactivates said validation means (30a, 30b) and applies a predetermined input address word ("0", "0") in the means for memorizing (37a, 37b) which is able to select said first input (Ei) of said switching means (40) to transmit said useful signal (IMAGE) to said output (S) of the switching means (40).

10 - System according to claim 9, characterized in that said validation means (30a, 30b) comprise AND gates having first inputs receiving bits of the input address word (C0, C1) respectively, and second inputs connected to a terminal (+ Vcc) in a high state ("1"), said terminal being grounded by means of a switch (31) to deactivate said validation means (30a, 30b), and said storage means comprise flip-flops (37a, 37b) having data inputs (D) respectively connected to the outputs of said AND gates, and clock inputs receiving said output address validation signal . 11 - System according to any one of claims 4 to 10 and integrated in a switching node in the form of a local network (RL) to which is attached a plurality of stations (POL, POD) transmitting first messages (MESSAGE ) each including a crossing point address word (m, n) detected by a control unit (UCG) of the grid (GC) to establish a corresponding crossing point, characterized in that the stations (POL, POD ) transmit second messages including, in addition, a control word for access mode to the useful signal leaving the output (ST _n ) of the grid (GC) connected to said crossing point (E _m , ST _n ) during the establishment of said crossing point, said access mode control word being received by a coupling means (CO) to the local network (RL) to be transmitted to said control means (UC) with the address (n) of said outlet (ST _n ) connected to said crossing point (E _m ST _n ).

12 - System according to claim 11, characterized in that each of said second messages further includes a user identification word which is received by a central supervision means (OS) connected to said local area network (RL) for incrementing a respective tax counter in said central means (OS), addressed by said user identification word, with an increment frequency dependent on said access mode control word in said each of the second messages.

13 - System according to claim 4, characterized in that the useful signals are image signals and the control means (53,54,55) comprises a synchronization extraction means (53) connected to said each gate output for detecting and extracting a synchronization signal in one of said image signals in order to selectively connect said first and second inputs (© _i , e ₂ ) to said output of the switching means

(51) alternately, depending on the detection and the failure to detect said synchronization signal in the image signal or depending on the presence or absence of the image signal.

14 - System for implementing the method according to claim 1 in a digital distribution network, characterized in that it comprises a serial-parallel conversion means (7a) for converting data words to serialized bits from an incoming digital useful signal (IMAGE) in parallel bit data words of said useful signal, first and second validation means (60a; 60b) for validating respectively said parallel bit data words of useful signal and data words disturbance signal parallels, in outgoing parallel bit words, means (61,61i; 64) for selectively activating either said first validation means (60a), or alternately and periodically said first validation means (60a) and said second validation means (60b), in response to a user access control command signal (CDE), and parallel-serial conversion means (7b) for converting said words outgoing parallel bits in words to serialized bit (IMAGE) transmitted to the user. 15 - System according to claim 14, characterized in that the incoming digital useful signal is a digital image signal, and said words with parallel bits of disturbance signal are stored in a memory (63) which is read cyclically by a counter (62) incremented at the frequency (HP) of the serialized bit words of the image signal (IMAGE) and reset to zero at the frame frequency (SYT) of the image signal.