Demodulation method and apparatus for calibrating data modulated by modulating amplitude of optical signal

Abstract

The 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 device (20) for capturing a digital image having related sensitivity and exposure time parameters, and an electronic computing device (23) implementing modules of:-automatically adjusting (24) the sensitivity and exposure time parameters,-acquiring (26) the digital image by the digital image capturing device,-extracting (28) a series of samples from the acquired digital image,-determining (28) the series of samples,-determining (28) the series of samples, and-determining (28) the series of samples. Each extracted sample takes one of two predetermined values,-calculating (30) a plurality of sampling factors from the series of samples, each sampling factor relating to a predetermined pattern and indicating a sample number representing the same value of the pattern, and storing the calculated sampling factors.

Description

Technical Field

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

[0002] The present invention also relates to a related device for calibration of demodulation.

[0003] The present invention belongs to the technical field of wireless communication using visible light, which uses a VLC (Visible Light Communication) system.

[0004] The present invention is more specifically applied within a data communication framework using an encoder / transmitter implemented on an industrial product, the industrial product including a light-emitting diode (LED) light indicator in the front, such as a screen or one or more light indicators, intended to provide an indication of the product state, and the data can be decoded by a VLC receiver / decoder integrated into, for example, a portable device (such as a mobile phone).

[0005] For example, the present invention is applied to products for monitoring and protecting electrical systems, but more generally to any type of product including an LED light indicator in the front. Background Art

[0006] Wireless communication technology using visible light (VLC) has recently developed and found many applications.

[0007] A VLC system includes an encoder / transmitter device and a receiver / decoder device that are positioned substantially opposite each other. The encoder / transmitter device includes a light source, such as one or more light-emitting diode (LED) lights, and the receiver / decoder device includes an image sensor device, such as a CMOS camera.

[0008] The light source emits an optical signal, which is amplitude-modulated according to the data to be transmitted. The data is encoded into symbols by an encoding method, and each symbol represents a bit to be modulated. The symbols are then encapsulated in a formatted transmission packet to form a bit stream including a predetermined synchronization word (or synchronization sequence), followed by a formatted transmission packet including a header, useful data, and an error detection code. This bit stream is converted into an electrical signal, which controls the activation or deactivation of the light source at a selected frequency so that the resulting flicker is not perceptible to the human eye. For example, when the light source is turned on (corresponding to the high state of the optical signal), binary "1" is transmitted, and when the light source is turned off (corresponding to the low state of the optical signal), binary "0" is transmitted. Without changing the principle of this method, another correspondence between the high and low states and the transmitted bits can be achieved.

[0009] The receiver / decoder device applies digital image processing to the image acquired by the image sensor device in order to perform demodulation of the modulated data and then perform decoding, thereby obtaining the decoded data.

[0010] The acquired digital image includes a pixel matrix, the values of which represent the high or low state of the light source, or a transition between states, in at least one area illuminated by the light source. Thus, the acquired digital image includes areas with bright and dark stripes, the bright and dark stripes corresponding respectively to the high and low states of the optical signal. The performance of the demodulation depends more specifically on the operating parameters of the CMOS image sensor device used, in particular the sensitivity and the exposure time. The sensitivity (expressed in ISO or dB (gain)) defines the sensitivity of the sensor. The exposure time defines the "on" time of the sensor and thus represents the time during which each column of the digital image matrix is exposed to light.

[0011] In addition, the CMOS sensor implements a rolling shutter mechanism, which makes it possible to form the columns of the acquired digital image and to form clear vertical stripes when the light source is on (corresponding to the high state of the optical signal), and dark vertical stripes when the light source is off (corresponding to the low state of the optical signal).

[0012] The performance of the demodulation also depends on the sampling factor, which represents the ratio of the bright or dark stripes of the acquired image to the corresponding number of bits. The sampling factor depends in particular on the acquisition time of the digital image and the acquisition time of the pixel values and thus varies according to the model of the image sensor device.

[0013] In the envisaged applications, it is advantageous to use existing electronic devices having at least one integrated CMOS image sensor device, such as mobile phones called smartphones, tablets and laptop computers.

[0014] However, there are many models of such electronic devices, incorporating image sensor devices with operating specificities.

