EP0424201A1 - Decoder for decoding an encrypted television audio signal - Google Patents

Decoder for decoding an encrypted television audio signal Download PDF

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Publication number
EP0424201A1
EP0424201A1 EP90402682A EP90402682A EP0424201A1 EP 0424201 A1 EP0424201 A1 EP 0424201A1 EP 90402682 A EP90402682 A EP 90402682A EP 90402682 A EP90402682 A EP 90402682A EP 0424201 A1 EP0424201 A1 EP 0424201A1
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EP
European Patent Office
Prior art keywords
frequency
signal
sound
decoder
carrier
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EP90402682A
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German (de)
French (fr)
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EP0424201B1 (en
Inventor
Maurice Le Van Suu
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STMicroelectronics SA
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SGS Thomson Microelectronics SA
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04K—SECRET COMMUNICATION; JAMMING OF COMMUNICATION
    • H04K1/00—Secret communication
    • H04K1/003—Secret communication by varying carrier frequency at or within predetermined or random intervals

Definitions

  • the present invention relates to a decoder for decoding an encrypted sound, this sound being encrypted so that it cannot be easily decrypted, or decrypted, by listeners who would not otherwise have means to decrypt it. These means usually bear the name of decoder. It finds its application more particularly in the field of broadcasting where the use of encrypted sounds is implemented to reserve the hearing of messages to subscribers of pay-TV broadcasting channels. In the field of broadcasting, it relates more particularly to broadcasting.
  • the encryption coding relates to a variable delay in the appearance of the line video signal with respect to the standard line trigger synchronization of the television set.
  • the electrical signal representative of the sound it it is a modulation, usually of SSB (Single Side Band) type of a carrier by the sound to be heard.
  • SSB Single Side Band
  • the pay-TV broadcasting stations emit a sound which, once the high frequency demodulation (HF) has been carried out, comprises or not, depending on whether it is encrypted or not, an additional amplitude modulation of the SSB type.
  • HF high frequency demodulation
  • the broadcasting station broadcasts in the clear, for example when advertising messages are transmitted, the broadcasts are not encrypted. Under these conditions, everyone can receive them and understand the advertising message.
  • the radio signal is encrypted. While listeners who do not have a set-top box see their image become blurred and the sound emitted become inaudible at the time of the change, this change takes place without incident for the owners of the set-top box.
  • the decoder is capable of recognizing the presence of the encryption and of implementing its decryption function.
  • the pay channels want to complicate the task of fraudsters who would like to receive the sound in the clear by only subjecting it to an amplitude demodulation after the high frequency demodulation.
  • These pay channels then code by the use of a carrier signal whose frequency is unknown. In addition, this unknown frequency may vary over time during the same transmission. Under these conditions, fraudsters are unable to decode the sound using a simple amplitude demodulator.
  • This additional coding by this modulation by an unknown frequency of sound must lead to simple decoding in the decoders supplied by the pay-TV broadcasting chain.
  • a system is known with which these decoders are provided in order to make the diffusion of sound audible whatever the encryption state for the holders of the decoder.
  • This system essentially comprises a microprocessor which calculates, on the received signal, the frequency of the carrier. This microprocessor then controls an oscillator in frequency so that this oscillator emits a reconstructed carrier signal whose frequency is equal to that of the unknown carrier of the encrypted sound.
  • Such a system has the disadvantage that it requires the presence of a microprocessor and that such a microprocessor, although their use is generalized and that they are in themselves inexpensive, comes to increase by its cost the cost of decoder. So we are trying to make a decoder that is cheaper, while also having such great functionality. Indeed, a microprocessor, because it can be programmed, accepts a certain programmability of the demodulation parameters.
  • the oscillator When the two phases are identical (that is to say when the phase and the frequency of the reference signal are equal to the phase and the frequency of the encrypted signal to be demodulated) the oscillator is maintained at its demodulation frequency and it only changes when the modulation frequency of the encrypted signal itself changes.
  • the subject of the invention is therefore a decoder for decoding an encrypted sound, this encryption of the sound being effected by modulation, by this sound, of an alternating signal.
  • this decoder comprising a demodulator, this demodulator receiving on the one hand an electrical signal representative of the encrypted sound and on the other hand an alternating electrical signal whose frequency is that of the unknown carrier, this decoder delivering an demodulated electrical signal representative of this sound, characterized in that it comprises an oscillator controlled by a servo, this servo comprising a phase comparator, to produce an electrical signal representative of the unknown carrier, this phase comparator receiving at the input, on the one hand an electrical signal representative of the carrier of the encrypted sound, and on the other hand an electrical signal coming from the output of the oscillator and corresponding to the signal representative of the unknown carrier.
  • FIG. 1 schematically shows a decoder according to the invention.
  • This comprises a demodulator 1 comprising two inputs, respectively 2 and 3.
  • a first input 2 receives an electrical signal representative of the encrypted sound.
  • this signal when it is effectively encrypted (that is to say modulated by a single sideband BLU type modulation) is transmitted to input 2 via a low pass filter 4 at output high frequency demodulation.
  • the demodulator On its second input 3, the demodulator receives an alternating electrical signal whose frequency is that of the unknown carrier.
  • the decoder 1 delivers on its output 5 a demodulated electrical signal representative of the sound once the decryption (demodulation) has been carried out.
  • the demodulator is a product type demodulator.
  • the demodulator is connected to a voltage-controlled oscillator 6.
  • the oscillator 6 is voltage-controlled by a servo essentially comprising a phase comparator 7.
  • the phase comparator 7 produces a signal error V e representative, as a function of time and of the control state of oscillator 6, of the frequency of the unknown carrier.
  • the comparator 7 has two inputs 8 and 9 respectively. On a first input 8, it receives an electrical signal representative of the encrypted sound. On a second input 9 it receives an electrical signal coming from the output of the oscillator and corresponding to the signal representative of the unknown carrier.
  • FIG. 2a shows the frequency diagram of a signal 10, the sound to be received. This sound was used to amplitude modulation of a signal at an unknown carrier frequency (it is indicated in dashes) of frequency f1 so as to lead to a spectrum 11.
  • demodulation in demodulator 1 if a signal at frequency f1 is introduced on l input 3, a signal will be obtained on output 5, the spectrum of which is represented in FIG. 2b in baseband by profile 12.
  • the signal at output of the demodulator does not only contain the useful spectral components of the sound but it also has a component at the demodulation frequency.
  • the demodulation frequency has precisely been the frequency f1, this spectral component 13 appears under f1.
  • the signals available at input 2 and input 3 of the modulator 1 are shaped by circuits 14 and 15 of the same kind respectively.
  • the circuit 14 which is the only one detailed comprises a cascaded amplifier 16 followed by a clipper 17.
  • the clipper 17 can be constituted by a simple diode. The diode is connected in parallel between the output of amplifier 16 and ground.
  • a capacitor 18 is arranged in series with the output of amplifier 16. This capacitor 18 eliminates the DC component.
  • the available signal has the appearance of that shown in FIG. 3a. It is a rectangular signal whose duty cycle is exactly 1.
  • other forms of the shaping circuit 14 can be envisaged, the latter being given here only to simplify the explanation.
  • this phase comparator 7 in principle comprises an exclusive OR gate.
  • the output of this gate is worth, in a zero example, when the two signals transmitted at its input are both negative or positive. In the other cases, the exit from the exclusive OR is worth one.
  • FIG. 3c shows pulses 19 - 23 during which the output of the exclusive OR gate of the comparator 7 has passed to 1.
  • the output of the exclusive OR gate is connected to an integrating circuit 24-25.
  • the integrator circuit 24-25 comprises a resistor 24 in series and a capacitor 25 connected between the output of the resistor 23 and the ground.
  • the output of the integrator circuit is taken at the midpoint of this RC circuit.
  • the time constant of this RC circuit is large compared to the period of the pulses 19 - 23. It is for example 10 times greater.
  • the integrator circuit 24-25 transforms the pulse signals 19 to 23 into a substantially flat signal 26 (FIG. 3c). This signal 26 is the signal V e : the error signal admitted to the input of the oscillator.
  • the exclusive OR gate is slightly different. It is in fact connected in cascade with a sequential circuit having for object to determine which of the two signals (that coming from the entry or that coming from the output of the demodulator 1) arrives first. This makes it possible, by the direction of advance or delay of phase thus detected, to give a positive or negative direction to the signal V e . Under these conditions, the signal V e remains at the frequency f1.
  • Circuits comprising both the exclusive OR circuit and the sequential circuit thus described are known in the state of the art under the abbreviation of PLL (Phase Lock Loop) and make it possible to constitute the phase control loops.
  • PLL Phase Lock Loop
  • such a PLL circuit used is the circuit: Micro Power Phase Locked Loop: CD 40-46 A from the Company RCA.
  • the frequency f1 is of the order of a dozen KHz. This leads to maximum possible phase differences, expressed in time, of the order of 40 microseconds.
  • the oscillator receives signals error of a given polarity then of an inverse polarity depending on whether the sequential circuit thus described has detected a phase change of signal or another beforehand. This can result in erratic operation of the oscillator 6.
  • the values n are preferably of the order of 10 for the application indicated.
  • the voltage-controlled oscillator 6 is normally not stable and must in practice be controlled by a quartz 27 connected to the terminals of an oscillator 28.
  • the oscillator 28 is itself connected to a divider by m. This is made necessary by the fact that the crystals normally produce very high natural frequencies which are known to be far from a frequency f2 of the order of 12 Khz around which it will be necessary to control the oscillator controlled in voltage 6.
  • the signal delivered by the output 5 of the demodulator 1 is sent, by means of a correction amplifier 29 to an input 30 of a selection circuit 31.
  • the selection circuit 31 also comprises another input 32 which receives the signal present at the input 2 of the demodulator 1.
  • the purpose of the selection circuit is to deliver the sound in clear to the output 33 of the decoder according to the invention.
  • the selection circuit 31 includes in particular switches allowing the passage of the non-demodulated signal or of the signal having undergone the first demodulation by f1.
  • the selection circuit 31 receives orders N, M or P issued by a decision circuit 34.
  • the decision circuit 34 receives the electrical signals representative of the state of encryption, double encryption or lack of encryption of the sound signal received.
  • the decision circuit 34 is a decoder analogous to an address decoder. It can also be constituted by a diode matrix or another wired circuit.
  • the signals representative of these states are signals V and W produced respectively by control circuits such as circuits 35 or 36. These control circuits 35 and 36 are given here only for information and only to explain the function that 'they are supposed to perform. Other circuits are easily accessible to those skilled in the art.
  • the circuit 35 is interposed between the input of the oscillator 6 and the input of the decision circuit 34 which receives the signal V.
  • the signal V is a logic signal: it is supposed to be at 1 constantly when the its sound is emitted in clear by the chain of broadcasting.
  • the phase comparator 7 receives on the one hand a signal produced by the oscillator 6 which is fixed by default at the control frequency which the oscillator 28 delivers to it and on the other hand a constant signal. In other words, the phase comparator delivers a signal which oscillates between + 1 and - 1 at the proper oscillation rhythm of the oscillator 6.
  • the decision circuit 34 transmits, as a function of the value of the orders V and W it receives, orders by N, M and P indicated by the decision table of FIG. 5.
  • V is 1
  • N is 1
  • M and P are 0.
  • the signal N which is introduced on the control gate of an N-type transistor 40 of the selection circuit 31 authorizes the passage through this selection circuit 31 of the clear signal available on input 32 of this circuit.
  • This clear signal is then transmitted to a low pass filter 41 whose purpose is to prevent crosstalk.
  • the low pass filter is in connection with an output amplifier 42.
  • FIGS. 4a to 4c show the spectral diagram of a sound signal having undergone a double modulation and having to undergo a double demodulation.
  • the double demodulation is not, however, too complicated. It must subscribe to a certain number of constraints. For example, it is assumed that the unknown carrier frequency must be in a certain range, for example between 12 and 14.8 Khertz.
  • the frequency f2 being known, and even in a preferred example being equal to 12.8 Khertz, the frequency f1 in the event of double demodulation, must be in another range. In a corresponding example it must be between 24.8 Khertz and 27.6 Khertz.
  • the demodulation is first carried out by the unknown carrier f1 (when it is a simple modulation) or by a combination of the unknown carrier (
  • the signal 10, FIG. 4a first modulating a carrier at frequency f2 can produce a modulated signal 47.
  • the modulated signal 47 modulating the unknown carrier f1 produces on the one hand a doubly modulated signal of which a component 48 is located outside band, and of which another component 49 is located in the useful band.
  • the signal 48 is eliminated and the signal 49 is demodulated.
  • the voltage-controlled oscillator 6 is calibrated at the frequency
  • This signal, once demodulated, is located around the frequency f2.
  • a low pass filter 51 placed at the output of the modulator 1, the unnecessary high frequency components resulting from this first demodulation are eliminated.
  • a second demodulator 52 placed downstream of the filter 51, receiving on the one hand the signal 50 delivered by the filter 51, and on the other hand a carrier signal at frequency f2 produced by the oscillator 28, the final demodulation so as to find the clear sound at the output 53 of the demodulator 52.
  • the signal delivered by the output 53 is itself filtered in a filter, preferably with switched capacitors 54, in order to remove the demodulation noise and in order to avoid tape folding problems.
  • N is necessarily zero and the orders M and P then take mutually inverse values to authorize, by their application on N-type transistor gates 43 and 44 respectively, the passage of the simply demodulated signal available on input 30, or the passage of a doubly demodulated signal available on input 45 of the selection circuit 31.
  • the signal W is generated also for example by a control circuit 36 in relation to a band pass filter 46 centered on the frequency f2.
  • the bandpass filter 46 lets appear the high frequency components which would exist if the signal had been doubly modulated and if consequently after the first demodulation still existed high frequency components. This signal is detected in the same way as in the control circuit 36 and the signal W is worth 1 when there has been double modulation or is worth 0 when there has been no double modulation.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
  • Signal Processing For Digital Recording And Reproducing (AREA)
  • Television Receiver Circuits (AREA)

