Method and device for demodulating modulated data by amplitude modulation of a light signal emitted by a light source of a transmitting device

Abstract

Method and device for demodulating modulated data by amplitude modulation of a light signal emitted by a light source of a transmitting device This demodulation device acquires a digital image comprising an area illuminated by a light signal emitted by a source.It is configured to: - calculate (24) and store an average value per column of said digital image, in association with a column index, rows between a first and a second edge index of the image; - determine (26) a maximum value of said average values, and calculate a threshold from said maximum value, - determine (28) a first column index corresponding to the first average value greater than or equal to said threshold starting from the first edge index, and a second column index corresponding to the first average value greater than or equal to said threshold starting from the second edge index; - demodulate (30, 32) binary data from the average values ​​between the first column index and the second column index. Figure for the abstract: Figure 2.

Description

Title of the invention: Method and device for demodulating modulated data by amplitude modulation of a light signal emitted by a light source of a transmitting device

[0001] The invention relates to a method for demodulating modulated data by amplitude modulation of a light signal emitted by a light source of a transmitting device. It also relates to an associated demodulation device and an associated electronic apparatus.

[0002] The invention lies in the field of wireless communication technology using visible light, using a VLC (for “Visible Light Communication”) system.

[0003] It applies in particular in the context of data communication using an encoder / transmitter implemented on industrial products comprising on the front a light-emitting diode (LED) indicator light, e.g. a screen or one or more indicator lights, intended to provide indications on a state of the product, the data being decodable by a VLC receiver / decoder, integrated for example in a portable device, for example a mobile telephone.

[0004] For example, the invention applies to products for monitoring and protecting electrical systems, but applies more generally to any type of product comprising a light indicator on the front.

[0005] The use of wireless communication technology using visible light, also called VLC, has developed recently and has found many applications.

[0006] A visible light communication system VLC comprises an encoder / transmitter device and a receiver / decoder device, positioned substantially opposite each other. The encoder / transmitter device comprises a light source, for example one or more light-emitting diode or LED lamps (for "Light-Emitting Diode", and the receiver / decoder device comprises an image capture device, e.g. a CMOS camera.

[0007] The light source emits a light signal which is amplitude modulated as a function of data to be transmitted, the data being coded into symbols by a coding method, each symbol being representative of a bit to be modulated. The symbols are then encapsulated in formatted transmission packets, to form a binary train comprising a predetermined synchronization word (or synchronization sequence), followed by a formatted transmission packet comprising a header, useful data and an error detection code. This binary train is transformed into an electrical signal that controls activation or deactivation of the light source, at a frequency chosen so that the flickering caused is imperceptible to the human eye. For example, when the light source is on (high state of the corresponding light signal), a binary '1' is transmitted, and when the light source is off (low state of the corresponding light signal), a binary '0' is transmitted. Another correspondence between high and low states and transmitted bits could be implemented, without changing the principle of the method.

[0008] The receiver / decoder device applies digital image processing to the images acquired by the image capture apparatus to perform demodulation of the modulated data, then decoding to obtain decoded data.

[0009] The acquired digital images comprise matrices of pixels whose values ​​are representative, in an area illuminated by the light source, of the high or low state of the light source, or of a transition between these states. Thus, an acquired digital image comprises an area comprising light and dark fringes, corresponding respectively to the high and low states of the light signal.

[0010] The known systems are particularly suitable for VLC communication with transmitters comprising powerful light sources with a large surface area.

[0011] In the context of an application such as mentioned above, the products incorporating light indicators or screens mentioned are relatively small in size, and the light sources are also small in size, for example between 2mm and 6cm. When considering the transmission of data by VLC communication from such a product, for reading the data by a portable electronic device, for example a mobile phone of the smart phone type (or "smartphone"), an electronic tablet or a laptop, the area of ​​the acquired image representative of the light source occupies a variable surface, and decreasing with the distance between the image sensor and the product considered.

[0012] [Fig.l] illustrates as an example images of a 2.5mm diameter LED type light source captured at different distances, ranging from 2.5mm to 5cm, and it can be observed that in the image captured at 2.5mm vertical fringes are observed corresponding to the high and low states of the light source corresponding to the modulation values, and that the more the distance between the image sensor and the light source increases, the more the corresponding area decreases in the captured image, being reduced to an almost point-like area on a dark, potentially noisy image background. Thus, the amount of background noise increases with distance, while the light intensity of the corresponding area decreases with distance, which risks introducing errors in the demodulation and decoding of the data.

[0013] There is therefore a need to improve the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source.

