EP0276332A1 - Method and device to decode a signal code - Google Patents

Abstract

After sampling and conversion into digital values, the digitized time samples of the received signal code (SC) are subjected to a fast Fourier transformation and the information (FFT) representing the lines of the transform are processed automatically so as to compare the ratios of the lines at their theoretical values stored for each possible signal-code in order to determine a set of information called "bets" (M1-Mn) among which a selector (12) then selects a particular "bet" (MSO) which will identify so reliable the received signal-code and will activate a display device (15)

Description

The present invention relates to a method and a device for decoding a code signal produced by modulation of a carrier current at a predetermined rate and for recognizing this signal code among several possible signals with a low probability of error.

A typical example of a signal code concerned by the invention is the signal produced by a coded track circuit used in a network of railways to signal to the driver of a train the speed limit authorized for the convoy at the place where is the train. In this application, the signal code is produced by a carrier current modulated in amplitude at a determined rate. Each modulation rate is associated with a determined speed limit.

The modulation rates are for example 75, 96, 120, 147, 180 and 220 pulses per minute with tolerance of plus or minus two pulses per minute on a carrier current having a frequency of 75 ± 3 hertz. There are then six code signals which can for example be associated with the following speed limits:
220 pulses per minute: maximum 60 km / h
180 pulses per minute: 80 km / h maximum
147 pulses per minute: 120 km / h maximum
120 pulses per minute: 130 km / h maximum
96 pulses per minute: 140 km / h maximum
75 pulses per minute: automatic triggering.

The code signals which circulate in the track circuits are received by an antenna on board the train and transmitted to the cockpit where a decoder decodes them in order to display the authorized speed limit in clear on the control panel.

The problem posed in this kind of application consists in that each signal-code must be decoded and recognized by the decoder with a very low probability of error despite the irregularities which the received signal-code may present and despite the inevitable presence of spurious signals.

The disturbances which can alter the signal code are divided into four groups according to their origin: change in the modulation rate (discontinuity of the signal code), phase rotation of the carrier current when passing a switch or a section from channel to next, momentary variation of the level of the signal code or momentary phase jump, presence of current flowing in the channel and coming from external sources (traction return currents, circulation current, crosstalk).

The apparatuses known for decoding the code signals of the kind described above use analog demodulation and filtration circuits. However, the precision and stability of decoding ensured by these known devices are variable both according to the operating circumstances and from one device to another, which makes it necessary to periodically carry out maintenance measurements and calibrations of installed equipment. In addition, the complexity and hence the size of these devices increases with the performance achieved in terms of the reliability of the decoding.

The invention relates to a decoding method and device which overcomes the drawbacks of the prior art.

This objective is achieved according to the invention thanks to a decoding method comprising the following steps:
after sampling at an adequate sampling frequency, the amplitudes of the time samples in successive blocks of samples of determined length are converted into digital values;
the digital values are transposed in the frequency domain by means of a fast Fourier transformation so as to produce and store a set of digital data (FFT) representing the frequency lines of the transform;
the relationships between the digital data (FFT) representing the measured frequency lines are compared with theoretical memorized reports in order to generate a set of information (M1-Mn) called "bets" whose values represent the differences between the measured lines and theoretical lines for each signal code;
in the set of memorized "bets", selection, for each signal code, of the bet which has the greatest value and memorization of these particular "bets" (MS₁-MS _N );
in the set of stored particular bets data (MS₁-MS _N ), the one with the greatest value (MSO) is selected and stored;
in response to the selected particular bet (MSO), a signaling message (MSC) is generated which identifies the signal code (SC) received, this signaling message (MSC) being intended to actuate a display device (15) .

