EP1205070A1

EP1205070A1 - Method and device for restoring a digital television signal

Info

Publication number: EP1205070A1
Application number: EP00958694A
Authority: EP
Inventors: Jean-Jacques Chevreul
Original assignee: Sagem SA
Current assignee: Sagemcom Broadband SAS
Priority date: 1999-08-20
Filing date: 2000-08-17
Publication date: 2002-05-15
Also published as: WO2001015446A1; AU7013600A; FR2797735B1; FR2797735A1

Abstract

The invention concerns a method for restoring a digital television signal in clear from a scrambled signal which consists in: transferring the demodulated scrambled signal containing messages for obtaining the control word of a television receiver providing demodulating and decoding functions to an access control module (12) via a physical interface. At least in one particular operating embodiment, the method consists in re-transferring the module controlling access to the receiver (10) with at least one of the components of the selected scrambled programme, simultaneously with the corresponding control words, computed by said module, and in carrying out the descrambling process in the receiver.

Description

METHOD AND DEVICE FOR RENDERING A DIGITAL TELEVISION SIGNAL

The present invention relates to methods and devices for rendering a clear digital television signal from a scrambled signal containing at least both the video and audio components and the messages making it possible to recover a control word necessary for descrambling. .

It is applicable to all assemblies associating a receiver which it groups together in the same unit the functions of decoding and restitution of the image on screen and of sound or which it fulfills only the function of decoding with restitution on a terminal of independent display) and an external or pluggable access control module. It finds a particularly important, but by no means exclusive, application in the case of receiver-decoders associated with an access control module conforming to the DVB-CL standard, of a type which, at the present time, renders clear to the receiver the components of the program viewed by the user, in digital form, on the interface connector of the module, so that it is possible to record the works broadcast, and this without any degradation in quality due to the digital nature of the signal.

The problem treated by the invention will now be explained in the particular case of broadcasting, on a conditional access network, according to the DVB standard, given by way of simple example. This standard allows any user with a digital television decoder receiver as defined in the previous paragraph and designated below by the term "receiver", suitable for a type of network (satellite, cable, terrestrial radio, etc.) to receive without restriction all the programs broadcast in the clear on any network of this type. However, each receiver is generally only able to process a single type of access control while operators / broadcasters use several. Even if the network can receive bouquets from many operators, a receiver given will, in general, only be able to receive scrambled programs from a single operator.

To remedy this situation, a technique has been devised which allows the receiver to adapt to any access control system. To do this, these receivers have a standard interface, through which in particular the digital stream received from the network passes and which is intended to receive different modules, each of them being specific to an access control system, or even of 'an operator. This interface is subject to standardization allowing interoperability between receiver and module, thanks to a common interface called DVB-CI (DVB Common Interface).

The usual architecture of such a receiver equipped with an access control module is given in FIG 1. The receiver 10 has a physical interface 13 for receiving a module 12. This module is controlled by a microprocessor 14 which is in communication with a microprocessor 16 of the receiver through a control link 48 of the control interface 13. The receiver can thus indicate to the module 12 which program has been selected by the user, for example from the program identifier and PIDs for the “packet identifier” and transmitting the access control descriptors relating to these components to it. The receiver also provides the module with the entire transport stream transmitted by the network, on the channel tuned in at this time. The module can then, using its digital filter 18, select the access control messages relating to this program. In particular, it will select the corresponding access title control messages or ECM and decrypt them (if the user has the necessary rights) to obtain the control words CW allowing the control of the descrambler 20 of the module. The decryption is carried out either by the microprocessor 14 of the module, which must then be a security microprocessor, or by means of an optional insertable smart card 22.

The module returns to the receiver the transport stream received, but in which the transport packets of the components of the program (s) selected by the user are in clear (descrambled) format.