[0014] To this end, the invention proposes a method for demodulating modulated data by amplitude modulation of a light signal emitted by a light source of a transmitting device, the method comprising an acquisition of a digital image comprising an area illuminated by said light signal, said area comprising fringes corresponding to high and low states of the light signal. This method further comprises steps of:

[0015] -calculation of an average value per column of said digital image, and storage of the average values ​​per column in association with a column index, arranged between a first edge index corresponding to a first edge of said image and a second edge index corresponding to a second edge of said image;

[0016] -determination of a maximum value of said average values, and calculation of a threshold from said maximum value,

[0017] - determination of a first column index corresponding to the first value average greater than or equal to said threshold starting from the first edge index, and a second column index corresponding to the first average value greater than or equal to said threshold starting from the second edge index;

[0018] - extraction of a subset of average values ​​between the first column index and the second column index;

[0019] -demodulation of binary data from said subset of average values.

[0020] Advantageously, the proposed method allows dynamic adaptation, without prior knowledge, to extract and process an image area corresponding to the light source. Thus, advantageously, this makes it possible to improve demodulation and decoding while having a margin of positioning of the image sensor of the receiving device relative to the light source.

[0021] The method for demodulating data modulated by amplitude modulation of a light signal according to the invention may have one or more of the characteristics below, taken independently or in all acceptable combinations.

[0022] The threshold is a percentage of said maximum value, preferably between 30% and 50%.

[0023] The threshold is equal to 40% of said maximum value.

[0024] In the storage step, the average values ​​per column are stored in a table or in a list ordered according to the column indices.

[0025] The binary data demodulation step comprises a calculation of a thresholding curve by a segmentation method.

[0026] According to another aspect, the invention relates to a device for demodulating data modulated by amplitude modulation of a light signal modulated by mo amplitude modulation of a light signal emitted by a light source of a transmitting device, the demodulation device being configured to acquire a digital image comprising an area illuminated by said light signal, said area comprising fringes corresponding to high and low states of the light signal. This device further comprises modules configured to:

[0027] -calculate an average value per column of said digital image, and store average values ​​per column in association with a column index, arranged between a first edge index corresponding to a first edge of said image and a second edge index corresponding to a second edge of said image;

[0028] -determine a maximum value of said average values, and calculate a threshold from said maximum value,

[0029] - determine a first column index corresponding to the first value average greater than or equal to said threshold starting from the first edge index, and a second column index corresponding to the first average value greater than or equal to said threshold starting from the second edge index;

[0030] -extract a subset of average values ​​between the first column index and the second column index;

[0031] -demodulate binary data from said subset of average values.

[0032] In one embodiment, the threshold is a percentage of said maximum value, preferably between 30% and 50%.

[0033] According to another aspect, the invention relates to an electronic apparatus comprising an image sensor and a demodulation device as briefly described above.

[0034] According to a particular characteristic, the electronic device is a mobile phone or an electronic tablet.

[0035] Other characteristics and advantages of the invention will emerge from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which:

[0036] [Fig-1] [Fig.l], already described, illustrates a plurality of images from the same source light captured at increasing distances;

[0037] [Fig.2] [Fig.2] is a schematic representation of a transmitter and a receiver of a VLC communication system;

[0038] [Fig.3] [Fig.3] is a synopsis of the main stages of a demo process dulation according to the invention;

[0039] [Fig.4] [Fig.4] illustrates examples of one-dimensional signals corresponding to average values ​​per column of a digital image acquired by a receiver.

[0040] [Fig.2] illustrates a VLC communication system 2, comprising a transmitter device 4 in unidirectional communication with a receiver device 6.

[0041] The transmitter device 4, also simply called transmitter, is configured to encoding, modulating and transmitting digital data D using amplitude modulation of a light signal emitted by a light source 8, for example formed of one or more lamps, adapted to emit waves of wavelength in the visible spectrum, the wavelength being between 380nm and 780nm.

[0042] The transmitter 4 comprises a coding module 10 and a modulation module 12, which controls the amplitude of a light signal emitted by the light source 8.

[0043] The coding module 10 implements for example a coding which consists of transforming bits into code words, also called symbols. In the VLC 2 system, the coding module 10 implements Manchester coding, and formatting in the form of transmission packets formatted according to a chosen protocol.

[0044] Manchester encoding, according to the IEEE 802.3 standard, consists of encoding a “1” by “01” and a “0” by “10”.

[0045] The symbols are then encapsulated in formatted transmission packets, to form a bit stream comprising a predetermined synchronization word, followed by a formatted transmission packet comprising a header, useful data and an error detection code.

[0046] For example, when Manchester encoding is used, the synchronization word is the pattern '1111' because by definition, this pattern is not part of the Manchester code. In other words, no series of symbols in the Manchester code forms a sequence of four '1's.