This method is implemented in an exemplary device which, according to a second aspect of the invention, is characterized in that it comprises a sampler for sampling the signal-code and producing a series of temporal samples in successive blocks of determined length;
an analog / digital converter for converting the amplitudes of the time samples of each block of samples to digital values;
a storage element for storing said digital values;
an organized transformation element, under the control of a stored program, to subject the stored digital values to a rapid Fourier transformation and produce a set of digital signals (FFT) representing the frequency lines of the transform;
a logical organizational element (8, 9) for comparing, under the direction of a stored program, the digital FFT signals corresponding to each line situated in the range in which the carrier frequency can vary with the digital FFT signals representing the lines corresponding to the harmonics of the modulation frequency in order to produce a set of information representing the position deviations of the measured frequency lines, and organized means for comparing the information representing the position deviations with stored data which represent the theoretical amplitudes of the lines in order to generate a set of information (M1-Mn) called "bets" for each signal-code;
means for selecting from the set of bets (M1-Mn) and memorizing, for each signal code, the particular bet which has the greatest value (MS₁-MS _N );
means for selecting from the "bets" MS1-MSN) corresponding to the different code signals, the bet data having the greatest value (MSO);
a device for generating a signaling message in response to the reception of the selected bet data and for transmitting this message to a display device.

• - la figure 1 est un schéma par blocs linéaire du décodeur selon l'invention,
• - la figure 2 est un organigramme de chaîne illustrant le processus d'analyse selon l'invention,
• - la figure 3 est un diagramme montrant une forme d'onde de signal-code typique, et un décalage exemplaire de blocs d'échantillons temporels dérivés de ce signal-code,
• - la figure 4 est un diagramme montrant un spectre de transformée de Fourier exemplaire,
• - la figure 5 est un diagramme illustrant les performances d'un mode d'exécution exemplaire du décodeur selon l'invention.

The invention will appear more clearly on reading the description which follows, made with reference to the appended drawings in which:

• FIG. 1 is a linear block diagram of the decoder according to the invention,
• FIG. 2 is a chain flow diagram illustrating the analysis process according to the invention,
• FIG. 3 is a diagram showing a typical signal-code waveform, and an exemplary offset of blocks of time samples derived from this signal-code,
• FIG. 4 is a diagram showing an exemplary Fourier transform spectrum,
• - Figure 5 is a diagram illustrating the performance of an exemplary embodiment of the decoder according to the invention.

Referring to Figure 1 we see a linear block diagram of the device according to the invention. In this figure, the elements represented symbolize elements which intervene and cooperate to achieve the same function in the decoding process which will be described, but not necessarily separate material elements. Thus, for example, certain elements in FIG. 1 represent elements for receiving and / or storing signals or data; these elements may, as is usual in the art, consist of zones or cells of storage reserved on the same support.

The device according to the invention is intended for decoding code signals produced by modulation of a carrier current at predetermined distinct rates and for recognizing a signal code among several possible signals. An example of a typical signal-code is shown in Figure 3. It is a signal obtained by amplitude modulation by all or nothing. However, it is understood that the decoding method according to the invention is applicable to other forms of modulation (for example, frequency modulation or phase modulation).

In the device according to the invention, the analog code signal SC, after usual filtering in a filter 1, is received in a sampling device 2 to be sampled at a sampling frequency ECH adequate to satisfy the Nyquist criterion, that is to say a sampling frequency equal to at least twice the highest frequency present in the composite signal. The sampling device is a device known per se. The amplitudes of the time samples are converted into digital values in an analog / digital converter 3.

Blocks of digitized time samples of given length are taken and transmitted successively in a time window 4 so as to favor, in each block, the central samples over the marginal samples, this in order to avoid the appearance of spurious signals due to the discontinuity of the signal at the limits of each time block if the latter does not contain an integer number of alternations of the carrier. The length of a block of time samples is for example 2.44 seconds with a shift of 0.2 seconds from one block to another. In Figure 3 is shown the shift of three blocks of time samples B1, B2, B3 derived from the exemplary signal code SC.

Each temporal sample of a block is multiplied by a coefficient determined by the position of the sample in the block, the multiplication coefficients CPE being advantageously located on a half-sinusoid.

The digital values of the digitized samples of each successive block are received in a storage element 5 so as to be then transposed in the frequency domain by a fast Fourier transformation in a manner known per se. The Fourier transformation means is shown diagrammatically at 6. The Fourier transformation makes it possible to determine the amplitude of each harmonic of a frequency in a composite signal from the form of this signal.

