EP1908197A1 - Procede et dispositif de reception d'un signal numerique transmis au voisinage d'un signal analogique - Google Patents
Procede et dispositif de reception d'un signal numerique transmis au voisinage d'un signal analogiqueInfo
- Publication number
- EP1908197A1 EP1908197A1 EP06777524A EP06777524A EP1908197A1 EP 1908197 A1 EP1908197 A1 EP 1908197A1 EP 06777524 A EP06777524 A EP 06777524A EP 06777524 A EP06777524 A EP 06777524A EP 1908197 A1 EP1908197 A1 EP 1908197A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- signal
- masking
- digital signal
- digital
- components
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04H—BROADCAST COMMUNICATION
- H04H20/00—Arrangements for broadcast or for distribution combined with broadcast
- H04H20/28—Arrangements for simultaneous broadcast of plural pieces of information
- H04H20/36—Arrangements for simultaneous broadcast of plural pieces of information for AM broadcasts
Definitions
- the field of the invention is that of the reception of signals, which may be of audio type, emitted in particular by DAB type systems.
- the invention relates to the reception of digital signals transmitted simultaneously with analog signals in the same propagation channel, and the processing of these digital signals in a receiver.
- the invention thus applies in particular, but not exclusively, to the reception of digital signals coming from a "simulcast" signal, corresponding to the simultaneous broadcasting, from the same transmission point, of a digital signal of the DRM type. , implementing an OFDM type modulation, and an analog signal carrying an audio signal modulating the amplitude of an AM carrier ("Amplitude Modulation" in English, “Amplitude Modulation” in French).
- OFDM modulation consists in distributing in the time / frequency space symbols of duration Tu (called useful symbol time) on a plurality of carrier frequencies modulated independently, for example in QPSK (of the English “Quadrature Phase Shift Keying” for “Quadrature Phase Shift Keying") or QAM (Quadrature Amplitude Modulation for Quadrature Amplitude Modulation) including 16, 64, 256, .. ., states.
- QPSK of the English "Quadrature Phase Shift Keying" for "Quadrature Phase Shift Keying”
- QAM Quadrature Amplitude Modulation
- the OFDM thus cuts the channel into cells along the axes of time 11 and frequencies 12, as illustrated in FIG. 1. Each of the carriers is orthogonal to the previous one.
- the channel is decomposed into a succession of frequency sub-bands 14 and a sequence of time segments 15 (also called time slots).
- Each time / frequency cell is assigned a dedicated carrier. We will therefore distribute the information to be transported on all of these carriers, each modulated at a low rate, for example by a QPSK or QAM type modulation.
- An OFDM symbol comprises all the information carried by all the carriers at a time t.
- This modulation technique is particularly effective in situations where multipaths are encountered.
- FIG. 2 which presents a set of OFDM symbols 21
- the same sequence of symbols arriving at a receiver by two different paths is presented as the same information arriving at two different times and which add up.
- These echoes cause two types of defects: intra-symbol interference: addition of a symbol with itself slightly out of phase;
- intersymbol interference addition of a symbol with the following and / or the preceding slightly out of phase.
- guard interval 22 Between each transmitted symbol, a "dead" zone called guard interval 22 is inserted, the duration ⁇ of which is chosen to be sufficiently large relative to the spreading of the echoes.
- each OFDM symbol 21 includes a guard interval 22 and data 23.
- the carriers may have undergone either an attenuation (destructive echoes), an amplification (constructive echoes) and / or a phase rotation.
- synchronization pilot carriers which are often of a higher amplitude than the useful data carriers
- reference pilots also called reference pilots.
- the value and location of these reference drivers in the time / frequency space are predefined and known to the receivers. After interpolation in time and frequency, we obtain an estimate of the response of the channel, more or less precise as a function of the number of reference pilots and their distribution in the time / frequency domain.
- the reference pilots inserted into the multicarrier signal are used to estimate the propagation channel.
- the estimation of the propagation channel notably makes it possible to correct the received data, also called data pilots, at the receiver (equalization), and to obtain the impulse response of the propagation channel.
- the impulse response obtained can then be used to refine the temporal synchronization of the receiver (s).