[0047] The size of the packets varies, depending on the intended application. It is indicated in the packet header.

[0048] The error detection code is for example a cyclic redundancy check code such as CRC8 or CRC 16.

[0049] The modulation module 12 implements an “all or nothing” type modulation or OOK (from the English “On-Off Keying”) modulation. In this type of modulation, the light signal emitted by the light source is in the high state (i.e. light source on) to transmit a binary '1' or in the low state (i.e. light source off) to transmit a '0', with a frequency high enough to prevent the flickering from being visible to the human eye.

[0050] In one embodiment, the coding module 10 and the modulation module 12 are implemented by a calculation processor 15. For example, the processor implemented by the transmitter device 4.

[0051] As a variant, each of the coding 10 and modulation 12 modules is a dedicated module produced in the form of a programmable logic component such as an FPGA (Field Programmable Gate Array) or in the form of a dedicated integrated circuit, such as an ASIC (Application Specific Integrated Circuit).

[0052] For example, the emitting device 4 is integrated into an industrial product and the light source 8 comprises one or more LED lamps, forming a light indicator initially intended for signaling an operating state of the industrial product. The light source is preferably integrated so as to illuminate a portion of a face of the industrial product, visible to a user when the industrial product is in the operating position, for example when the industrial product is fixed on a support, the illuminated portion is located on the front face of the product.

[0053] The modulation of the emitted light signal is indistinguishable to the naked eye. Advantageously, the modulation of the light signal to transmit additional digital data does not disturb the initial function of the light indicator of the industrial product.

[0054] For example, the digital data D includes additional information relating to the industrial product, for example a unique identifier of the product, an IP address (Internet Protocol) or a URL (acronym for "Uniform Resource Locator" or web address), a key or a code, the state of the registers, a BLE pairing password (acronym for "Bluetooth Low Energy") or dynamic keys for commissioning Zigbee. More generally, the digital data D includes information on commissioning or pairing wireless communication, measurements carried out by the product, the state of the product. This makes it possible, for example, to facilitate the installation and commissioning, or the maintenance of a product by a user. This also makes it possible to improve the cybersecurity of a product.

[0055] The modulated light signal is emitted by the light source 8.

[0056] The receiving device 6, also simply called receiver, comprises an image sensor 20, for example an optical camera with CMOS sensors (for “Complementary metal-oxide-semiconductor”), adapted to capture light signals in the form of a digital image, a digital image being composed of one or more pixel matrices, each pixel of a pixel matrix having an associated digital value.

[0057] Depending on calibration parameters of the image sensor 20, in particular the sensitivity (expressed in ISO or in gain in dB), the exposure time and the acquisition time of an image (or frame), the acquired digital images comprise values ​​representative of the high or low state, or of a transition between these states in an image zone illuminated by the light source. Thus, an acquired digital image comprises an area comprising light and dark fringes, corresponding respectively to the high and low states of the light signal.

[0058] The acquired digital image is transmitted to a demodulation device 22, and the result of the demodulation device 22 is transmitted to a decoding module 23 to obtain as output a set of decoded digital data D*.

[0059] In the absence of loss or error, the decoded digital data D* is identical to the digital data D.

[0060] In one embodiment, the demodulation device 22 and the decoding module 23 are implemented by a calculation processor 25.

[0061] As a variant, each of the demodulation 22 and decoding 23 devices is a dedicated module produced in the form of a programmable logic component such as an FPGA (Field Programmable Gate Array) or in the form of a dedicated integrated circuit, such as an ASIC (Application Specific Integrated Circuit).

[0062] In one embodiment, the receiving device 6 is a portable electronic device, such as a mobile phone or smartphone, an electronic tablet, a laptop, or any other portable electronic device equipped with a camera and an electronic computing device.

[0063] When the receiving device 6 is placed by a user so that the image sensor 20 of the receiving device is placed substantially opposite the light source 8, at a distance from the light source 8 chosen by the user and for example between 0 cm (i.e. stuck) and 6 cm, and the receiving device 6 is put in a VLC reception mode, the or each captured image comprises, in an area illuminated by the light signal emitted by the light source, fringes, arranged vertically, representative of the high or low states of the emitted signal.

[0064] Thus, a wireless transmission of data is carried out between the transmitter 4 and the receiver 6, using VLC communication.

[0065] The demodulation device 22 is an electronic calculation device implementing the method of demodulating data modulated by amplitude modulation of a light signal emitted by the light source 8 of the emitting device 4.

