Method and device for calibrating the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source

Abstract

Method and device for calibrating the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source The present invention relates to a method and a device for calibrating the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source.The device comprises a digital image capture apparatus (20) having associated sensitivity and exposure time parameters, and an electronic calculation device (23) implementing modules for: automatic adjustment (24) of the sensitivity and exposure time parameters, acquisition (26) of a digital image by said digital image capture apparatus, extraction (28) of a series of samples from the acquired digital image, each extracted sample taking one of two predetermined values, calculation (30) of a plurality of sampling factors from said series of samples, each sampling factor being associated with a predetermined pattern and being indicative of a number of samples of the same value representative of said pattern, and storage of the calculated sampling factors. Figure for the abstract: Figure 1.

Description

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

[0001] The present invention relates to a method for calibrating the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source.

[0002] It also relates to an associated demodulation calibration device.

[0003] The invention lies in the field of wireless communication technology. using visible light, using a VLC system (for “Visible Light Communication”).

[0004] 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.

[0005] For example, the invention applies to products for monitoring and protecting electrical systems, but applies more generally to any type of product comprising an LED indicator light on the front.

[0006] The use of wireless communication technology using visible light (VLC) has developed recently and found many applications.

[0007] 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.

[0008] 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 which controls an activation or deactivation of the source light, at a frequency chosen so that the flicker 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 set up, without changing the principle of the method.

[0009] 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.

[0010] The acquired digital images comprise pixel matrices whose values, in at least one area of ​​the image illuminated by the source, are representative 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. The performance of the demodulation depends in particular on the operating parameters of the CMOS image capture device used, in particular the sensitivity and the exposure time. The sensitivity, expressed in ISO or in dB (gain), defines the sensitivity of the sensors. The exposure time defines the “open” time of the sensors, and consequently represents the time during which each column of the matrix of a digital image is exposed to light.

[0011] In addition, CMOS sensors implement a “Rolling Shutter” mechanism which makes it possible to form columns of the acquired digital image, and to form light vertical fringes when the light source is on (high state of the corresponding light signal), and dark vertical fringes when the light source is off (low state of the corresponding light signal).

[0012] The demodulation performance further depends on a sampling factor indicating the ratio between the light or dark fringes of an acquired image and the number of corresponding bits. The sampling factor(s) depend in particular on the acquisition time of a digital image, and on an acquisition time of the pixel values, and therefore vary depending on the models of image capture device.

[0013] In the applications envisaged, it is advantageous to use existing electronic devices with at least one integrated CMOS image capture device, such as so-called smart mobile phones (“smartphones”), electronic tablets, portable computers.

[0014] However, there are many models of such electronic devices, integrating image capture devices with specific operating features.

[0015] It is not practical to consider manual calibration of each model image capture device integrated into various models of portable electronic devices.

[0016] The invention aims to remedy this drawback by proposing an improved demodulation calibration method integrating automatic calibration and automatic calculation of a plurality of sampling factors from the acquired digital images.

[0017] For this purpose, the invention proposes a method for calibrating the demodulation of modulated data by amplitude modulation of a light signal emitted by a light source of an encoder device, the modulated data being encapsulated in formatted transmission packets, the method being implemented by a demodulation calibration device comprising a digital image capture apparatus having associated sensitivity and exposure duration parameters, and an electronic computing device configured to receive digital images acquired by said capture apparatus. This method comprises steps of: - A) automatic adjustment of the sensitivity and exposure duration parameters for placing the image capture apparatus in an image acquisition mode suitable for decoding data transmitted by visible light communication, - B) acquisition of a digital image by said digital image capture device, - C) extraction of a series of samples from the acquired digital image, each extracted sample taking one of two predetermined values, - D) calculating a plurality of sampling factors from said series of samples, each sampling factor being associated with a predetermined pattern and being indicative of a number of samples of the same value representative of said pattern, and storing the calculated sampling factors.

[0018] Advantageously, the proposed method allows for the adjustment of sensitivity and exposure duration parameters and the calculation of a plurality of sampling factors, without prior knowledge of the model of image capture device used.