- FIG. 3 thus shows the OFDM structure in A mode of a set of DRM symbols, illustrating the distribution of the reference pilots 31 in the time / frequency space. This structure is described in particular in the ETSI ES 201 980 DRM standard.
- Digital broadcasting for example of the DRM type, gradually replaces the analog AM broadcast in the frequency bands below 30 MHz, which requires, during at least one transition period, the broadcasting (possibly simultaneous) of digital and analogue signals carrying a digital signal. similar information from the same site.
- a digital signal and an analog signal are combined in a so-called simulcast signal, the digital signal being modulated on a sideband of a carrier of the propagation channel and a correction signal being modulated on the other sideband.
- the correction signal is determined so that the time envelope of the received signal corresponds to the time envelope of a double band analog signal.
- This technique thus makes it possible to broadcast in a same channel a digital signal and an analog signal, by modifying the analog bet of the simulcast signal so that the overall envelope of this signal is seen as a dual band AM signal for a conventional analog receiver.
- the reception of signals broadcast in the context of such a simulcast broadcast then requires the installation of digital receivers capable of demodulating the received simulcast signal composed of an analog signal and a digital signal carried by the same carrier frequency.
- the receivers used for the OFDM demodulation essentially exploit the response of the channel calculated from the reference pilots, after having separated the digital and analog parts of the received "simulcast" signal.
- the accuracy of this estimate depends on the proportion of reference pilots inserted in the OFDM symbols.
- the quality of the reproduction of the audio signal by the digital receivers is dependent on the number of reference pilots inserted in the OFDM symbols.
- the simulcast signal is obtained, before transmission, by successive iterations, so as to preserve the envelope modulation of the analog signal, that is to say that is, the global simulcast signal is comparable to a dual-band time domain AM signal.
- a receiver demodulating the signal envelope is thus only slightly disturbed by the digital part of the simulcast signal.
- the invention particularly aims to overcome these disadvantages of the prior art.
- an object of the invention is to provide a technique making it possible to optimize the reception of a digital signal during the broadcasting of this digital signal and of an analog signal in the same propagation channel.
- an object of the invention is to provide such a technique for improving the reception performance of a digital signal.
- the invention proposes to determine or recover, in reception, a masking signal, derived from the analog signal carrying an audio signal, taken into account during the equalization step of the received digital signal to improve the quality of the decoding.
- Such a masking signal representative of a psycho-acoustic masking curve of the source audio signal may also have been previously applied before transmission to the digital signal, to mask this digital signal in the analog signal carrying a source audio signal modulating the amplitude of the signal.
- an AM carrier during a simulcast broadcast may also have been previously applied before transmission to the digital signal, to mask this digital signal in the analog signal carrying a source audio signal modulating the amplitude of the signal.
- the invention thus makes it possible to improve the reception of a digital signal modulated by a masking signal in the context of a simulcast broadcast.
- the invention is particularly remarkable in that the digital and analog signals are transmitted in the same propagation channel and are carried by the same carrier frequency AM.
- the invention comprises a demodulation step at the carrier frequency AM delivering a complex signal, and a step of separating the digital signal and the analog signal AM, from the complex signal.
- the simulcast signal is demodulated at the carrier frequency of the analog signal AM at the receiver, to provide a complex signal composed of the digital signal and the analog signal.
- the separation step then makes it possible to process the complex signal to separate the digital signal, for example of the DRM type, from the analog signal AM.
- the obtaining step implements a step of determining the masking signal from the masking curve, and low-pass filtering in the frequency domain and / or in the frequency domain. the time domain.
- Such low-pass filtering makes it possible in particular to smooth the masking curve, and therefore to improve (or at least not to degrade). the channel estimation implemented in reception of the digital signal.
- the masking curve is obtained according to at least one of the following steps: mathematical transformation of the source audio signal from the time domain to the frequency domain;
- the masking curve is obtained by adapting the conventional method to the bandwidth of the audio signal in the AM bands.
- a "classical" psychoacoustic masking curve that can be used is described in particular in the MPEG1 or MPEG2 specifications.
- the obtaining step implements a step of recovering the masking signal transmitted together with the analog signal AM.
- the recovery step notably implements a step of demodulating a quadrature channel with the analog signal carrying the masking signal.