In the case of amplitude modulation by all or nothing of a carrier current of frequency f = ω / 2π, for example, with a modulation amplitude A, the instantaneous value of the composite signal is given by the relation ci- after:
a = (Ao + A1 cos Ω t + A3 cos 3Ω t + ...) sin ωt (1)
in which Ω is the pulse of the modulation signal and A1, A2, A3 ... are the amplitudes of the harmonics of the modulation frequency F = Ω / 2 π.

In the example of a 2.44 second time sample block, the lines of the transform are spaced about 25 pulses per minute. FIG. 4 shows the spectrum of the transform for a modulation rate of 75 pulses per minute on a carrier frequency of 75 Hz. It will be noted that the harmonics of the modulation frequency correspond to certain lines of the transform around the carrier of 75 hertz, which can evolve in a frequency range going from 72 to 78 hertz.

The fast Fourier transformation is carried out automatically in a transformation element 6, known per se, under the direction of a stored program. After transformation, each block of samples is represented by a set of digital signals which define the amplitudes of frequency lines of the transform. In the following, these digital signals will be called "FFT data".

The FFT data are received in a storage cell 7 with a view to then being automatically processed according to the invention in order to check whether the distribution of the lines corresponding to the carrier frequency and to the harmonics of the modulation frequency is close to the theoretical distribution for a given signal-code.

The automatic processing of FFT data in accordance with the present invention is carried out in a logical organizational element represented diagrammatically at 8, under the direction of a stored analysis program. The logical organizational element 8 can of course be combined with the transformation element 6 and the analysis program can be integrated with the FFT transformation program in general digital processing software.

The process for processing the FFT data is illustrated by the chain flow diagram in FIG. 2. The starting state A represents the storage of all the FFT data in the storage cell. 7 of FIG. 1. For each block of time samples, the FFT data corresponding to each of the lines of the frequency range of the carrier is multiplied (function 21) by the FFT data representing the lines corresponding to the harmonics for each signal- coded. In the example of a time sample block length of 2.44 seconds with a carrier that can vary in the range 72 to 78 hertz, the FFT data corresponding to each of the seventeen lines is thus multiplied by the data FFT corresponding to harmonics. At the end of step 21, the processing process has determined a set of information linked to the position deviations of the measured frequency lines, these data being hereinafter called "EFM information". With the seventeen lines covering the frequency range in which the carrier can move and taking into account the odd harmonics 1 to 7, for example, a set of 102 EFM pieces of information is thus determined.

Each EFM information is multiplied (function 22) by a digital data (less than one) called "spectral shape coefficient" (CFS) which penalizes the EFM information in proportion to the difference between the value of this information and its value theoretical, that is to say in proportion to the difference between the amplitude of each measured line and its theoretical amplitude as it would result from the relation (1) mentioned above.

The spectral shape coefficient is determined (function 23), for each measured line, from the stored FFT data (state A) by comparison of this FFT data with the corresponding theoretical stored data deduced from relation (1) and residing in a memory forming part of the logical organizational element 8 of FIG. 1.

At the end of step 22, the logical organizational element 8 d determines a set M1, M2 ... Mn (n = 102 in the example cited above) which represent values which are all the more large that the set of lines to which they relate corresponds well to relation (1) for a given code. This information will be called "bets". Stakes M1-Mn are stored (function 24) in a storage cell 9 (fig. 1). Among this set of bets, a selector 10 (fig. 1) then selects (function 25), for each signal code, the bet which has the greatest value in the frequency range considered. For each block of samples analyzed, this information MS₁-MS _N (N = 6 in the example considered here) is received (function 26) in a storage cell 11 (fig. 1).

It will be observed that, according to the invention, there is a one-to-one correspondence between each of the data MS₁-MS _N and the signal-code from which the analyzed sample block has been extracted. Then, among these stored data, a comparator 12 identifies (function 27) the one with the largest value (MSO) and this value is stored (function 28) in a storage cell 13. This MSO data, which identifies the received SC code signal reliably, validates an individual counter for this signal code. The group of counters is represented as a whole at 14 in FIG. 1. As explained below, in response to the data MSO applied to its validation input, each counter retrograde by one unit and only one of them is then incremented by a value defined automatically by the decoder. The output of the one of the counters 14 which has the highest value delivers in line 100 going to a display device 15, a coded message MSC which identifies the signal-code and which is used to indicate this signal code on the display device 15.