[0019] Thus, advantageously, this makes it possible to integrate the proposed demodulation calibration method into any portable electronic device, in particular of the smartphone or electronic tablet type, and therefore to implement applications using the reading of data transmitted by the VLC communication system.

[0020] The calibration method for the demodulation of 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.

[0021] The method further comprises a step of applying the calculated sampling factors to recover at least one packet of transmitted data.

[0022] It comprises, after recovery of at least one packet of transmitted data, a validation of said data by application of an error detection code.

[0023] The method comprises the application of steps B) to D) on a predetermined number P of acquired digital images, in order to obtain a plurality of sampling factors per acquired digital image, and a calculation of an average sampling factor per pattern, equal to the average value of the sampling factors associated with said pattern for each acquired digital image.

[0024] It further comprises an application of the average sampling factors for a recovery of transmitted data, for a plurality of digital images, of at least one packet of data transmitted per digital image, a validation of the recovered data by application of an error detector code, and a calculation of a validity indicator.

[0025] If the validity indicator is lower than a validity threshold, steps B) to D) are iterated on said predetermined number of subsequent acquired digital images.

[0026] Step D) of calculating a plurality of sampling factors from said series of samples comprises, for each sample value, a count of the numbers of successive samples taking said value, and an arrangement of the counted numbers in descending order in a list associated with said sample value.

[0027] The method comprises, for each list, a filtering to retain only the distinct numbers having an occurrence greater than or equal to two.

[0028] The method comprises a grouping of the counted numbers distant by at most the predetermined distance threshold, the grouping comprising a replacement, in the corresponding list, of said counted numbers distant by at most the predetermined distance threshold by an updated number equal to the average value of said counted numbers distant by at most the predetermined distance threshold.

[0029] The method further comprises detecting the presence in one of said lists of an updated number corresponding to a predetermined synchronization word.

[0030] The method comprises, in the event of positive detection, a calculation of said sampling factors by multiplying each updated number by a predetermined uncertainty coefficient.

[0031] The sensitivity is adjusted to 55% of a maximum sensitivity of the image capture device.

[0032] According to another aspect, the invention relates to a device for calibrating the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source of an encoder device, the modulated data being in encapsulated in formatted transmission packets, comprising a digital image capture apparatus having associated sensitivity and exposure time parameters, and an electronic computing device configured to receive digital images acquired by said capture apparatus, configured to implement a method as briefly described above.

[0033] In one embodiment, the calibration device is a telephone or an electronic tablet.

[0034] 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:

[0035] [Fig-1] [Fig.l] a schematic representation of a communication system VLC integrating a demodulation calibration device according to one embodiment;

[0036] [Fig.2] [Fig.2] is a synopsis of the main steps of a calibration process of demodulation according to one embodiment;

[0037] [Fig.3] [Fig.3] is a block diagram of the steps for calculating a plurality of factors sampling according to one embodiment;

[0038] [Fig.4] [Fig.4] is an example of a data series, lists and factors sampling obtained at the output of the steps in [Fig.3].

[0039] [Fig.l] illustrates a VLC communication system 2, comprising an encoder / transmitter device 4 in unidirectional communication with a receiver / decoder device 6.

[0040] The device 4 is configured to code, modulate and transmit digital data D using amplitude modulation of a light signal emitted by an LED 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.

[0041] The device 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.

[0042] 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.

[0043] Manchester coding, according to the IEEE 802.3 standard, consists of coding a “1” by “01” and a “0” by “10”.

[0044] The symbols are then encapsulated in formatted transmission packets, to form a binary train, i.e. a packet comprising a predetermined synchronization word, followed by a series of formatted symbols comprising a header, data useful and an error detection code.

[0045] 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.

[0046] The size of the packets varies, depending on the intended application. It is indicated in the packet header.

[0047] The error detection code is for example a cyclic redundancy check code such as CRC8 or CRC 16.

[0048] 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.

[0049] In one embodiment, the coding module 10 and the modulation module 12 are implemented by a calculation processor 15.

[0050] 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).

[0051] For example, the 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.

[0052] 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 to a support by one of its faces called the rear face, the illuminated portion is located on the front face of the product.