- the invention comprises a step of applying a delay on the digital signal before the equalization, so that the steps of equalization and obtaining the masking signal are synchronized.
- the delay compensation introduced when the masking signal is obtained is applied to the digital signal so that the masking signal and the digital signal are synchronized.
- the extracted digital signal for example of the DRM type, is then equalized by the masking signal, by applying the inverse transfer function of the obtained masking signal.
- the invention is particularly remarkable in that it comprises a step of calculating a confidence for each data element of the digital signal taking into account the masking signal.
- the invention comprises a channel decoding step which takes account of the calculation of trusts.
- the channel decoding step implements multi-level decoding (or MLC).
- MLC multi-level decoding
- the step of calculating a confidence, which corresponds to the power of the channel at each point, is notably implemented to improve the efficiency of the channel decoder.
- the variations introduced at the emission by the masking curve are taken into account in this calculation of the confidences.
- the invention also relates to a device for receiving a digital signal formed of corresponding data elements.
- a device for receiving a digital signal formed of corresponding data elements comprises means for obtaining a masking signal representative of a psycho-acoustic curve of the source audio signal, and means of equalizing the digital signal as a function of the masking signal.
- Such a reception device is particularly adapted to implement the reception method as described above.
- the invention further relates to a computer program which comprises program code instructions for performing the steps of the method of receiving a digital signal according to the invention when this program is executed in or by a microprocessor.
- FIGS. 6A to 6D illustrate, in the frequency domain, the different substeps of the calculation of the masking curve involved in the demodulation of the digital signal;
- FIG. 7 presents the algorithm for receiving a modulated DRM signal according to a masking signal using a block diagram, according to a second embodiment of the invention
- Figure 8 illustrates the simplified structure of a receiving device according to a particular embodiment of the invention.
- the invention therefore relates to the reception of a simulcast signal corresponding to the transmission, in the same propagation channel, of a signal digital and an analog signal.
- the digital signal is modulated before transmission by a masking signal from an analog signal carrying the audio signal.
- a masking signal (which may correspond to that modulating the digital signal before transmission) is obtained in reception, either by calculating it or by recovering it when it is transmitted jointly to the analog signal, and is taken into account. account when equalizing the digital signal, after separation of the analog and digital components of the simulcast signal, to optimize the digital signal processing.
- the signal received by the reception device is a simulcast signal composed of a digital signal and an analog signal carrying the same source audio signal and transmitted in a same propagation channel.
- Figure 4 illustrates the simulcast signal as it was emitted.
- the digital signal 41 modulated in amplitude by a masking signal, and the analog signal 42 are simultaneously broadcast (simulcast mode), and are carried by the same carrier frequency 43, in the AM bands.
- a first sideband 44 of the carrier 43 carries the analog signal
- the other sideband 45 carries the modulated digital signal.
- a first demodulation step 51 consists of demodulating, at the carrier frequency 43 of the analog signal AM, the simulcast signal transmitted at the receiver. This demodulation step provides a complex signal 511 composed of the digital signal and the analog signal.
- the separation step 52 thus notably ensures the extraction of the digital signal 521 from the complex signal obtained 511.
- A.2.3 Determination of masking curve (53) Once the digital signal and the analogue signal are separated, a global masking curve of the analog signal carrying the source audio signal modulating the amplitude of the carrier AM is calculated during a third step 53 of determining a masking signal.
- the bandwidth of the audio signal is 4.5 or 5kHz.
- FIGS. 6A to 6D The various sub-steps of the calculation of the masking curve according to a preferred embodiment are presented in relation to FIGS. 6A to 6D. This calculation is based on an adaptation of the "classical" psycho-acoustic masking curve model from the MPEG1 or MPEG2 specification, to adapt to the bandwidth of the audio signal in the AM bands.
- This step 53 of calculation of the masking curve thus implements the following substeps: - time-frequency conversion by Fourier transform; determination of the absolute mask; highlighting tonal and non-tonal components; decimation of the different components; calculation of individual masks; - calculation of the global mask.