In the receiver, the stream is received by the demultiplexer 24 which filters and distributes according to their nature (audio, video, data) the various selected components which it has received in clear. In typical architecture, the receiver 10, although having a DVB-CI interface, generally comprises an additional circuit 26 making it possible to bypass the module and an integrated DVB descrambler 28 and this because the receiver generally processes at least one system of integrated access control (practically free, whereas an access control module is necessarily expensive due to its material constitution).

To take account of the appearance on the general public market of devices allowing the recording and the reading of audiovisual works in digital form, that is to say without any degradation of the quality compared to the transmitted signal, the companies of distribution of works introduce into their contracts with digital television operators a clause prohibiting the presence, in clear and in digital form, of the components (especially the video component) of works subject to copyright on an external connector of receivers in order to avoid direct digital recording.

DVB-CI compliant access control modules do not meet this requirement because the selected program components all appear in plain text on the physical interface.

The invention proposes, in order to remedy this defect, a method of restoring a digital television program signal in the clear from a scrambled signal according to which the demodulated, scrambled signal is transferred and containing messages for obtaining a control word, from a television receiver performing the demodulation and decoding functions to an access control module via a physical interface, characterized in that, at less in a particular operating mode, the signal received from the access control module is retransferred to the receiver with at least, in the scrambled state, one of the components of the selected program, together with the corresponding control words , calculated by the module, and in that the complementary descrambling is carried out in the receiver.

The invention remedies the above-mentioned defect, essentially by a simple software modification of both the decoder and the module on existing receivers already having a descrambler; this will allow a very economical update of the fleet already in place, insofar as the receiver and the module can be downloaded through the broadcasting network. If in the future the descrambler is not provided for on all receivers, the presence or addition of such a descrambler will be necessary.

It can therefore be seen that the adaptation to be carried out is simple and even that, on existing decoders, there is not even to modify the hardware part.

The invention also proposes an access control module intended to be connected by a physical interface to a digital television receiver ensuring demodulation and decoding functions, said module comprising means for recovering control words from '' a stream received from the receiver, means for descrambling of components of the stream and for restoring descrambled components to the receiver, characterized in that the descrambling means are provided for, in response to an instruction contained in the stream, to restore one every month components to the receiver in scrambled form and transfer to the receiver the control words associated with this scrambling. It also proposes a receiver making it possible to implement the above method, comprising a descrambler and a physical interface for demodulated scrambled signal output to an access control and signal return module from the module, the descrambler being provided to descramble at least one of the components of the signal, returned still scrambled by the module, by using a control word transmitted by the module.

In its application to scrambled streams according to the DVB algorithm, the method involves no longer descrambling in this module at least the video component of the program (s) selected when they are subject to the copyright requirement.

A variant consists in generalizing the behavior to all of the programs, whether or not they are subject to copyright.

According to other variants, provision is made, in possible combination with the previous one, for not descrambling several, or even all of the components, of the program (s) concerned.

The module continues, as in the current situation, to carry out all of the functions related to access control (acquisition and processing of access title management messages or EMM, acquisition and decryption of access title control messages or ECM, possible descrambling of components ...) except descrambling of certain components. The module allows this or these components to pass transparently but then transmits to the microprocessor of the receiver the control words which it will have deciphered and which relate to this or these components.

Control words can be transmitted in clear because the level of security required is not very high; they can also be encrypted from a shared secret shared between the module and the receiver, developed for example according to the Diffie-Hellman principle. As indicated above, the receiver can in particular be a decoder receiver which includes a DVB type descrambler; it can then simply, while not embedding any other specificity linked to the type of access control used, descramble the component or components of the program transmitted transparently by the module. The indication that the transport packets of the components concerned are scrambled will be provided to the receiver by the headers of these packets. The transition from "normal mode / copyright mode" is therefore seamless; the synchronization between the descrambling stop by the module and the beginning of descrambling by the receiver being naturally ensured by the content of a “transport scrambling control” field of the transport packets transmitted by the module to the receiver on the link 40 of interface 13.