Abstract

An encrypted sound decoder is obtained by feeding back the output from a demodulator (1) (used in the decryption) to an input (9) of a phase comparator (7) whose output controls a voltage-controlled oscillator (6) which drives the demodulator. The phase comparator moreover receives on another input (8) the signal to be decrypted (to be demodulated). The signal to be decrypted is a signal modulated by a carrier whose frequency is unknown. It is shown that this system makes it possible to avoid employing a microprocessor tasked with calculating the unknown frequency and with producing a signal at a demodulation frequency equal to this unknown frequency. <IMAGE>

Description

La présente invention a pour objet un décodeur pour décoder un son crypté, ce son étant crypté de manière à ne pas pouvoir être facilement décrypté, ou déchiffré, par des auditeurs qui ne possèderaient pas par ailleurs des moyens pour le déchiffrer. Ces moyens portent usuellement le nom de décodeur. Elle trouve son application plus particulièrement dans le domaine de la radiodiffusion où l'utilisation de sons cryptés est mise en oeuvre pour réserver l'audition de messages à des abonnés de chaînes de radiodiffusion à péage. Dans le domaine de la radiodiffusion, il concerne plus particulièrement la radiotélévision.The present invention relates to a decoder for decoding an encrypted sound, this sound being encrypted so that it cannot be easily decrypted, or decrypted, by listeners who would not otherwise have means to decrypt it. These means usually bear the name of decoder. It finds its application more particularly in the field of broadcasting where the use of encrypted sounds is implemented to reserve the hearing of messages to subscribers of pay-TV broadcasting channels. In the field of broadcasting, it relates more particularly to broadcasting.