[0053] 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 (Internet Protocol) address or a URL (acronym for “Uniform Resource Locator” or web address), a key or a code, the state of the registers, a BLE (acronym for “Bluetooth Low Energy”) pairing password or dynamic keys for the implementation of the protocol. Zigbee service. More generally, digital data D includes information on wireless communication commissioning or pairing, measurements taken by the product, and the product's status. This makes it possible, for example, to facilitate installation and commissioning, or maintenance of a product by a user. In addition, it improves the cybersecurity of a product.

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

[0056] The receiver / decoder device 6 comprises an image capture device 20 and an electronic calculation device 23.

[0057] For example, the image capture apparatus is an optical camera with CMOS (for "Complementary metal-oxide-semiconductor) sensors adapted to capture light signals and to transform them into a digital image composed of one or more pixel matrices, each pixel of a pixel matrix having an associated digital value.

[0058] When the device 6 is placed by a user so that the image capture apparatus 20 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 (stuck) and 2 meters, and the capture apparatus 20 is put in a VLC reception mode, the or each digital image acquired 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 signal emitted, or of a transition between these states.

[0059] The performance of the demodulation of the modulated data transmitted by the light source depends in particular on the operating parameters of the image capture device used, i.e. the sensitivity parameter, defining the sensitivity of the sensors; the exposure time, defining the “open” time of the sensors, and the sampling factors indicating the ratio between the light or dark fringes of an acquired image and the number of corresponding symbols. Advantageously, the calculation of several sampling factors is proposed, as described in more detail below.

[0060] The digital image acquired by the apparatus 20 is transmitted to the device 23 which carries out operations of calibration of the demodulation of modulated data, and optionally, a demodulation and a decoding to obtain at output a set of decoded digital data D*.

[0061] In the absence of loss or error, the decoded digital data D* is identical to the digital data D.

[0062] The device 23 is an electronic calculation device which implements a demodulation calibration method as described below according to its various embodiments.

[0063] The electronic calculation device 23 comprises a calculation unit 22, e.g. one or more processors and an associated electronic memory unit 21. The electronic memory unit 21 comprises in particular memories of the RAM, ROM type, any type of non-volatile memory (for example EPROM, EEPROM, FLASH, NVRAM).

[0064] The image capture apparatus 20, the memory unit 21 and the computing unit 22 are adapted to communicate via a data communication bus.

[0065] In one embodiment, the receiver / decoder 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 CMOS camera and an electronic computing device.

[0066] The electronic computing device 23 is configured to implement a module 24 for automatically adjusting the sensitivity and exposure duration parameters for placing the image capture apparatus in an image acquisition mode suitable for decoding data transmitted by visible light communication. The parameters are implemented by the image capture apparatus 20 for acquiring digital images, at a given frequency, for example 30 frames per second.

[0067] An acquired digital image is provided to the electronic computing device by the capture apparatus 20. The electronic computing device 23 also implements:

[0068] - a module 26 for extracting a series of samples from the acquired digital image, each extracted sample taking one of two predetermined values, in practice a series of '1' and '0';

[0069] - a module 28 for calculating and storing a plurality of sampling factors from the series of samples, each sampling factor being associated with a predetermined pattern and being indicative of a number of samples of the same value representative of the pattern and

[0070] -a module 30 for applying the calculated sampling factors to demodulate and decode at least one packet of transmitted data.

[0071] In one embodiment, the modules 24, 26, 28, 30 are produced in the form of software instructions forming a computer program, which, when executed by a computer, implements a method for calibrating the demodulation of data modulated by amplitude modulation of a light signal as described.

[0072] The computer program comprising software instructions is further capable of being recorded on a non-transitory computer-readable medium. The computer-readable medium is, for example, a medium capable of storing the electronic instructions and of being coupled to a bus of a computer system. For example, the readable medium is an optical disc, a magneto-optical disc, a ROM memory, a RAM memory, any type of non-volatile memory (e.g. EPROM, EEPROM, FLASH, NVRAM), a magnetic card or an optical card.