- the analog signal carrying a source audio signal modulating the amplitude of the AM carrier is first transposed into the frequency domain, from a Fourier Transform (FFT) type mathematical transformation. More precisely, the calculation of the masking curve requiring an estimation of the power spectral density for each of the frequencies of the audio signal, is implemented a Fourier transformation at 1024 samples for a sampling frequency of 48 kHz in order to obtain the different frequencies and their power spectral densities. The transform is calculated from the input analog signal to which a Hanning window is applied.
- FFT Fourier Transform
- N is the width of the bandwidth (here N is equal to 9 or 10 kHz), and the index k is the position of the component relative to the center frequency AM of the bandwidth.
- the second sub-step of the calculation of the masking curve of the analog signal requires the determination of an absolute mask, corresponding to the hearing threshold for each of the frequencies corresponding to the components delivered by the Fourier transform.
- the hearing thresholds for the different frequencies denoted LT q (k), come from the MPEGl standard, and are presented in appendix 1.
- the third sub-step implements a discrimination between the tonal components and the non-tonal components.
- the tone of a component has an influence on the contribution of the component in the calculation of the masking curve.
- the separation of the tonal and non-tonal components takes place in several phases: determination of the local maxima, extraction of the tonal components, and calculation of the intensity of the non-tonal components of a critical band.
- the frequency components are analyzed in packets of three, and if the central component has a higher level than the other two, it is considered as a local maximum.
- FIG. 6A illustrates, in particular, the technique of determining local maxima, based on an example of a frequency spectrum 61 of the analog signal.
- the first three frequency components (carriers 1, 2 and 3) of a portion 61 of the spectrum of the analog signal are first analyzed.
- the central component (carrier 2) having a level, that is to say a spectral power density, higher than the other two (carriers 1 and 3), it is considered a local maximum.
- This analysis is then repeated for the other frequency components (carriers 2, 3 and 4, then carriers 3, 4 and 5, ).
- the local maxima correspond to the carriers 2, 5, 7, 13, 15 and 17.
- the tonal components correspond to the grouping of the lines contributing to the same harmonic component.
- a local maximum is considered to belong to the list of tonal components if the following conditions are met: X (k) - X (k + j) ⁇ 7 dB with j such that:
- the width of the analysis band is modified around the local maxima determined according to the position of the maxima in the spectrum, which implies that the value of j varies according to the values of k.
- the sound pressure level X tm (k) is determined by the following relation:
- FIG. 6B illustrates in particular the determination of a tonal component, from the local maxima as determined previously.
- the spectrum components that were used for the determination of the tonal components are set to an infinitely low level: the index components 4, 5, 6, 8, 9 , 10 and 11 disappear.
- the non-tonal components correspond to the grouping of lines that have no connection with harmonic components. As illustrated in FIG. 6C, all spectral density components remaining after calculating the tonal components are used to determine the level of the non-tonal components in a critical band.
- Bc is the critical band considered
- k corresponds to the index of the component closest to the geometric center of the critical band.
- Figure 6D shows the remaining spectrum portion after extracting the tonal and non-tonal components, and determining their sound pressure levels.
- the fourth sub-step of the calculation of the masking curve of the analog signal implements a reduction of the number of components of the individual masks, eliminating certain tonal and non-tonal components of the remaining spectrum according to the following conditions: the tonal and non-tonal components are conserved if their levels (ie their spectral power densities) are greater than the absolute mask (where the absolute mask corresponds to the hearing threshold for the different components, and is presented in annex 1):
- LT lm [z (j) 4i)] X lm [z (j)] + av tMj)] + VfHDM dB
- LT nm [z (j) 40] ⁇ nm Wi)] + av nm [z (j)] + vf [z (j), z (i)] dB
- - LT tm and LT 111n are the individual masks (tonal and non-tonal components) of the masking component z (j) in z (i);
- - X tm and X 111n are the sound pressure levels of the tonal and non-tonal components
- av of the component has a different value depending on whether it is a tonal component or a non-tonal component
- av tm -6.025- 0.275z (j) dB
- av nm -2.025- 0.175 z (j) dB.
- masking from components at values greater than 8 barks or less than -3 barks of the masked component is not used, for reasons of complexity. implementation and irrelevance, these components being too far from the masking component to disrupt its spectral power density.