On connaît les chaînes de radiotélévision à péage. Celles-ci émettent des signaux radioélectriques représentatifs d'images et de sons qui peuvent être soit en clair soit cryptés. Ces images et ces sons sont dits en clair lorsqu'ils peuvent être reçu sur n'importe quel récepteur de télévision, pourvu au minimum d'une antenne et de moyens de démodulation. Par contre, lorsque les images et les sons sont émis d'une manière cryptée, seul les propriétaires d'un décodeur peuvent démoduler les signaux radioélectriques émis et les démoduler ultérieurement avec succès. Le décodage dont il est question s'effectue normalement après une démodulation haute fréquence du signal radioélectrique émis.We know the pay TV channels. These emit radio signals representative of images and sounds which can be either in clear or encrypted. These images and these sounds are said to be clear when they can be received on any television receiver, provided at least with an antenna and demodulation means. On the other hand, when the images and the sounds are emitted in an encrypted way, only the owners of a decoder can demodulate the radio signals emitted and demodulate them later successfully. The decoding in question is carried out normally after a high frequency demodulation of the radio signal emitted.

Pour la présentation des images, le codage de cryptage concerne un retard variable de l'apparition du signal vidéo ligne par rapport à la synchronisation standard de déclenchement ligne du poste de télévision. Pour le signal électrique représentatif du son, il s'agit d'une modulation, habituellement de type BLU (Bande Latérale Unique) d'une porteuse par le son à entendre. Autrement dit, les chaînes de radiotélévision à péage émettent un son qui, une fois que la démodulation haute fréquence (HF) a été effectuée, comporte ou non, selon qu'il est crypté ou non, une modulation en amplitude supplémentaire de type BLU.For the presentation of the images, the encryption coding relates to a variable delay in the appearance of the line video signal with respect to the standard line trigger synchronization of the television set. For the electrical signal representative of the sound, it it is a modulation, usually of SSB (Single Side Band) type of a carrier by the sound to be heard. In other words, the pay-TV broadcasting stations emit a sound which, once the high frequency demodulation (HF) has been carried out, comprises or not, depending on whether it is encrypted or not, an additional amplitude modulation of the SSB type.

Lorsque la chaîne de radiotélévision émet en clair, par exemple au moment du passage des messages publicitaires, les émissions ne sont pas cryptées. Dans ces conditions, tout le monde peut les recevoir et comprendre le message publicitaire. Par contre, lorsqu'un programme de loisir ou d'information est émis le signal radioélectrique et crypté. Alors que les auditeurs qui ne possèdent pas de décodeur voient à la fois leur image se brouiller et le son émis devenir inaudible au moment du changement, ce changement s'effectue sans incident pour les propriétaires de décodeur. Autrement dit, le décodeur est capable de reconnaître la présence du cryptage et de mettre en oeuvre sa fonction de décryptage.When the broadcasting station broadcasts in the clear, for example when advertising messages are transmitted, the broadcasts are not encrypted. Under these conditions, everyone can receive them and understand the advertising message. On the other hand, when a leisure or information program is transmitted, the radio signal is encrypted. While listeners who do not have a set-top box see their image become blurred and the sound emitted become inaudible at the time of the change, this change takes place without incident for the owners of the set-top box. In other words, the decoder is capable of recognizing the presence of the encryption and of implementing its decryption function.

Compte tenu du caractère simple de la modulation supplémentaire du son, les chaînes à péage veulent compliquer la tâche des fraudeurs qui voudraient recevoir le son en clair en ne lui faisant subir qu'une démodulation d'amplitude après la démodulation haute fréquence. Ces chaînes à péage codent alors par l'utilisation d'un signal de porteuse dont la fréquence est inconnue. De plus, cette fréquence inconnue peut varier dans le temps au cours d'une même émission. Dans ces conditions, les fraudeurs sont dans l'incapacité de décoder le son au moyen d'un simple démodulateur en amplitude.Given the simple nature of the additional modulation of the sound, the pay channels want to complicate the task of fraudsters who would like to receive the sound in the clear by only subjecting it to an amplitude demodulation after the high frequency demodulation. These pay channels then code by the use of a carrier signal whose frequency is unknown. In addition, this unknown frequency may vary over time during the same transmission. Under these conditions, fraudsters are unable to decode the sound using a simple amplitude demodulator.

Cependant, ce codage supplémentaire par cette modulation par une fréquence inconnue du son, doit conduire à un décodage simple dans les décodeurs fournis par la chaîne de radiotélévision à péage. On connaît un système dont sont munis ces décodeurs dans le but de rendre la diffusion du son audible quel que soit l'état de cryptage pour les titulaires du décodeur. Ce système comporte essentiellement un microprocesseur qui calcule, sur le signal reçu, la fréquence de la porteuse. Ce microprocesseur commande alors en fréquence un oscillateur afin que cet oscillateur émette un signal de porteuse reconstitué et dont la fréquence est égale à celle de la porteuse inconnue du son crypté. Un tel système présente l'inconvénient qu'il nécessite la présence d'un microprocesseur et qu'un tel microprocesseur, bien que leur emploi se généralise et qu'ils soient en eux-mêmes peu coûteux, vient augmenter par son coût le coût du décodeur. On cherche donc à réaliser un décodeur qui soit moins cher, tout en ayant par ailleurs une aussi grarde fonctionnalité. En effet, un microprocesseur, parce qu'il peut être programmé, accepte une certaine programmabilité des paramètres de démodulation.However, this additional coding by this modulation by an unknown frequency of sound, must lead to simple decoding in the decoders supplied by the pay-TV broadcasting chain. A system is known with which these decoders are provided in order to make the diffusion of sound audible whatever the encryption state for the holders of the decoder. This system essentially comprises a microprocessor which calculates, on the received signal, the frequency of the carrier. This microprocessor then controls an oscillator in frequency so that this oscillator emits a reconstructed carrier signal whose frequency is equal to that of the unknown carrier of the encrypted sound. Such a system has the disadvantage that it requires the presence of a microprocessor and that such a microprocessor, although their use is generalized and that they are in themselves inexpensive, comes to increase by its cost the cost of decoder. So we are trying to make a decoder that is cheaper, while also having such great functionality. Indeed, a microprocessor, because it can be programmed, accepts a certain programmability of the demodulation parameters.

En outre, et parce qu'une phase de démodulation par un seul signal même si la fréquence de ce signal est inconnue, n'est pas suffisamment disuasive vis-à-vis des fraudeurs astucieux, l'habitude a été prise de soumettre le son modulé en bande latérale unique à une deuxième modulation supplémentaire. Cette deuxième modulation est également une modulation en bande latérale unique, mais à une autre fréquence de porteuse. Cette autre fréquence de porteuse est elle, pour ne pas trop compliquer les affaires, à fréquence fixe.In addition, and because a phase of demodulation by a single signal even if the frequency of this signal is unknown, is not sufficiently dissuasive vis-à-vis clever fraudsters, the habit has been taken of submitting the sound modulated in single sideband to a second additional modulation. This second modulation is also a single sideband modulation, but at another carrier frequency. This other carrier frequency is it, not to complicate matters too much, at fixed frequency.

Ceci signifie qu'en définitive le son peut être modulé trois fois ; une fois le son module le signal de porteuse à fréquence fixe, une deuxième fois le résultat de cette première modulation module la porteuse dont la fréquence est dite inconnue, et une troisième fois le signal issu de cette seconde modulation module en fréquence une porteuse HF pour pouvoir être émis radioélectriquement. Ceci justifie la présence du microprocesseur qui doit être capable de faire la différence entre non pas deux situations, claire et cryptée, mais trois situations, claire, simplement cryptée ou doublement cryptée. Dans l'invention, et bien qu'un tel microprocesseur apparaisse alors comme presque indispensable, on réussit à s'en passer.This means that ultimately the sound can be modulated three times; once the sound modulates the signal carrier at fixed frequency, a second time the result of this first modulation modulates the carrier whose frequency is said to be unknown, and a third time the signal from this second modulation modulates in frequency an HF carrier in order to be able to be emitted radioelectrically. This justifies the presence of the microprocessor which must be able to differentiate between not two situations, clear and encrypted, but three situations, clear, simply encrypted or doubly encrypted. In the invention, and although such a microprocessor then appears to be almost essential, we manage to do without it.