[0073] As a variant, each of the modules 24, 26, 28, 30 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).

[0074] [Fig.2] is a block diagram of the main steps of an embodiment of a method for calibrating the demodulation of data modulated by amplitude modulation of a light signal emitted by a light source.

[0075] The method comprises a step 40 of automatic adjustment, also called calibration, of the sensitivity and exposure duration parameters for placing the image capture device in an image acquisition mode suitable for decoding data transmitted by visible light communication.

[0076] In particular, automatic self-adjustment for shooting is inhibited.

[0077] Preferably, the sensitivity is set at a percentage of the maximum sensitivity Gmax of the image capture device, for example at 55% of Gmax.

[0078] Preferably, the exposure duration is set at approximately 30 microseconds. Such an exposure duration is a short duration, corresponding to a high sampling rate, so as to promote the recovery of the transmitted symbols.

[0079] The sampling factors to be applied are calculated and stored, as explained in more detail below with reference to Figures 3 and 4.

[0080] Step 40 is followed by a step 42 of acquiring a digital image, obtained by the image capture device implementing the fixed sensitivity and exposure duration parameters.

[0081] Step 42 is followed by a step 44 of extracting a series of samples from the acquired digital image, each extracted sample taking one of two predetermined values, for example '1' and '0'. For example, this is a series of binary data, extracted by applying a thresholding curve, from average values ​​per column, calculated from the luminance of the acquired digital image.

[0082] In one embodiment, step 44 implements a calculation of average values ​​per column of the luminance of the acquired digital image, a calculation of a thresholding curve from these average values, and an application of the thresholding curve. Several methods of calculating such a thresholding curve are known in the state of the art, and are applicable here.

[0083] A series of samples taking values ​​of '1' or '0' depending on whether the average value of the corresponding column is above or below the corresponding value threshold curve latent. The sample series has as many samples as the number of columns in the digital image.

[0084] In an optimized embodiment, step 44 implements:

[0085] - a calculation of an average value per column of the luminance of the digital image, and a storage of the average values ​​per column in association with a column index, arranged between a first edge index corresponding to a first of said image and a second edge index corresponding to a second edge of said image;

[0086] - a determination of a maximum value of said average values, and calculation of a threshold from said maximum value,

[0087] - a determination of 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;

[0088] - an extraction of a subset of average values ​​between the first index of column and the second column index, and

[0089] - a calculation of a thresholding curve on the subset of average values ​​and an application of the calculated thresholding curve to obtain a series of binary samples.

[0090] The method then comprises a step 46 of calculating a plurality of sampling factors.

[0091] Each sampling factor is associated with a predetermined pattern and being indicative of a number of samples of the same value ('1' or '0') representative of the pattern in the series of samples.

[0092] In practice, when Manchester coding is applied, and the synchronization word '1111' is used, the patterns to be considered are:

[0093] -M^'1111'

[0094] -M2='ll'

[0095] -M3= '1'

[0096] -M4='OO'

[0097] -M5= '0'

[0098] Indeed, the patterns correspond to the set of groupings of symbols of the same value capable of being validly modulated and transmitted in transmission packets by the encoding and transmitting device, when Manchester coding is applied and when the synchronization word is '1111'.

[0099] A sampling factor E is associated with each pattern M;, F; being a number indicating the number of samples of the same value from the series of samples are used to modulate the pattern.

[0100] An embodiment of step 46 of calculating a plurality of sampling factors is described with reference to [Fig.3], while [Fig.4] illustrates an example of intermediate results obtained during the implementation of this calculation.

[0101] Thus, starting from a series 45 of samples taking binary values ​​(or symbols), respectively '1' or '0', an example of which is provided in [Fig.4], the method comprises a step 54 of counting the numbers of successive samples of each value, i.e. '1' or '0', and of storing these numbers of samples, sorted in descending order, in a list associated with the value. The result of step 54 is referenced 55.

[0102] As shown in the example of [Fig.4], at the end of step 54, the following are obtained respectively: a list L1 of successive numbers of '1' in the series 45, and a list L2 of successive numbers of '0' in the series 45.