- LT (i) is equal to the linear sum of the individual masks of the samples situated in the range [8; -3] barks and of the absolute mask:
- LT (i) represents the level of the masking curve for sample i (for the calculation of the overall mask, 72 or 75 samples are used, depending on the audio bandwidth, according to the MPEGl specification).
- the samples do not have the same frequency width as a function of their positions in the spectrum (from 62.5 to 250 Hz according to the MPEG1 specification).
- a masking signal is obtained by filtering 54 in the frequency and / or time domain of the masking curve corresponding to the global mask.
- a conventional "envelope detection” type receiver demodulates the so-called “simulcast” signal (analog signal on a first sideband, and amplitude modulated digital signal by the masking signal on the second sideband). ), and delivers a base signal corresponding to the sum of the modulating signals on each of the two lateral bands of the carrier.
- the frequency masking makes it possible to render inaudible any signal that follows the amplitude variations of the masking curve.
- the time and frequency variations of the masking curve would be too great for the DRM signal, because of its multicarrier structure ( OFDM). It therefore appears necessary to smooth the masking curve.
- the invention thus proposes to limit these variations in time and in frequency by implementing a low-pass filtering of the masking curve in the time and frequency domains delivering the masking signal, according to a preferred embodiment of the invention. in order to improve (or at least not to degrade) the channel estimation implemented in reception of the digital signal. The channel estimate thus follows the evolution of the masking signal.
- A.2.5 Frequency domain transposition (55) In parallel, once the digital signal and the analog signal are separated during the separation step 52, the digital signal 521 is transposed (55) in the frequency domain from a transformation of the Fourier transform type, for example over 1024 points, as described in paragraph A.2.3.
- H k the transfer function related to the propagation channel
- M k the transfer function related to the masking curve applied to the transmission
- Compensation 56 of the delay introduced during the parallel processing of determination of the masking signal 53, 54 of the different functions is then applied to the digital signal DRM, so that this signal is synchronized with the masking signal determined in order to achieve the equalization of the digital signal extracted.
- the digital signal may be equalized during a step 57 as a function of the masking signal determined during the filtering step 54 as described in the preceding paragraph.
- the equalization step 57 of the digital signal received may be in the following mathematical form:
- This step 58 relies, for example, on an analysis, known per se, of the reference pilots of the OFDM signal.
- a step 59 for calculating the trusts C k is implemented. These trusts correspond to the power of the channel at each point, and can be written in the following form:
- the transmitted simulcast signal can itself carry the masking signal.
- the masking signal is transmitted in conjunction with the analog signal 42, which simplifies the reception since it is no longer necessary to calculate the masking curve.
- the masking signal representative of a psycho-acoustic masking curve of the source audio signal modulating the digital signal is transmitted in quadrature with the analog signal carrying the source audio signal.
- the transmitted simulcast signal comprises the digital signal modulated in amplitude by the masking signal, and the analog signal and the masking signal transmitted in quadrature.
- the quadrature signal may carry other information, and not only the masking signal (for example a portion of the digital signal DRM, ancillary information - images, title, enhancement data, especially to improve the quality of the signal , ).
- FIG. 7 illustrates in particular the reception of such a simulcast signal comprising the masking signal.
- step 51, 52 and 55 to 59 identical to those of the first embodiment and already described in relation to FIG. 5, are not detailed further.
- the masking signal is recovered during a step 71 for recovering the masking signal.
- the masking signal transmitted together with the analog signal is recovered from the analog part AM 522 extracted from the complex signal 511, after a transposition in the frequency domain from a Fourier transform type transformation, for example on 1024 points, as described in paragraph A.2.3.
- This recovery step 71 of the masking signal, transmitted jointly to the analog signal AM, is performed at the receiver.
- the invention thus proposes a technique for receiving a digital signal DRM modulated by a masking signal in the context of a simulcast broadcast, based on the main stages of separation of the digital signal and the analog signal, for obtaining a masking signal, representative of a psycho-acoustic masking curve of the source audio signal by calculation of the masking curve or recovery of the masking signal transmitted together with the analog signal.
- the invention thus provides an improvement in the reception of a DRM signal originating from a simulcast broadcast (corresponding to the transmission of a digital signal of the DRM type and of an analog signal carrying an audio signal modulating the amplitude of the signal.