[0066] In one embodiment, the demodulation device 22 comprises:

[0067] -a module 24 configured to calculate an average value per column of said digital image, and store average values ​​per column in association with a column index, arranged between a first edge index corresponding to a first edge of said image and a second edge index corresponding to a second edge of said image;

[0068] -a module 26 configured to determine a maximum value of said average values, and calculate a threshold from said maximum value,

[0069] - a module 28 configured to determine a first column index cor corresponding to the first average value greater than or equal to said threshold starting from the first edge index, and a second column index corresponding to the first average value greater than or equal to said threshold starting from the second edge index;

[0070] - a module 30 configured to extract a subset of average values ​​between the first column index and the second column index;

[0071] -a module 32 configured to demodulate binary data from said subset of average values.

[0072] Preferably, the modules 24, 26, 28, 30, 32 are produced in the form of software instructions forming a computer program which, when executed by a calculation processor 25, execute the method of demodulating modulated data as described below. This software code is stored in an electronic memory of the receiving device 6.

[0073] When the method is carried out in the form of one or more software programs, that is to say in the form of a computer program, also called a computer program product, it is furthermore capable of being recorded on a medium, not shown, readable by a computer. The computer-readable medium is for example a medium capable of storing electronic instructions and of being coupled to a bus of a computer system. By way of example, the readable medium is an optical disk, a magneto-optical disk, a ROM memory, a RAM memory, any type of non-volatile memory (for example FLASH or NVRAM) or a magnetic card. A computer program comprising software instructions is then stored on the readable medium.

[0074] As a variant, each of the modules 24, 26, 28, 30, 32 is a dedicated module produced in the form of a programmable logic component such as an FPGA (Field Programmable Gate Array) or in the form of a dedicated integrated circuit, such as an ASIC (Application Specific Integrated Circuit).

[0075] [Fig. 3] is a block diagram of the main steps of an embodiment of a method for demodulating modulated data by amplitude modulation of a light signal emitted by a light source of a transmitting device.

[0076] The method comprises a step 40 of acquiring a digital image, comprising an area illuminated by the light source, the area comprising fringes corresponding to high and low states of the light signal, and fringes corresponding to a transition between these states.

[0077] The acquired digital image is a grayscale image, corresponding to the luminance of the light signal. It is formed from a matrix of pixels, each pixel having a value representative of an associated grayscale level. For example, each pixel takes a value coded on 8 or 16 bits.

[0078] For example, when the luminance is coded on 256 gray levels, the light fringes correspond to a high state of the light signal, and the dark fringes correspond to a low state of the light signal, and the gray fringes correspond to the transitions between these states.

[0079] The method then comprises a step 42 of calculating an average value by column of the digital image, and storage of the average values ​​calculated in association with a column index.

[0080] For example, the calculated average values ​​are stored in a table or in an ordered list of values, the list being ordered according to the column indices. The calculated average values ​​form a one-dimensional SI signal (or 1D signal) representative of the columns of the digital image.

[0081] The signal SI thus formed is illustrated in graph G1 of [Fig.4], which represents column index values ​​on the abscissa and calculated average values ​​on the ordinate.

[0082] For a digital image comprising N columns, the column indices are integer indices between a first edge index, for example i=0, corresponding to a first edge of the acquired digital image and a second edge index, for example i=Nl, corresponding to a second edge of the acquired digital image. In other words, the first edge is a first vertical edge of the digital image, and the second edge is the second vertical edge, parallel to the first edge.

[0083] The method then comprises a step 44 of determining the maximum value V max of the calculated and stored average values, and a step 46 of calculating a threshold Th from the maximum value Vmax.

[0084] Preferably, the threshold Th is equal to a percentage of Vmax, for example 40% of V max*

[0085] More generally, the threshold Th is equal to a percentage P% of Vmax between 30% and 50%.

[0086] The threshold Th is also stored.

[0087] The method then comprises a step 48 of determining a first column index, cb, corresponding to the first average value greater than or equal to the threshold Th, starting from the first edge index and scanning the column indices in the increasing direction, and of determining a second column index, c2, corresponding to the first average value greater than or equal to the threshold Th, starting from the second edge index and scanning the column indices in the decreasing direction.

[0088] In other words, the second index corresponds to the last average value greater than or equal to the threshold Th of the stored 1D signal, by scanning the indices in increasing direction.

[0089] Thus, all the values ​​of the samples of the signal S1 (mean value signal) between 0 (first edge index) and crl and all the values ​​of the signal SI between c2+l and Nl (second edge index) are lower than the threshold Th.

[0090] A subset of average values, forming a sub-signal S2, is then extracted in the extraction step 50, the sub-signal S2 being formed from the average values ​​included between the first column index Ci and the second column index c2.