[0103] The method then comprises a filtering step 56, during which only the distinct numbers having an occurrence greater than or equal to 2 are retained in each list. The result of step 56, referenced 57, is illustrated in the example of [Fig.4],

[0104] For example, in list L1, the numbers 18, 8, 4 and 3 are retained, and the number “1” corresponding to a single occurrence is eliminated.

[0105] Advantageously, this makes it possible to eliminate possible isolated errors.

[0106] The method comprises a grouping step 58 comprising a replacement, in the corresponding list, of the counted numbers distant from at most a predetermined distance threshold, by an updated number equal to the average value of said counted numbers distant from at most the predetermined distance threshold.

[0107] For example, the predetermined distance threshold is equal to 2.

[0108] The result of step 58, referenced 59, is illustrated in the example of [Fig.4]. In this example, in list L1, the numbers 4 and 3 are replaced by 3.5 and in list L2, the numbers 8 and 7 are replaced by 7.5.

[0109] The numbers retained at the end of step 58 are called updated numbers.

[0110] As can be seen from the example, the updated numbers are real numbers, while the numbers initially counted are whole numbers.

[0111] The updated numbers are representative of adjacent merged values.

[0112] Of course, variations in carrying out the steps described above within the reach of those skilled in the art are conceivable.

[0113] Other calculation variants are conceivable, for example the calculation of an updated number by calculating the average value of the numbers distant by at most the predetermined distance threshold.

[0114] The method then comprises a step 60 of detecting the presence of the predetermined synchronization word in the series.

[0115] In one embodiment, the synchronization word is '1111', which corresponds to the pattern Mp

[0116] In one embodiment, step 60 implements the calculation of the ratio of the first two numbers of the list L1 (list relating to the successive numbers of '1' counted).

[0117] When the calculated ratio is within a given range of values, for example between 1.7 and 2.3, the detection is considered positive.

[0118] In the event of positive detection in step 60, the method comprises a calculation 62 of the sampling factors by multiplying each updated number by a predetermined uncertainty coefficient.

[0119] For example, the predetermined uncertainty coefficient is K=1.3 which corresponds to an uncertainty margin of 30%. Other uncertainty coefficient values ​​are conceivable, for example between 1.1 and 1.4 corresponding to an uncertainty margin varying between 10% and 40%.

[0120] Thus, we obtain table 61 illustrated as an example in [Fig.4], which contains the calculated sampling factor F associated with each pattern considered.

[0121] Of course, other storage structures can be used to store the sampling factors associated with each pattern considered.

[0122] In one embodiment, the effective sampling factors, applied for the demodulation, are integer factors obtained from the calculated sampling factors. For example, each integer factor is chosen as being the closest integer to the calculated sampling factor. The respective steps of acquiring 42 a digital image, extracting 44 a series of samples from the acquired digital image and calculating 46 a plurality of sampling factors from the series of samples are implemented for a plurality of P images, P being a predetermined number, greater than or equal to one, preferably greater than or equal to two, for example equal to 25. The sampling factors, for each pattern and for each processed digital image, are stored.

[0123] The method comprises, when the number P is greater than or equal to two, after the predetermined number P of images is reached (verification 70), a step 72 of calculating an average sampling factor per pattern equal to the average value of the sampling factors associated with said pattern calculated for each acquired digital image.

[0124] Then the method comprises a step 74 of applying the sampling factors for the recovery of at least one data packet.

[0125] Preferably, the sampling factors are applied for the recovery of data packets from a plurality of P' acquired digital images, P' being a predetermined number, greater than or equal to 2, for example 10.

[0126] Then, a validation of the recovered data is carried out by applying an error detector code and a percentage of validated demodulations is calculated.

[0127] More generally, a validity indicator is calculated from all the P' processed images.

[0128] This validity indicator is compared to a predetermined validity threshold, for example equal to 80% when the indicator is the percentage of validated demodulations.

[0129] More generally, the validity threshold is for example between 60% and 100% when the indicator is the percentage of validated demodulations.

[0130] If the validity threshold is reached or exceeded (test of step 76), the average sampling factors are stored in storage step 78 for the image capture device used.