Dans l'invention, pour remédier aux inconvénients cités, on propose tout simplement de réaliser un décodage, une démodulation du signal modulant la porteuse à fréquence inconnue, dans un démodulateur qui reçoit d'une part le signal crypté, c'est-à-dire modulant cette fréquence inconnue, et d'autre part, un signal émis par un oscillateur commandé en tension. Le signal émis par l'oscillateur résulte d'une boucle d'asservissement dans laquelle on effectue la comparaison de phase entre le signal crypté (porteuse reçue) et un signal correspondant au signal de démodulation (de référence) dans ce démodulateur. Lorsque les deux phases sont identiques (c'est-à-dire lorsque la phase et la fréquence du signal de référence sont égales à la phase et à la fréquence du signal crypté à démoduler) l'oscillateur est maintenu à sa fréquence de démodulation et il n'en change que lorsque la fréquence de modulation du signal crypté elle-même change.In the invention, to remedy the drawbacks mentioned, it is quite simply proposed to carry out a decoding, a demodulation of the signal modulating the carrier at an unknown frequency, in a demodulator which receives on the one hand the encrypted signal, that is to say say modulating this unknown frequency, and on the other hand, a signal emitted by a voltage controlled oscillator. The signal emitted by the oscillator results from a servo loop in which the phase comparison is carried out between the encrypted signal (carrier received) and a signal corresponding to the demodulation signal (reference) in this demodulator. When the two phases are identical (that is to say when the phase and the frequency of the reference signal are equal to the phase and the frequency of the encrypted signal to be demodulated) the oscillator is maintained at its demodulation frequency and it only changes when the modulation frequency of the encrypted signal itself changes.

L'invention a donc pour objet un décodeur pour décoder un son crypté, ce cryptage du son étant effectué par modulation, par ce son, d'un signal alternatif oscillant à une fréquence inconnue d'une porteuse, ce décodeur comportant un démodulateur, ce démodulateur recevant d'une part un signal électrique représentatif du son crypté et d'autre part un signal électrique alternatif dont la fréquence est celle de la porteuse inconnue, ce décodeur délivrant en sortie un signal électrique démodulé représentatif de ce son, caractérisé en ce qu'il comporte un oscillateur commandé par un asservissement, cet asservissement comportant un comparateur de phase, pour produire un signal électrique représentatif de la porteuse inconnue, ce comparateur de phase recevant en entrée, d'une part un signal électrique représentatif de la porteuse du son crypté, et d'autre part un signal électrique provenant de la sortie de l'oscillateur et correspondant au signal représentatif de la porteuse inconnue.The subject of the invention is therefore a decoder for decoding an encrypted sound, this encryption of the sound being effected by modulation, by this sound, of an alternating signal. oscillating at an unknown frequency of a carrier, this decoder comprising a demodulator, this demodulator receiving on the one hand an electrical signal representative of the encrypted sound and on the other hand an alternating electrical signal whose frequency is that of the unknown carrier, this decoder delivering an demodulated electrical signal representative of this sound, characterized in that it comprises an oscillator controlled by a servo, this servo comprising a phase comparator, to produce an electrical signal representative of the unknown carrier, this phase comparator receiving at the input, on the one hand an electrical signal representative of the carrier of the encrypted sound, and on the other hand an electrical signal coming from the output of the oscillator and corresponding to the signal representative of the unknown carrier.

L'invention sera mieux comprise à la lecture de la description qui suit et à l'examen des figures qui l'accompagnent. Celles-ci ne sont données qu'à titre indicatif et nullement limitatif de l'invention. Les figures montrent :

  • - Figure 1 : une représentation schématique d'un décodeur de son crypté selon l'invention ;
  • - Figures 2a et 2b : des spectres de fréquence de signaux cryptés et simplement décodés en utilisant le décodeur selon l'invention ;
  • - Figures 3a à 3c : des diagrammes temporels de signaux intervenant dans le circuit de l'invention ;
  • - Figures 4a à 4c : des spectres de signaux décodés avec le décodeur selon l'invention mais ayant subi au préalable deux modulations de codage imbriquées ;
  • - Figure 5 : la table de vérité d'un circuit de multiplexage permettant la diffusion du son en fonction des traitements qu'il a dû subir selon son état de cryptage.
The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are given for information only and in no way limit the invention. The figures show:
  • - Figure 1: a schematic representation of an encrypted sound decoder according to the invention;
  • - Figures 2a and 2b: frequency spectra of signals encrypted and simply decoded using the decoder according to the invention;
  • - Figures 3a to 3c: time diagrams of signals involved in the circuit of the invention;
  • - Figures 4a to 4c: spectra of signals decoded with the decoder according to the invention but having previously undergone two nested coding modulations;
  • - Figure 5: the truth table of a multiplexing circuit allowing the diffusion of sound according to the treatments it had to undergo according to its state of encryption.

La figure 1 montre schématiquement un décodeur selon l'invention. Celui-ci comporte un démodulateur 1 comportant deux entrées, respectivement 2 et 3. Une première entrée 2 reçoit un signal électrique représentatif du son crypté. En pratique, ce signal, quand il est effectivement crypté (c'est-à-dire modulé par une modulation de type bande latérale unique BLU) est transmis sur l'entrée 2 par l'intermédiaire d'un filtre passe bas 4 en sortie d'une démodulation haute fréquence. Sur sa deuxième entrée 3 le démodulateur reçoit un signal électrique alternatif dont la fréquence est celle de la porteuse inconnue. Le décodeur 1 délivre sur sa sortie 5 un signal électrique démodulé représentatif du son une fois que le décryptage (la démodulation) a été effectuée. De préférence, le démodulateur est un démodulateur de type produit. Selon la caractéristique essentielle de l'invention, le démodulateur est raccordé à un oscillateur commandé en tension 6. L'oscillateur 6 est commandé en tension par un asservissement comprenant essentiellement un comparateur de phase 7. Le comparateur de phase 7 produit un signal d'erreur Ve représentatif, en fonction du temps et de l'état de commande de l'oscillateur 6, de la fréquence de la porteuse inconnue. Le comparateur 7 comporte deux entrées respectivement 8 et 9. Sur une première entrée 8, il reçoit un signal électrique représentatif du son crypté. Sur une deuxième entrée 9 il reçoit un signal électrique provenant de la sortie de l'oscillateur et correspondant au signal représentatif de la porteuse inconnue.Figure 1 schematically shows a decoder according to the invention. This comprises a demodulator 1 comprising two inputs, respectively 2 and 3. A first input 2 receives an electrical signal representative of the encrypted sound. In practice, this signal, when it is effectively encrypted (that is to say modulated by a single sideband BLU type modulation) is transmitted to input 2 via a low pass filter 4 at output high frequency demodulation. On its second input 3, the demodulator receives an alternating electrical signal whose frequency is that of the unknown carrier. The decoder 1 delivers on its output 5 a demodulated electrical signal representative of the sound once the decryption (demodulation) has been carried out. Preferably, the demodulator is a product type demodulator. According to the essential characteristic of the invention, the demodulator is connected to a voltage-controlled oscillator 6. The oscillator 6 is voltage-controlled by a servo essentially comprising a phase comparator 7. The phase comparator 7 produces a signal error V e representative, as a function of time and of the control state of oscillator 6, of the frequency of the unknown carrier. The comparator 7 has two inputs 8 and 9 respectively. On a first input 8, it receives an electrical signal representative of the encrypted sound. On a second input 9 it receives an electrical signal coming from the output of the oscillator and corresponding to the signal representative of the unknown carrier.