[0015] It is not practical to consider manually calibrating each model of image sensor device integrated into various models of portable electronic devices. Summary of the Invention

[0016] The object of the present invention is to remedy this drawback by proposing an improved demodulation calibration method, which integrates automatic calibration and automatic calculation of multiple sampling factors from the acquired digital image.

[0017] To this end, the present invention proposes a method for calibrating the demodulation of data modulated by amplitude modulation of an optical signal emitted by a light source of an encoder device, the modulated data being encapsulated in a formatted transmission packet, the method being implemented by a demodulation calibration device comprising a digital image capture device having associated sensitivity and exposure time parameters, and an electronic computing device configured to receive the digital image acquired by said image capture device. This method comprises the following steps:

[0018] -A) Automatically adjust the sensitivity and exposure time parameters to place the image capture device in an image acquisition mode suitable for decoding data transmitted by visible light communication,

[0019] -B) Obtain a digital image by the digital image capture device, where the obtained digital image includes a region having bright stripes and dark stripes, and the bright stripes and dark stripes respectively correspond to the high state and low state of the received optical signal,

[0020] -C) Extract a series of samples from the obtained digital image, and each extracted sample takes one of two predetermined values,

[0021] -D) Calculate a plurality of sampling factors from the series of samples, each sampling factor is related to a predetermined pattern and indicates the number of samples representing the same value of the pattern, and for each predetermined pattern, store the calculated sampling factor related to the pattern.

[0022] Advantageously, the proposed method allows adjusting the sensitivity and exposure time parameters and allows calculating a plurality of sampling factors without prior knowledge of the model of the image capture device used.

[0023] Therefore, advantageously, it enables integrating the proposed demodulation calibration method into any portable electronic device, particularly a smart phone or an electronic tablet, and thus implementing an application for reading data transmitted by a VLC system.

[0024] The method for calibrating the demodulation of data modulated by amplitude modulation according to the present invention may have one or more of the following features, which may be adopted independently or in all technically feasible combinations.

[0025] The method further includes a step of applying the calculated sampling factors to recover at least one transmitted data packet.

[0026] The method includes, after recovering at least one transmitted data packet, verifying the data by applying an error detection code.

[0027] The method includes applying steps B) to D) to the digital images obtained for a predetermined number P to obtain a plurality of sampling factors for each obtained digital image, and calculating the average sampling factor for each pattern, where the average sampling factor is equal to the average of the sampling factors related to the pattern of each obtained digital image.

[0028] The method further includes applying the average sampling factor to recover the transmitted data of a plurality of digital images, at least one transmitted data packet of each digital image, verifying the recovered data by applying an error detection code, and calculating a validity indicator.

[0029] If the validity indicator is less than the validity threshold, steps B) to D) are iterated for the digital images subsequently acquired for the predetermined number.

[0030] Step D) of calculating a plurality of sampling factors from the series of samples includes, for each sample value, counting the consecutive sample numbers taking that value, and sorting the counted numbers in descending order in a list associated with the sample value.

[0031] The method includes, for each list, filtering to retain only the distinct numbers that occur more than or equal to 2 times.

[0032] The method includes grouping the counted numbers that are at most a predetermined distance threshold apart, which grouping includes, in the corresponding list, replacing the counted numbers that are at most a predetermined distance threshold apart with an updated number equal to the average of the counted numbers that are at most a predetermined distance threshold apart.

[0033] The method further includes detecting the presence of an updated number corresponding to a predetermined synchronization word in one of the lists.

[0034] In the case of a positive detection, the method includes calculating the sampling factor by multiplying each updated number by a predetermined uncertainty factor.

[0035] The sensitivity is adjusted to 55% of the maximum sensitivity of the image capture device.

[0036] According to another aspect, the present invention relates to a device for calibrating the demodulation of data modulated by amplitude modulation of an optical signal emitted by a light source of an encoder device, the modulated data being encapsulated in a formatted transmission packet, the device including a digital image capture device having associated sensitivity and exposure time parameters, and an electronic computing device configured to receive the digital images acquired by the image capture device, the device being configured to implement the method briefly described above.