So that it is certain that the signal code identified by the MSO setting selected by the decoding process described in the foregoing, is indeed a stable and undisturbed signal code not momentarily a modulation phase jump or changes in codes reconciled, the output message MSC is delayed until a sufficient number of confirmations is given by the analysis of several blocks of successive time samples.

The duration of this delay is determined here by the increment given to the counter 14 corresponding to the identified signal-code. In the set of "bets" MS₁-MS _N stored in cell 11, a selector 16 selects (function 29) the second largest value MSʺ, and this datum MSʺ is stored (function 30) in a storage cell 17 A comparator 18 determines (function 30) the ratio between the first largest value MSO and the second value MSʺ. The value of this ratio is called "momentary confidence coefficient" CCM. This coefficient is applied to the counter 14 corresponding to the identified signal code and is used to increment this counter.

In the absence of disturbance, the selected counter 14 is incremented in response to the CCM signal. The counter 14 is thus incremented during the analysis of one or more blocks of successive samples and the signaling message MSC can then be transmitted to the display device 15.

On the other hand, when there is a momentary disturbance, after having been incremented as a function of the CCM confidence coefficient, the selected counter 14 is demoted during the analysis of a block of samples ulterior. The decoding process according to the invention thus ensures reliable identification of a code signal among several possible code signals.

The invention even provides very reliable decoding during a code change or in the event of a disturbance. Fig. 5 illustrates for example the reactions of a decoder according to the invention when changing from a code of 96.15 pulses per minute on a carrier at 75 hertz (code 96) to a code of 220.6 pulses per minute (code 220) . code 96 is maintained until time t = 3.75 seconds. During all this time, the code 96 is the only one to have a significant value (horizontal line at 100% level). The confidence coefficient is very high. At time t = 3.75 seconds, code 96 is replaced by code 220 until time t = 4.3 seconds where there is a momentary return to code 96 until time t = 4.9 seconds. At this moment, there is a definitive change to code 220. We observe that the decoder perfectly filters this "hiccup" between times t = 4.3 seconds and t = 4.9 seconds since the output of the decoder (indicated near the 100% level line) shows a clear transition between the two codes: first the "block" value at time t = 6.05 seconds, then the filtered output at time t = 6.85 seconds. Similar transitions have been observed in other cases.

In the foregoing, the decoding processing was carried out on the odd harmonics of the modulation only, hence supposing a symmetry of the signal-code. If the received signal-code has a significant asymmetry between the activation time to and the activation time T of the carrier, certain values of the duty cycle to / T may cause the cancellation of one of the harmonics used in the determination of the "bet" and the appearance of even harmonics. In the case where the signal code transmitted is likely to have a duty cycle of less than 0.45 or greater than 0.55, it will be useful to also provide for a spectral analysis as described above but based on even harmonics.

In applications where it would be necessary to improve the rejection of crosstalk in railway tracks, it may be advantageous to provide for a sampling of the signal-code synchronously in each rail: we then have two blocks information in which the components of the useful signal are in phase opposition while the crosstalk signals are frequently in phase there. Depending on the particular case, we can:
- either process one of the two blocks of information as described above and check in the other block that the spectral lines which constitute the selected code are found in phase opposition,
- either first select the lines in phase opposition in the two information blocks and apply only the processing according to the invention to these lines,
- Either subtract the two blocks of information from each other and apply the processing method as described above to the resulting block.