Le fonctionnement de ce circuit est le suivant. Sur la figure 2a on a représenté le diagramme fréquentiel d'un signal 10, le son à recevoir. Ce son a servi à moduler en amplitude, un signal à une fréquence porteuse inconnue (elle est indiquée en tirets) de fréquence f₁ de manière à conduire à un spectre 11. Lors de la démodulation dans le démodulateur 1, si on introduit un signal à la fréquence f₁ sur l'entrée 3 on réobtiendra sur la sortie 5 un signal dont le spectre est représenté sur la figure 2b en bande de base par le profil 12. En fait, on considère que le signal en sortie du démodulateur ne comporte pas que les composantes spectrales utiles du son mais qu'il comporte également une composante à la fréquence de démodulation. Lorsque la fréquence de démodulation a justement été la fréquence f₁ cette composante spectrale 13 apparaît sous f₁.The operation of this circuit is as follows. FIG. 2a shows the frequency diagram of a signal 10, the sound to be received. This sound was used to amplitude modulation of a signal at an unknown carrier frequency (it is indicated in dashes) of frequency f₁ so as to lead to a spectrum 11. During demodulation in demodulator 1, if a signal at frequency f₁ is introduced on l input 3, a signal will be obtained on output 5, the spectrum of which is represented in FIG. 2b in baseband by profile 12. In fact, it is considered that the signal at output of the demodulator does not only contain the useful spectral components of the sound but it also has a component at the demodulation frequency. When the demodulation frequency has precisely been the frequency f₁, this spectral component 13 appears under f₁.

Avant d'être introduits sur les entrées 8 et 9 du comparateur de phase 7, les signaux disponibles à l'entrée 2 et l'entrée 3 du modulateur 1 sont mis en forme par des circuits respectivement 14 et 15 de même nature. Le circuit 14 qui est seul détaillé comporte un amplificateur en cascade 16 suivi d'un écrêteur 17. En pratique l'écrêteur 17 peut être constitué par une simple diode. La diode est connectée en parallèle entre la sortie de l'amplificateur 16 et la masse. En série avec la sortie de l'amplificateur 16 est disposé un condensateur 18. Ce condensateur 18 permet d'éliminer la composante continue. Dans ces conditions, en sortie du circuit 14, le signal disponible a l'allure de celui représenté sur la figure 3a. C'est un signal rectangulaire dont le rapport cyclique est égal à 1 exactement. Bien entendu, d'autres formes du circuit de mise en forme 14 sont envisageables, celle-ci n'étant donnée ici que pour simplifier l'explication.Before being introduced on the inputs 8 and 9 of the phase comparator 7, the signals available at input 2 and input 3 of the modulator 1 are shaped by circuits 14 and 15 of the same kind respectively. The circuit 14 which is the only one detailed comprises a cascaded amplifier 16 followed by a clipper 17. In practice the clipper 17 can be constituted by a simple diode. The diode is connected in parallel between the output of amplifier 16 and ground. In series with the output of amplifier 16, a capacitor 18 is arranged. This capacitor 18 eliminates the DC component. Under these conditions, at the output of circuit 14, the available signal has the appearance of that shown in FIG. 3a. It is a rectangular signal whose duty cycle is exactly 1. Of course, other forms of the shaping circuit 14 can be envisaged, the latter being given here only to simplify the explanation.

Dans un premier temps, on peut admettre que les signaux issus des circuits de mise en forme 14 et 15 visibles respectivement sur les figures 3a et 3b sont admis directement sur les entrées du comparateur de phase 7. Dans un exemple, ce comparateur de phase 7 comporte dans son principe une porte OU exclusif. La sortie de cette porte vaut, dans un exemple zéro, lorsque les deux signaux émis à son entrée sont tous les deux ou négatifs ou positifs. Dans les autres cas, la sortie du OU exclusif vaut un.At first, we can admit that the signals from the shaping circuits 14 and 15 visible respectively in FIGS. 3a and 3b are admitted directly to the inputs of the phase comparator 7. In one example, this phase comparator 7 in principle comprises an exclusive OR gate. The output of this gate is worth, in a zero example, when the two signals transmitted at its input are both negative or positive. In the other cases, the exit from the exclusive OR is worth one.

La figure 3c montre des impulsions 19 - 23 pendant lesquelles la sortie de la porte OU exclusif du comparateur 7 est passée à 1.FIG. 3c shows pulses 19 - 23 during which the output of the exclusive OR gate of the comparator 7 has passed to 1.

La sortie de la porte OU exclusif est reliée à un circuit intégrateur 24-25. Dans un exemple simplifié, le circuit intégrateur 24-25 comporte une résistance 24 en série et un condensateur 25 raccordé entre la sortie de la résistance 23 et la masse. La sortie du circuit intégrateur est prise au point milieu de ce circuit RC. La constante de temps de ce circuit RC est grande devant la période des impulsions 19 - 23. Elle est par exemple 10 fois plus grande. Le circuit intégrateur 24-25 transforme les signaux impulsionnels 19 à 23 en un signal sensiblement plat 26 (figure 3c). Ce signal 26 est le signal Ve : le signal d'erreur admis sur l'entrée de l'oscillateur.The output of the exclusive OR gate is connected to an integrating circuit 24-25. In a simplified example, the integrator circuit 24-25 comprises a resistor 24 in series and a capacitor 25 connected between the output of the resistor 23 and the ground. The output of the integrator circuit is taken at the midpoint of this RC circuit. The time constant of this RC circuit is large compared to the period of the pulses 19 - 23. It is for example 10 times greater. The integrator circuit 24-25 transforms the pulse signals 19 to 23 into a substantially flat signal 26 (FIG. 3c). This signal 26 is the signal V e : the error signal admitted to the input of the oscillator.

De l'examen des figures 3a et 3b, il ressort que plus l'écart de phase entre les signaux admis sur les entrées des comparateurs 8 et 9 est grand, plus le signal 26 sera élevé et plus l'oscillateur 6 se rapprochera rapidement de la fréquence f₁ qu'il doit atteindre. Lorsque le signal mesuré en sortie du démodulateur 1 est à une fréquence égale à la fréquence f₁, et lorsque sa phase est également la même que celle du signal introduit sur l'entrée 2 du démodulateur, la porte OU exclusif reçoit en entrée des signaux qui valent en même temps 0 ou en même temps 1. Elle délivre dans ces conditions un signal 0. Ce signal intégré dans l'intégrateur 24-25 se transmet en un signal d'erreur nul : l'oscillateur 6 reste à la fréquence qu'il a atteint.From the examination of FIGS. 3a and 3b, it appears that the greater the phase difference between the signals admitted to the inputs of the comparators 8 and 9, the higher the signal 26 and the faster the oscillator 6 will approach the frequency f₁ which it must reach. When the signal measured at the output of the demodulator 1 is at a frequency equal to the frequency f₁, and when its phase is also the same as that of the signal introduced on the input 2 of the demodulator, the exclusive OR gate receives at the input signals which are equal to 0 or at the same time 1. Under these conditions, it delivers a signal 0. This signal integrated in the integrator 24-25 is transmitted in a zero error signal: oscillator 6 remains at the frequency that he reached.