In infrequent cases, scrambling is carried out at the level of so-called PES packets in the MPEG standard. In this case the fields containing the information are the "PES scrambling control" in the PES headers.

In an advantageous mode of implementation of the invention, a specific signal is transmitted in the digital stream making it possible to distinguish the works (events in the MPEG or DVB sense) protected by copyright or more generally those for which it is contractually obligated not to transfer them in clear by a physical interface. One possibility is already provided by the MPEG2 standard at the level of the headers of PES which include a bit of copyright. It can be used, but in general we will prefer signaling in the SI / PSI service identifiers whose access will be easier by the microprocessor of the receiver. Many solutions are possible. Standardization of this signaling is advantageous. It can be provided by introducing an identifier constituting a copyright descriptor in the program definition tables, known as the Program Map Table or PMT.

When selecting a program by the user, the receiver, which systematically accesses the PMT, will determine, depending on the presence or not of this descriptor, whether it should enter copyright mode or not. In if so, it gives this information to the module during the transmission of the command containing the partial program definition table ca_pmt which will integrate, in addition in this case, the copyright descriptor.

Furthermore, still in order to guarantee interoperability between modules and receivers, the protocol of the following two functions must be made common for all the operators and broadcasters implementing the process:

- identification by the receiver of the module's capacity to process copyright mode,

- the transmission of control words from the module to the receiver.

The identification by the receiver of a module suitable for the mode will be done, during the initialization of the module by the response to the request for listing of resources, called the "Profile Inquiry" command. The module responds by listing the resources it contains. A suitable access control module will respond by adding, to the current access control resource (class 3, type 1), the “copyright mode” resource which will be qualified, for example, as class 4 typical.

The transmission of the control words to the host microprocessor of the receiver will be done using a new command, which can be called for example cr_cw. An example of such a command is as follows:

Possible cr_cw_tag value: 9F 80 40

We see that the invention makes it possible to satisfy the new imperative introduced by the work distribution companies by simple evolutions, generally only software, of the receivers and of the existing modules. In general, it will suffice, on already existing devices, to make the software evolve and this evolution can often be carried out by downloading in the data stream contained in the program signal. Only current receivers which do not integrate an internal DVB descrambler, which is very rare, at least in Europe, could not support this evolution.

The above characteristics as well as others will appear better on reading the following description of particular embodiments given by way of nonlimiting examples. The description refers to the accompanying drawings, in which: FIG. 1, already mentioned, is a diagram of the elements of a decoder and of a module concerned by the invention; and fig. 2 shows a variant. The receiver 10 shown in FIG. 1 has a known material constitution. It comprises an input tuner 30 which receives the radio frequency signal and supplies a demodulator 32 supplying the scrambled stream as output. This flow arrives on the one hand at the interface 13 and possibly at the descrambler 28.

During operation in mode without copyright protection, the descrambler 28 is transparent and is limited to transmitting the descrambled stream, received from module 12 on link 40 of the interface, to demultiplexer 24 which directs the data to the microprocessor host 16 associated with working random access memories 34 and non-volatile program 36 and directs the packets containing the video and audio information towards the decoder 38, of MPEG type for example.

During operation in protection mode, the descrambler 20 of the module 12 is controlled by the microprocessor 14 so as to transmit at least one of the components in scrambled form on the link 40. The software necessary for this operating mode, if it is not 'is not originally provided, can be downloaded, for example in memory 36 and in a flash memory 42 of the module 12.

We will now describe a variant of the invention which can be used when the operator or an operator wants to be able to broadcast on its network streams protected by any X scrambling, but different from DVB scrambling (or more generally scrambling for which the receiver is basically equipped with). This variant of the invention relates only to the access control module, the receiver remaining unchanged and interoperable with access control modules conforming to one or the other of the two variants of the invention.