- a DRM signal originating from a simulcast broadcast (corresponding to the transmission of a digital signal of the DRM type and of an analog signal carrying an audio signal modulating the amplitude of the signal.
- an AM carrier in the same propagation channel of 9 or 10 kHz bandwidth, which corresponds to the bandwidth allocated by the ITU (International Telecommunication Union, in French "Union Internationale des Télécommunications") for AM broadcasting in the frequency bands below 30 MHz), by estimating the modulation introduced on transmission.
- the invention is not limited to a purely hardware implementation but that it can also be implemented in the form of a sequence of instructions of a computer program or any form mixing a hardware part and a software part.
- the corresponding instruction sequence can be stored in a removable storage means (such as for example a floppy disk, a CD-ROM or a DVD-ROM) or no, this storage means being partially or completely readable by a computer or a microprocessor.
- Such a reception device comprises a memory 81, a processing unit 82, equipped for example with a microprocessor ⁇ P, and driven by a computer program 83, implementing the reception method according to the invention.
- the code instructions of the computer program 83 are for example loaded into a RAM memory before being executed by the processor of the processing unit 82.
- the processing unit 82 receives as input a signal digital camera formed of data elements and transmitted in the vicinity of an analog signal carrying a source audio signal modulating the amplitude of an AM carrier.
- the microprocessor of the processing unit 82 implements the steps of the reception method described above, according to the instructions of the computer program 83.
- the receiving device comprises means for obtaining a masking signal representative of a psycho-acoustic masking curve of the source audio signal, and means for equalizing the digital signal as a function of the masking signal. These means are controlled by the microprocessor of the processing unit 82.
- the processing unit 82 outputs the equalized digital signal.
- Appendix 1 Table of absolute masks according to the audio frequencies
- Appendix 2 Table of Central Frequencies of the Critical Bands Established by E.
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- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Circuits Of Receivers In General (AREA)
Abstract
Description
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Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FR0507857A FR2889011B1 (fr) | 2005-07-22 | 2005-07-22 | Procede et dispositif de reception d'un signal numerique transmis au voisinage d'un signal analogique |
PCT/EP2006/063699 WO2007009864A1 (fr) | 2005-07-22 | 2006-06-29 | Procede et dispositif de reception d'un signal numerique transmis au voisinage d'un signal analogique |
Publications (1)
Publication Number | Publication Date |
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EP1908197A1 true EP1908197A1 (fr) | 2008-04-09 |
Family
ID=36128375
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
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EP06777524A Withdrawn EP1908197A1 (fr) | 2005-07-22 | 2006-06-29 | Procede et dispositif de reception d'un signal numerique transmis au voisinage d'un signal analogique |
Country Status (3)
Country | Link |
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EP (1) | EP1908197A1 (fr) |
FR (1) | FR2889011B1 (fr) |
WO (1) | WO2007009864A1 (fr) |
Family Cites Families (3)
Publication number | Priority date | Publication date | Assignee | Title |
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US5301363A (en) * | 1992-06-29 | 1994-04-05 | Corporate Computer Systems, Inc. | Method and apparatus for adaptive power adjustment of mixed modulation radio transmission |
US6144705A (en) * | 1996-08-22 | 2000-11-07 | Lucent Technologies Inc. | Technique for simultaneous communications of analog frequency-modulated and digitally modulated signals using precanceling scheme |
EP1370017A1 (fr) * | 2002-06-07 | 2003-12-10 | Sony International (Europe) GmbH | Réduction d'interférence pour signaux simulcast |
-
2005
- 2005-07-22 FR FR0507857A patent/FR2889011B1/fr active Active
-
2006
- 2006-06-29 WO PCT/EP2006/063699 patent/WO2007009864A1/fr not_active Application Discontinuation
- 2006-06-29 EP EP06777524A patent/EP1908197A1/fr not_active Withdrawn
Non-Patent Citations (1)
Title |
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See references of WO2007009864A1 * |
Also Published As
Publication number | Publication date |
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FR2889011B1 (fr) | 2007-09-14 |
FR2889011A1 (fr) | 2007-01-26 |
WO2007009864A1 (fr) | 2007-01-25 |
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