De manière à tenir compte de la possibilité que la fréquence du signal en entrée 3 soit supérieure à la fréquence du signal admis sur l'entrée 2, la porte OU exclusif est légèrement différente. Elle est montée en fait en cascade avec un circuit séquentiel ayant pour objet de déterminer lequel des deux signaux (celui provenant de l'entrée ou celui provenant de la sortie du démodulateur 1) arrive le premier. Ceci permet par le sens de l'avance ou du retard de phase ainsi détecté de donner un sens positif ou négatif au signal Ve. Dans ces conditions, le signal Ve reste à la fréquence f₁. Des circuits comportant à la fois le circuit OU exclusif et le circuit séquentiel ainsi décrits sont connus dans l'état de la technique sous l'abréviation de PLL (Phase Lock Loop) et permettent de constituer les boucles d'asservissement de phase. Dans un exemple, un tel circuit PLL utilisé est le circuit : Micro Power Phase Locked Loop : CD 40-46 A de la Société RCA.In order to take into account the possibility that the frequency of the signal at input 3 is greater than the frequency of the signal admitted on input 2, the exclusive OR gate is slightly different. It is in fact connected in cascade with a sequential circuit having for object to determine which of the two signals (that coming from the entry or that coming from the output of the demodulator 1) arrives first. This makes it possible, by the direction of advance or delay of phase thus detected, to give a positive or negative direction to the signal V e . Under these conditions, the signal V e remains at the frequency f₁. Circuits comprising both the exclusive OR circuit and the sequential circuit thus described are known in the state of the art under the abbreviation of PLL (Phase Lock Loop) and make it possible to constitute the phase control loops. In one example, such a PLL circuit used is the circuit: Micro Power Phase Locked Loop: CD 40-46 A from the Company RCA.

En pratique la fréquence f₁ est de l'ordre d'une douzaine de KHz. Ceci conduit à des écarts de phase maximum possibles, exprimés en terme temporel, de l'ordre de 40 microsecondes. Lorsque l'asservissement démarre, lorsque la fréquence propre de l'oscillateur 6 et la fréquence inconnue sont très éloignées l'une de l'autre, il est possible que dans son parcours pour rejoindre la fréquence f₁ inconnue, l'oscillateur reçoive des signaux d'erreur d'une polarité donnée puis d'une polarité inverse selon que le circuit séquentiel ainsi décrit a détecté un changement de phase d'un signal ou d'un autre au préalable. Il peut en résulter un fonctionnement erratique de l'oscillateur 6. Afin d'éviter une telle situation, on préfère utiliser une division, par n, des fréquences des signaux admis sur les entrées 8 et 9 du comparateur 7. Ceci permet en définitive de se trouver moins souvent dans des situations d'inversion de phase. Autrement dit, le ralliement par l'oscillateur 6 de la fréquence f₁ sera à la fois plus rapide et sans à-coup. Dans la pratique, les valeurs n sont de préférence de l'ordre de 10 pour l'application indiquée.In practice the frequency f₁ is of the order of a dozen KHz. This leads to maximum possible phase differences, expressed in time, of the order of 40 microseconds. When the control starts, when the natural frequency of the oscillator 6 and the unknown frequency are very far from each other, it is possible that in its path to reach the unknown frequency f₁, the oscillator receives signals error of a given polarity then of an inverse polarity depending on whether the sequential circuit thus described has detected a phase change of signal or another beforehand. This can result in erratic operation of the oscillator 6. In order to avoid such a situation, it is preferred to use a division, by n, of the frequencies of the signals admitted to the inputs 8 and 9 of the comparator 7. This ultimately makes it possible to be less often in phase reversal situations. In other words, the rallying by the oscillator 6 of the frequency f₁ will be both faster and smoothly. In practice, the values n are preferably of the order of 10 for the application indicated.

L'oscillateur 6 commandé en tension n'est normalement pas stable et doit être en pratique piloté par un quartz 27 raccordé aux bornes d'un oscillateur 28. L'oscillateur 28 est lui-même relié à un diviseur par m. Ceci est rendu nécessaire par le fait que les quartz produisent normalement des fréquences propres très élevées qui sont notoirement éloignées d'une fréquence f₂ de l'ordre de 12 Khz autour de laquelle il va falloir piloter l'oscillateur commandé en tension 6.The voltage-controlled oscillator 6 is normally not stable and must in practice be controlled by a quartz 27 connected to the terminals of an oscillator 28. The oscillator 28 is itself connected to a divider by m. This is made necessary by the fact that the crystals normally produce very high natural frequencies which are known to be far from a frequency f₂ of the order of 12 Khz around which it will be necessary to control the oscillator controlled in voltage 6.

Le signal délivré par la sortie 5 du démodulateur 1 est envoyé, par l'intermédiaire d'un amplificateur de correction 29 sur une entrée 30 d'un circuit de sélection 31. Le circuit de sélection 31 comporte en outre une autre entrée 32 qui reçoit le signal présent à l'entrée 2 du démodulateur 1. Le circuit de sélection a pour objet de délivrer à la sortie 33 du décodeur selon l'invention le son en clair. Le circuit de sélection 31 comporte notamment des interrupteurs permettant le passage du signal non démodulé ou du signal ayant subi la première démodulation par f₁. Le circuit de sélection 31 reçoit des ordres N, M ou P émis par un circuit de décision 34. Le circuit de décision 34 reçoit les signaux électriques représentatifs de l'état de cryptage, de double cryptage ou d'absence de cryptage du signal sonore reçu.The signal delivered by the output 5 of the demodulator 1 is sent, by means of a correction amplifier 29 to an input 30 of a selection circuit 31. The selection circuit 31 also comprises another input 32 which receives the signal present at the input 2 of the demodulator 1. The purpose of the selection circuit is to deliver the sound in clear to the output 33 of the decoder according to the invention. The selection circuit 31 includes in particular switches allowing the passage of the non-demodulated signal or of the signal having undergone the first demodulation by f₁. The selection circuit 31 receives orders N, M or P issued by a decision circuit 34. The decision circuit 34 receives the electrical signals representative of the state of encryption, double encryption or lack of encryption of the sound signal received.

Dans un exemple le circuit de décision 34 est un décodeur analogue à un décodeur d'adresse. Il peut aussi bien être constitué par une matrice à diodes ou un autre circuit câblé. Les signaux représentatifs de ces états sont des signaux V et W élaborés respectivement par des circuits de commande tels que les circuits 35 ou 36. Ces circuits de commande 35 et 36 ne sont donnés ici qu'à titre indicatif et uniquement pour expliquer la fonction qu'ils sont sensés exécuter. D'autres circuits sont facilement à la portée de l'homme de métier. Dans un exemple, le circuit 35 est interposé entre l'entrée de l'oscillateur 6 et l'entrée du circuit de décision 34 qui reçoit le signal V. Le signal V est un signal logique : il est sensé être à 1 constamment lorsque le son est émis en clair par la chaîne de radiotélévision. Lorsque ce cas se produit, aucune porteuse ne peut y être détecté, et après division par n la fréquence de ce signal basse fréquence (en bande de base) est si basse qu'on peut admettre qu'elle est nulle. Dans ces conditions, le comparateur de phase 7 reçoit d'une part un signal élaboré par l'oscillateur 6 qui se fixe par défaut à la fréquence de pilotage que lui délivre l'oscillateur 28 et d'autre part un signal constant. Autrement dit, le comparateur de phase délivre un signal qui oscille entre + 1 et ― 1 au rythme propre d'oscillation de l'oscillateur 6.In one example, the decision circuit 34 is a decoder analogous to an address decoder. It can also be constituted by a diode matrix or another wired circuit. The signals representative of these states are signals V and W produced respectively by control circuits such as circuits 35 or 36. These control circuits 35 and 36 are given here only for information and only to explain the function that 'they are supposed to perform. Other circuits are easily accessible to those skilled in the art. In one example, the circuit 35 is interposed between the input of the oscillator 6 and the input of the decision circuit 34 which receives the signal V. The signal V is a logic signal: it is supposed to be at 1 constantly when the its sound is emitted in clear by the chain of broadcasting. When this case occurs, no carrier can be detected there, and after division by n the frequency of this low frequency signal (in baseband) is so low that we can admit that it is zero. Under these conditions, the phase comparator 7 receives on the one hand a signal produced by the oscillator 6 which is fixed by default at the control frequency which the oscillator 28 delivers to it and on the other hand a constant signal. In other words, the phase comparator delivers a signal which oscillates between + 1 and - 1 at the proper oscillation rhythm of the oscillator 6.