In this variant, shown in fig. 2, where the members corresponding to those of FIG. 1 are designated by the same reference, the module 12 comprises both a descrambler 44 of type X and a scrambler 46 of type DVB by which will transit, after descrambling X, the component or components subject to copyright. When passing through this scrambler 44, the “transport scrambling control” field of the header of the transport packets is repositioned in the “scrambled” state and the control words necessary for descrambling DVB of this or these components, by the receiver are transmitted to it by the protocol described above. The necessary control messages are also transmitted between the module 12 and the receiver by the link 48 of the interface 13.

Creating local or transmitting control words over the network is a choice for the designer of the access control system and / or module. These may be locally developed hazards, possibly constant throughout the event. They can be derived from the control words necessary for descrambling X, or even be transmitted by private messages from the access control system in the stream broadcast on the network.

The invention is susceptible of numerous other embodiments as well. The nature of the interface can be either parallel, as in the case of DVB Cl, or fast serial, as in the case of the IEEE 1394 type.

Claims

1 - Method for rendering a clear digital television program signal on a receiver from a scrambled signal according to which the demodulated, scrambled signal containing messages for obtaining a control word, d 'a television receiver providing the demodulation and decoding functions to an access control module, via a physical interface,

characterized in that, at least in a particular mode of operation, the signal from the access control module is retransferred to the receiver with at least one of the components of the program selected in the scrambled state, at the same time as the corresponding control words, calculated by the module, and in that the complementary descrambling is carried out in the receiver.

2. - Method according to claim 1, in which we transfer back, from the access control module, the component or components of the selected program that said module leaves in the scrambled state to transmit them to the receiver.

3.- Method according to claim 1 or 2, wherein the video is the component or one of the components not descrambled by the module.

4. - Method according to claim 2 or 3, wherein the transmitted stream is scrambled in accordance with the DVB standard.

5. - Method according to claim 2, 3 or 4, in which a signaling of copyright is inserted in the signal to activate the particular operating mode, for example by means of a descriptor placed in definition tables program (PMT).

6. - Method according to claim 2, 3, or 4, wherein all the programs are treated identically with non-systematic descrambling of at least one of their components.

7. - Method according to any one of the preceding claims, in which software is downloaded to the receiver causing the sending of a non-descrambling instruction by the module and descrambling by the receiver.

8. - Method according to claim 1, according to which the access control module, after having descrambled the components of the selected program, re-scrambles in accordance with the DVB standard at least one of the components and transmits the corresponding control words to the receiver which provides the corresponding descrambling.

9 - Method according to claim 8, wherein the video is the component or one of the components re-scrambled by the module.

10. - A method according to claim 8 or 9, wherein one inserts into the signal a copyright signaling to activate the particular operating mode, for example by means of a descriptor placed in a program list (PMT ).

11. - A method according to claim 8, 9 or 10, not implementing specific signaling and in which all the programs are treated identically with systematic re-scrambling of at least one of their components.

12.- Method according to any one of the preceding claims, in which software is downloaded into the module allowing the implementation of the method according to any one of Claims 1 to 11.

13. - Method according to any one of the preceding claims, in which the control words are transmitted by the module to the receiver in clear or encrypted by means of a common secret developed for example on a diagram equivalent to that of Diffie- Hellman.

14.-Access control module implementing the method according to any one of claims 1 to 13.

15. Digital television receiver implementing the method according to any one of claims 1 to 13.

16.- Access control module intended to be connected by a physical interface to a digital television receiver ensuring demodulation and decoding functions, said module comprising means for recovering control words from a stream received from the receiver, means for descrambling of components of the flow and for restoring descrambled components to the receiver, characterized in that the descrambling means are provided for, either systematically or in response to an instruction transmitted by the receiver, at least one of the components, such as video, to the receiver in scrambled form and transferring to the receiver the control words corresponding to this scrambling.

17. Digital television receiver comprising a descrambler and a demodulated scrambled signal output physical interface to an access control and signal return module from the module, the descrambler being designed to descramble at least one of the components of the signal, such as the video, retransmitted scrambled by the module, using a control word supplied by the module through the interface, either systematically or according to protection information contained in the stream.