[0131] Subsequently, the stored sampling factors are used for demodulation and decoding of transmitted data.

[0132] If the validity threshold is not reached, steps 42 to 76 are iterated on subsequent digital images, on which a noise reduction or correction filter is applied, for example.

[0133] Advantageously, in one embodiment, the sampling factors are calculated on a plurality of acquired digital images, which allows good statistical representativeness of the operation of the image capture device.

[0134] Advantageously, the method automatically provides a fine adaptation to any possible specific non-linearity of the image capture device used. This allows application with any image capture device already installed in a model of electronic devices already marketed.

Claims

Claims

1. Method for calibrating the demodulation of modulated data by amplitude modulation of a light signal emitted by a light source of an encoder device, the modulated data being encapsulated in formatted transmission packets, the method being implemented by a demodulation calibration device comprising a digital image capture apparatus having associated sensitivity and exposure duration parameters, and an electronic computing device configured to receive digital images acquired by said capture apparatus, the method being characterized in that it comprises steps of: - A) automatic adjustment (40) of the sensitivity and exposure duration parameters for placing the image capture apparatus in an image acquisition mode suitable for decoding data transmitted by visible light communication,- B) acquisition (42) of a digital image by said digital image capture apparatus, - C) extraction (44) of a series of samples from the acquired digital image, each extracted sample taking one of two predetermined values, - D) calculation (46) of a plurality of sampling factors from said series of samples, each sampling factor being associated with a predetermined pattern and being indicative of a number of samples of the same value representative of said pattern, and storage of the calculated sampling factors.,

2. The method of claim 1, further comprising a step (74) of applying the calculated sampling factors to recover at least one packet of transmitted data.

3. Method according to claim 2, comprising after recovery of at least one packet of transmitted data, a validation of said data by application of an error detection code.

4. Method according to any one of claims 1 to 3 comprising the application of steps B) to D) on a predetermined number P of images digital images acquired, in order to obtain a plurality of sampling factors per acquired digital image, and a calculation (72) of an average sampling factor per pattern, equal to the average value of the sampling factors associated with said pattern for each acquired digital image.

5. The method of claim 4, further comprising applying the average sampling factors for recovery of transmitted data, for a plurality of digital images, of at least one packet of data transmitted per digital image, validating the recovered data by applying an error detection code, and calculating a validity indicator.

6. A method according to claim 5, wherein if the validity indicator is lower (76) than a validity threshold, steps B) to D) are iterated on said predetermined number of subsequent acquired digital images.

7. Method according to any one of claims 1 to 6, in which step D) of calculating a plurality of sampling factors from said series of samples comprises: - for each sample value, a count (54) of the numbers of successive samples taking said value, and an arrangement of the counted numbers in descending order in a list associated with said sample value.

8. Method according to claim 7, comprising, for each list, a filtering (56) to retain only the distinct numbers having an occurrence greater than or equal to two.

9. Method according to claim 8, comprising a grouping (58) of the counted numbers distant by at most the predetermined distance threshold, the grouping comprising a replacement, in the corresponding list, of said counted numbers distant by at most the predetermined distance threshold by an updated number equal to the average value of said counted numbers distant by at most the predetermined distance threshold.

10. Method according to claim 9, further comprising a detection (60) of presence in one of said lists of an updated number corresponding to a predetermined synchronization word.

11. Method according to claim 10, comprising, in the event of positive detection, a calculation (62) of said sampling factors by multiplying each updated number by a predetermined uncertainty coefficient.

12. A method according to any preceding claim, wherein the sensitivity is adjusted to 55% of a maximum sensitivity of the image capturing apparatus.

13. Device for calibrating the demodulation of modulated data by amplitude modulation of a light signal emitted by a light source of an encoder device, the modulated data being encapsulated in formatted transmission packets, comprising a digital image capture apparatus having associated sensitivity and exposure duration parameters, and an electronic calculation device configured to receive digital images acquired by said capture apparatus, configured to implement a method according to claims 1 to 12.

14. A demodulation calibration device according to claim 13, said device being a mobile phone or an electronic tablet.

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