Ce signal oscillant entre + 1 et ― 1 est détecté dans le circuit 35 par une diode 37 et est filtré par un circuit résistance 38 - condensateur 39. Dans ces conditions, la sortie du circuit 35 se porte à 1. Par contre, lorsque le signal sonore module en amplitude la porteuse inconnue, et que l'asservissement joue son rôle, le signal d'erreur Ve est nul. Par conséquent, le signal en sortie du circuit de commande 35 est nul aussi : V = 0.This signal oscillating between + 1 and - 1 is detected in circuit 35 by a diode 37 and is filtered by a resistance circuit 38 - capacitor 39. Under these conditions, the output of circuit 35 increases to 1. However, when the sound signal amplitude modulates the unknown carrier, and the servo plays its role, the error signal V e is zero. Consequently, the signal at the output of the control circuit 35 is also zero: V = 0.

Le circuit de décision 34 transmet, en fonction de la valeur des ordres V et W qu'il reçoit, des ordres par N, M et P indiqués par la table de décision de la figure 5. Quand V vaut 1, N vaut 1 et M et P valent 0. Dans ce cas, le signal N qui est introduit sur la grille de commande d'un transistor 40 de type N du circuit de sélection 31 autorise le passage à travers ce circuit de sélection 31 du signal en clair disponible sur l'entrée 32 de ce circuit. Ce signal en clair est alors transmis à un filtre passe bas 41 dont le but est d'empêcher la diaphonie. Le filtre passe bas est en liaison avec un amplificateur de sortie 42.The decision circuit 34 transmits, as a function of the value of the orders V and W it receives, orders by N, M and P indicated by the decision table of FIG. 5. When V is 1, N is 1 and M and P are 0. In this case, the signal N which is introduced on the control gate of an N-type transistor 40 of the selection circuit 31 authorizes the passage through this selection circuit 31 of the clear signal available on input 32 of this circuit. This clear signal is then transmitted to a low pass filter 41 whose purpose is to prevent crosstalk. The low pass filter is in connection with an output amplifier 42.

Les figures 4a à 4c montrent le diagramme spectral d'un signal sonore ayant subi une double modulation et devant subir une double démodulation. Comme on l'a indiqué précédemment, la double démodulation n'est cependant pas trop compliquée. Elle doit souscrire à un certain nombre de contraintes. Par exemple on admet que la fréquence de porteuse inconnue doit se trouver dans une certaine gamme, par exemple entre 12 et 14,8 Khertz. En outre, la fréquence f₂ étant connue, et même dans un exemple préféré étant égale à 12,8 Khertz, la fréquence f₁ en cas de double démodulation, doit se trouver dans une autre gamme. Dans un exemple correspondant elle doit être comprise entre 24,8 Khertz et 27,6 Khertz. Ainsi on fait apparaître une composante spectrale à une fréquence |f₁ - f₂| qui est également comprise dans la même première gamme de 12 à 14,8 Khertz. Quel que soit l'ordre dans lequel sont effectués pour le cryptage les deux modulations, on effectue en premier la démodulation par la porteuse inconnue f₁ (quand il s'agit d'une simple modulation) ou par une combinaison de la porteuse inconnue (|f₁ - f₂| dans l'autre cas). Ainsi, le signal 10, figure 4a, modulant d'abord une porteuse à fréquence f₂ peut produire un signal modulé 47. Le signal modulé 47 modulant la porteuse inconnue f₁ produit d'une part un signal doublement modulé dont une composante 48 est située hors bande, et dont une autre composante 49 est située dans la bande utile. A la réception, avec le filtre passe bas 4, on élimine le signal 48 et on effectue la démodulation du signal 49.FIGS. 4a to 4c show the spectral diagram of a sound signal having undergone a double modulation and having to undergo a double demodulation. As previously indicated, the double demodulation is not, however, too complicated. It must subscribe to a certain number of constraints. For example, it is assumed that the unknown carrier frequency must be in a certain range, for example between 12 and 14.8 Khertz. In addition, the frequency f₂ being known, and even in a preferred example being equal to 12.8 Khertz, the frequency f₁ in the event of double demodulation, must be in another range. In a corresponding example it must be between 24.8 Khertz and 27.6 Khertz. Thus we show a spectral component at a frequency | f₁ - f₂ | which is also included in the same first range from 12 to 14.8 Khertz. Whatever the order in which the two modulations are carried out for the encryption, the demodulation is first carried out by the unknown carrier f₁ (when it is a simple modulation) or by a combination of the unknown carrier (| f₁ - f₂ | in the other case). Thus, the signal 10, FIG. 4a, first modulating a carrier at frequency f₂ can produce a modulated signal 47. The modulated signal 47 modulating the unknown carrier f₁ produces on the one hand a doubly modulated signal of which a component 48 is located outside band, and of which another component 49 is located in the useful band. On reception, with the low pass filter 4, the signal 48 is eliminated and the signal 49 is demodulated.

Naturellement, l'oscillateur commandé en tension 6 vient se caler à la fréquence |f₁ - f₂|. Il effectue donc la démodulation du signal modulé 49 et on dispose à la sortie 5 du démodulateur 1 d'un signal 50 une fois démodulé. Ce signal une fois démodulé est situé autour de la fréquence f₂. Avec un filtre passe bas 51 placé en sortie du modulateur 1, on élimine les composantes inutiles haute fréquence résultant de cette première démodulation. Au moyen d'un deuxième démodulateur 52 placé en aval du filtre 51, recevant d'une part le signal 50 délivré par le filtre 51, et d'autre part un signal de porteuse à fréquence f₂ élaboré par l'oscillateur 28, on effectue la démodulation finale de manière à retrouver le son en clair à la sortie 53 du démodulateur 52.Naturally, the voltage-controlled oscillator 6 is calibrated at the frequency | f₁ - f₂ |. It therefore performs the demodulation of the modulated signal 49 and there is at the output 5 of the demodulator 1 a signal 50 once demodulated. This signal, once demodulated, is located around the frequency f₂. With a low pass filter 51 placed at the output of the modulator 1, the unnecessary high frequency components resulting from this first demodulation are eliminated. By means of a second demodulator 52 placed downstream of the filter 51, receiving on the one hand the signal 50 delivered by the filter 51, and on the other hand a carrier signal at frequency f₂ produced by the oscillator 28, the final demodulation so as to find the clear sound at the output 53 of the demodulator 52.

Avant d'être introduit sur l'entrée 45 du circuit de sélection 31, le signal délivré par la sortie 53 est lui-même filtré dans un filtre, de préférence à capacités commutées 54, dans le but d'enlever le bruit de démodulation et dans le but d'éviter les problèmes de repliement de bande.Before being introduced on the input 45 of the selection circuit 31, the signal delivered by the output 53 is itself filtered in a filter, preferably with switched capacitors 54, in order to remove the demodulation noise and in order to avoid tape folding problems.

Lorsque V est nul, N est obligatoirement nul et les ordres M et P prennent alors des valeurs mutuellement inverses pour autoriser, par leur application sur des grilles de transistor de type N respectivement 43 et 44, le passage du signal simplement démodulé disponible sur l'entrée 30, ou le passage d'un signal doublement démodulé disponible sur une entrée 45 du circuit de sélection 31. Le signal W est élaboré lui aussi par exemple par un circuit de commande 36 en relation avec un filtre passe bande 46 centré sur la fréquence f₂. Le filtre passe bande 46 laisse apparaître les composantes haute fréquence qui existeraient si le signal avait été doublement modulé et si par conséquent après la première démodulation existaient encore des composantes haute fréquence. Ce signal est détecté de la même façon que dans le circuit de commande 36 et le signal W vaut 1 lorsqu'il y a eu double modulation ou vaut 0 quand il n'y a pas eu double modulation.When V is zero, N is necessarily zero and the orders M and P then take mutually inverse values to authorize, by their application on N-type transistor gates 43 and 44 respectively, the passage of the simply demodulated signal available on input 30, or the passage of a doubly demodulated signal available on input 45 of the selection circuit 31. The signal W is generated also for example by a control circuit 36 in relation to a band pass filter 46 centered on the frequency f₂. The bandpass filter 46 lets appear the high frequency components which would exist if the signal had been doubly modulated and if consequently after the first demodulation still existed high frequency components. This signal is detected in the same way as in the control circuit 36 and the signal W is worth 1 when there has been double modulation or is worth 0 when there has been no double modulation.