Claims (10)

1. Method for decoding a signal code (SC) produced by modulation of a carrier current at a predetermined rate and for recognizing this signal code among several possible signals, characterized by the following steps:
after sampling at an adequate sampling frequency, the amplitudes of the time samples in successive blocks of samples of determined length are converted into digital values,
the digital values are transposed in the frequency domain by means of a fast Fourier transformation so as to produce and store a set of digital data (FFT) representing the frequency lines of the transform;
the relationships between the digital data (FFT) representing the measured frequency lines are compared with theoretical memorized reports in order to generate a set of information (M1-Mn) called "bets" whose values represent the differences between the measured lines and theoretical lines for each signal code;
in the set of memorized "bets", selection, for each signal code, of the bet which has the greatest value and memorization of these particular bets (MS₁-MS _N ), is found and memorized the one which has the greatest value (MSO);
in response to the selected particular bet (MSO), a signaling message (MSC) is generated which identifies the signal code (SC) received, this message signaling (MSC) being intended to actuate a display device (15). 2. Method according to claim 1, characterized in that each of the digital signals (FFT) corresponding to a frequency situated in the range in which the carrier frequency can vary is compared to the digital signals (FFT) corresponding to the harmonics of the modulation frequency in order to produce a set of information (EFM) representing the position deviations of the measured frequency lines; and
in that the information (EFM) representing the line position differences are compared with stored data representing the theoretical amplitudes of the lines in order to generate all of the "bets" (M1-Mn). 3. Method according to claim 1 or 2, characterized in that the blocks of digitized time samples are transmitted in a time window before being subjected to the fast Fourier transformation. 4. Method according to any one of claims 1 to 3, characterized in that the signaling message (MSC) is transmitted when it is maintained after receipt of the selected "bet" (MSO) at the end of the analysis several blocks of successive time samples (B1, B2 ...). 5. Method according to claim 4, characterized in that the blocks of successive time samples (B1, B2 ...) overlap. 6. Device for decoding a signal code produced by modulation of a carrier current at a predetermined rate and for recognizing this signal code among several possible signals, characterized in that it comprises a sampler (2) for sampling the signal code (SC) and producing a series of samples temporal in successive blocks (B1, B2 ...) of determined length;
an analog / digital converter (3) for converting the amplitudes of the time samples of each block of samples to digital values;
a storage element (5) for storing said digital values;
a transformation element (6) organized, under the control of a stored program, to subject the stored digital values to a fast Fourier transformation and produce a set of digital signals (FFT) representing the frequency lines of the transform;
a logical organizational element (8, 9) for comparing, under the direction of a stored program, the relationships between the digital signals (FFT) representing the measured frequency lines with the stored theoretical relationships in order to generate and store a set of information (M1-Mn) called "bets" representing the differences between the measured ratios and the theoretical ratios, for each signal-code;
means (10, 11) for selecting from the set of bets (M1-Mn), for each signal code, the bet which has the greatest value and for storing these particular bets (MS₁-MS _N );
means (12, 13) for selecting from the "bets" (MS₁-M _N ) corresponding to the different code signals, and for storing the one which has the greatest value (MSO);
a device (14) for generating a signaling message (MSC) in response to the reception of the selected bet data (MSO) and for transmitting this message (MSC) to a display device (15). 7. Device according to claim 6, characterized in that the logical organizational element (8, 9) for producing and memorizing the set of "bets" (M1-Mn) comprises organized means for comparing the digital signals ( FFT) corresponding to each line located in the range in which the carrier frequency can evolve with digital signals (FFT) representing the lines corresponding to the harmonics of the modulation frequency in order to produce a set of information (EFM) representing the deviations of position of the measured frequency lines, and organized means for comparing the information (EFM) representing the position differences with stored data which represent the theoretical amplitudes of the lines in order to generate all of the "bets" (M₁-M _N mentioned above. 8. Device according to claim 6 or 7 characterized in that it comprises a device for delaying the transmission of the signaling message (MSC) to the display device until it has been maintained for a period of time after receipt of at least two blocks of successive time samples. 9. Device according to claim 8, characterized in that the delay in the transmission of the signaling message (MSC) is determined by a counting device (14) responding to the selected particular "stake" (MSO) following the of each block time samples (B1, B2, B3 ...), this device being incremented by a value determined as a function of the ratio between the selected particular bet (MSO) and the second "bet" of greatest value (MSʺ) in the set of particular "bets" (MS₁-MS _N ) on each analysis of a block of successive samples until the output of the counting device produces the signaling message. 10. Device according to claim 9, characterized in that the counting device (14) comprises a separate counter by signal-code to be decoded, the outputs of these counters forming a code message.

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