[0091] The sub-signal S2 is represented in the graph G2 of [Fig.4].

[0092] Advantageously, the sub-signal S2 corresponds to the area actually illuminated by the light source.

[0093] Advantageously, the method makes it possible to dynamically adapt the size of the sub-signal S2 without prior knowledge of the size (e.g. the diameter) of the light source and the distance between the image sensor and the light source. Thus, the parasitic, potentially noisy values ​​corresponding to the image background (unlit) are automatically eliminated.

[0094] The method then comprises a demodulation of binary data from the subset of average values ​​extracted in step 50.

[0095] In one embodiment, the demodulation is carried out in two steps, respectively a step 52 of calculating a thresholding curve by a segmentation method, and a step 54 of extracting binary data by applying the thresholding curve to each column of the digital image.

[0096] In one embodiment, the segmentation method for calculating the thresholding curve is a polynomial curve fitting approximation.

[0097] The calculated threshold curve Cth is illustrated on graph G2 of [Fig.4].

[0098] For each column index, if the average value is above the cor point corresponding to the thresholding curve, a binary value equal to 1 is extracted, and if the average value lies below the point of the thresholding curve, a binary value equal to 0 is extracted.

[0099] The invention has been described above in the context of a vertical orientation of the fringes observed in the digital image. This orientation is linked to the direction of observation of the captured digital image. It is clear that it is easy for a person skilled in the art to transpose the method described for an application for digital images comprising horizontal fringes.

[0100] Advantageously, the method makes it possible to extract binary data from an illuminated area of ​​the digital image, by dynamic adaptation, and therefore the image background, potentially noisy, is not used in the demodulation step. The precision of the demodulation is thus increased.

[0101] Advantageously, the modulation of the light signal to transmit additional digital data does not disturb the initial function of the light indicator of the industrial product. Thus, it is possible to integrate the proposed functionality without requiring significant structural modifications to existing products, which makes it possible to limit costs.

Claims

Claims

1. Method for demodulating modulated data by amplitude modulation of a light signal emitted by a light source (8) of a transmitting device, the method comprising an acquisition (40) of a digital image comprising an area illuminated by said light signal, said area comprising fringes corresponding to high and low states of the light signal, the method being characterized in that it further comprises steps of: -calculation (42) of an average value per column of said digital image, and storage of the average values ​​per column in association with a column index, arranged between a first edge index corresponding to a first edge of said image and a second edge index corresponding to a second edge of said image;-determination (44) of a maximum value (Vmax) of said average values, and calculation (46) of a threshold from said maximum value, - determination (48) of a first column index (ci) corresponding to the first average value greater than or equal to said threshold starting from the first edge index, and of a second column index (c2) corresponding to the first average value greater than or equal to said threshold starting from the second edge index; -extraction (50) of a subset of average values ​​between the first column index and the second column index; -demodulation (52, 54) of binary data from said subset of average values.;

2. A method according to claim 1, wherein said threshold is a percentage of said maximum value, preferably between 30% and 50%.

3. The method of claim 2, wherein said threshold is equal to 40% of said maximum value.

4. A method according to any one of claims 1 to 3, wherein in the storing step (42), the average values ​​per column are stored in a table or in a list ordered according to the column indices.

5. A method according to any one of claims 1 to 4, wherein the step of demodulating binary data comprises calculating a thresholding curve by a segmentation method.

6. Device for demodulating modulated data by modulation of amplitude of a light signal emitted by a light source of a transmitting device, the demodulation device being configured to acquire a digital image comprising an area illuminated by said light signal, said area comprising fringes corresponding to high and low states of the light signal, the device being characterized in that it further comprises modules configured to: -calculate (24) an average value per column of said digital image, and store average values ​​per column in association with a column index, arranged between a first edge index corresponding to a first edge of said image and a second edge index corresponding to a second edge of said image;-determining (26) a maximum value of said average values, and calculating a threshold from said maximum value, - determining (28) a first column index corresponding to the first average value greater than or equal to said threshold starting from the first edge index, and a second column index corresponding to the first average value greater than or equal to said threshold starting from the second edge index; -extracting (30) a subset of average values ​​between the first column index and the second column index; -demodulating (32) binary data from said subset of average values.;

7. Demodulation device according to claim 6, wherein said threshold is a percentage of said maximum value, preferably between 30% and 50%.

8. Electronic apparatus comprising an image sensor and a device for demodulating data modulated by amplitude modulation of a light signal according to claims 6 and 7.

9. An electronic apparatus according to claim 8, said electronic apparatus being a mobile phone or a tablet computer with at least one integrated CMOS image capture apparatus.

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