Le système décrit jusqu'ici permet une simple démodulation ou une double démodulation automatique évitant que l'auditeur ait besoin d'intervenir en un quelconque endroit. Il permet la double démodulation ou la simple démodulation même lorsqu'une des modulations est à fréquence de porteuse inconnue. On constate qu'il ne fait pas usage de microprocesseur et que par conséquent sa réalisation est bien moins onéreuse que celle évoquée dans l'état de la technique.The system described so far allows a simple demodulation or a double automatic demodulation avoiding that the listener needs to intervene in any place. It allows double demodulation or simple demodulation even when one of the modulations has an unknown carrier frequency. It can be seen that it does not use a microprocessor and that consequently its production is much less expensive than that mentioned in the state of the art.

Claims (7)

1 - Décodeur pour décoder un son crypté, ce cryptage du son étant effectué par modulation, par ce son (10), d'un signal alternatif oscillant à une fréquence inconnue (f₁) d'une porteuse, ce décodeur comportant un démodulateur (1), ce démodulateur recevant d'une part (2) un signal électrique représentatif du son crypté et d'autre part (3) un signal électrique alternatif dont la fréquence est celle de la porteuse inconnue, ce décodeur délivrant en sortie (5) un signal électrique démodulé représentatif de ce son, caractérisé en ce qu'il comporte un oscillateur (6) commandé par un asservissement (15, 7), cet asservissement comportant un comparateur (7) de phase, pour produire un signal électrique représentatif de la porteuse inconnue, ce comparateur de phase recevant en entrée, d'une part (8) un signal électrique représentatif de la porteuse du son crypté, et d'autre part (9) un signal électrique provenant de la sortie de l'oscillateur et correspondant au signal représentatif de la porteuse inconnue, caractérisé en ce que le comparateur de phase comporte une porte logique OU exclusif en série avec un circuit séquentiel (PLL).1 - Decoder for decoding an encrypted sound, this encryption of the sound being effected by modulation, by this sound (10), of an alternating signal oscillating at an unknown frequency (f₁) of a carrier, this decoder comprising a demodulator (1 ), this demodulator receiving on the one hand (2) an electrical signal representative of the encrypted sound and on the other hand (3) an alternating electrical signal whose frequency is that of the unknown carrier, this decoder delivering at output (5) a demodulated electrical signal representative of this sound, characterized in that it comprises an oscillator (6) controlled by a servo (15, 7), this servo comprising a phase comparator (7), for producing an electrical signal representative of the carrier unknown, this phase comparator receiving as input, on the one hand (8) an electrical signal representative of the carrier of the encrypted sound, and on the other hand (9) an electrical signal coming from the output of the oscillator and corresponding to the s ignal representative of the unknown carrier, characterized in that the phase comparator comprises an exclusive OR logic gate in series with a sequential circuit (PLL). 2 - Décodeur selon la revendication 1, caractérisé en ce que l'oscillateur commandé en fréquence est piloté (27) par un signal à une fréquence fixe et connue, et dont la valeur de la fréquence est du même ordre (m) que la fréquence de la porteuse inconnue.2 - Decoder according to claim 1, characterized in that the frequency controlled oscillator is controlled (27) by a signal at a fixed and known frequency, and whose frequency value is of the same order (m) as the frequency of the unknown carrier. 3 - Décodeur selon la revendication 1 ou la revendication 2, caractérisé en ce qu'il comporte un circuit (14) de mise en forme du signal électrique représentatif du son crypté, interposé entre l' entrée correspondante du comparateur et une borne d'arrivée du signal électrique représentatif du son crypté.3 - Decoder according to claim 1 or claim 2, characterized in that it comprises a circuit (14) for shaping the electrical signal representative of the encrypted sound, interposed between the corresponding input of the comparator and an arrival terminal of the electrical signal representative of the encrypted sound. 4 - Décodeur selon l'une quelconque des revendications 1 à 3, caractérisé en ce que le comparateur de phase est connecté à l'oscillateur commandé en fréquence par l'intermédiaire d'un intégrateur (24-25) dont la constante de temps est au moins deux fois supérieure à la durée maximum de la période du signal alternatif de la porteuse inconnue et en ce que le comparateur de phase possède avant chacune de ses deux entrées un diviseur (n) de signal pour ne comparer les phases que de signaux de fréquences moins élevées que celles de ceux qui sont introduits sur les entrées de ces diviseurs.4 - Decoder according to any one of claims 1 to 3, characterized in that the phase comparator is connected to the frequency controlled oscillator via an integrator (24-25) whose time constant is at least twice the maximum duration of the AC signal period of the unknown carrier and in that the phase comparator has before each of its two inputs a signal divider (n) to compare the phases only of signal frequencies lower than those of those introduced on the inputs of these dividers. 5 - Décodeur selon l'une quelconque des revendications 1 à 4, caractérisé en ce que le comparateur de phase comporte une porte logique OU exclusif en série avec un circuit séquentiel (PLL).5 - Decoder according to any one of claims 1 to 4, characterized in that the phase comparator comprises an exclusive OR logic gate in series with a sequential circuit (PLL). 6 - Décodeur selon l'une quelconque des revendications 1 à 5, caractérisé en ce qu'il comporte un circuit de sélection (31) relié à deux circuits (35, 36) de commande pour orienter des sorties de ce décodeur vers un circuit de diffusion du son, ces circuits de commande comportant des moyens pour délivrer des signaux (V, W) de commande correspondant au caractéristiques de cryptage du son crypté.6 - Decoder according to any one of claims 1 to 5, characterized in that it comprises a selection circuit (31) connected to two control circuits (35, 36) for directing outputs of this decoder towards a circuit of sound broadcasting, these control circuits comprising means for delivering control signals (V, W) corresponding to the encryption characteristics of the encrypted sound. 7 - Décodeur selon l'une quelconque des revendications 1 à 6, dans lequel le son étant de plus crypté, d'une manière complexe, par une double modulation caractérisé en ce que la valeur de la deuxième fréquence (82) de modulation est égale à la valeur de la fréquence du signal à une fréquence fixe et connue qui pilote (28) l'oscillateur.7 - Decoder according to any one of Claims 1 to 6, in which the sound is moreover encrypted, in a complex manner, by a double modulation characterized in that the value of the second modulation frequency (82) is equal at the value of the frequency of the signal at a fixed and known frequency which controls (28) the oscillator.
EP90402682A 1989-10-02 1990-09-28 Decoder for decoding an encrypted television audio signal Expired - Lifetime EP0424201B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR8912844A FR2652698A1 (en) 1989-10-02 1989-10-02 DECODER FOR DECODING A CRYPT SOUND.
FR8912844 1989-10-02

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EP0424201A1 true EP0424201A1 (en) 1991-04-24
EP0424201B1 EP0424201B1 (en) 1992-08-12

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EP (1) EP0424201B1 (en)
JP (1) JPH03158033A (en)
DE (1) DE69000257T2 (en)
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EP0424201B1 (en) 1992-08-12
JPH03158033A (en) 1991-07-08
US5144666A (en) 1992-09-01
FR2652698A1 (en) 1991-04-05
DE69000257D1 (en) 1992-09-17
DE69000257T2 (en) 1993-04-08

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