[0037] In one embodiment, the calibration device is a telephone or an electronic tablet. Description of the Drawings

[0038] With reference to the accompanying drawings, other features and advantages of the present invention will become apparent from the description given below as a non-limiting example, in which:

[0039] - Figure 1 is a schematic diagram of a VLC system including a demodulation calibration device according to one embodiment;

[0040] - Figure 2 is a flowchart of the main steps of a calibration method of demodulation according to one embodiment;

[0041] - Figure 3is a flowchart of steps for calculating multiple sampling factors according to one embodiment;

[0042] - Figure 4 is an example of a series of data, lists, and sampling factors obtained at the output of the steps shown in Figure 3 Specific embodiments Specific embodiments

[0043] Figure 1 shows a VLC system 2, including an encoder / transmitter device 4 that communicates unidirectionally with a receiver / decoder device 6.

[0044] The transmitter device 4 is configured to encode, modulate, and transmit digital data D by using amplitude modulation of the optical signal emitted by the LED light source 8, which is formed, for example, by one or more lamps and is adapted to emit wavelengths in the visible spectrum, with the wavelengths being included between 380 nm and 780 nm.

[0045] The device 4 includes an encoding module 10 and a modulation module 12, and the modulation module 12 controls the amplitude of the optical signal emitted by the light source 8.

[0046] The encoding module 10 implements, for example, encoding that includes converting bits into codewords, also known as symbols. In the VLC system 2, the encoding module 10 implements Manchester encoding and formatting in the form of transmission packets formatted according to a selected protocol.

[0047] According to the IEEE standard 802.3, Manchester encoding includes encoding "1" with "01" and encoding "0" with "10".

[0048] These symbols are then encapsulated in a formatted transmission packet to form a bit stream, that is, a packet including a predetermined synchronization word, followed by a formatted sequence of symbols including a header, useful data, and an error detection code.

[0049] For example, when using Manchester encoding, the synchronization word is the pattern "1111" because, by definition, the pattern is not part of the Manchester code. In other words, no series of Manchester code symbols forms a four "1" sequence.

[0050] The size of the packet is variable, depending on the intended application. The size is indicated in the header of the packet.

[0051] The error detection code is, for example, a cyclic redundancy check code such as CRC8 or CRC16.

[0052] The modulation module 12 implements modulation such as "On-Off Keying" (OOK). In this type of modulation, the optical signal emitted by the light source is in a high state (i.e., the light source is on) to transmit binary "1", or in a low state (i.e., the light source is off) to transmit "0", with a frequency high enough to prevent the human eye from seeing the flicker.

[0053] In one embodiment, the encoding module 10 and the modulation module 12 are implemented by the computing processor 15.

[0054] In a variant, each of the encoding module 10 and the modulation module 12 is a dedicated module generated in the form of a programmable logic component (e.g., FPGA (Field Programmable Gate Array)) or in the form of an application-specific integrated circuit (e.g., ASIC (Application-Specific Integrated Circuit)).

[0055] For example, the device 4 is integrated into an industrial product, and the light source 8 includes one or more LED lights, forming an optical indicator originally intended to indicate the operating state of the industrial product.

[0056] The light source is preferably integrated so that when the industrial product is in the operating position, for example, when the industrial product is fastened to a support by one of its faces called the back, it irradiates a part of the face of the industrial product that is visible to the user, and the irradiated part is on the front of the product.

[0057] Advantageously, the modulation of the optical signal for transmitting additional digital data does not interfere with the initial function of the optical indicator of the industrial product.

[0058] For example, the digital data D includes additional information related to the industrial product, such as the unique identifier of the product, IP (Internet Protocol) address or URL (Uniform Resource Locator or the acronym of the web address), key or code, register status, BLE (Bluetooth Low Energy acronym) coupling password or dynamic key for Zigbee debugging. More generally, the digital data D includes information about debugging or pairing wireless communication, measurements made by the product, and the product status. As a result, it facilitates, for example, the installation and debugging of the product, or the maintenance of the product by the user. In addition, as a result, the network security of the product is improved.

[0059] The modulated optical signal is emitted by the light source 8.

[0060] The receiver / decoder device 6 includes an image capture device 20 and an electronic computing device 23.

[0061] For example, the image capture device is a CMOS sensor (Complementary Metal Oxide Semiconductor) camera, suitable for capturing the optical signal and converting it into a digital image composed of one or more pixel matrices, and each pixel in the pixel matrix has a related numerical value.

[0062] When the user places the device 6 in such a way that the image capture device 20 is placed substantially opposite the light source 8, at a distance chosen by the user, for example between 0 cm (i.e., glued together) and 2 meters, and the receiver device 20 is placed in VLC reception mode, the acquired image or each acquired image includes, in the area illuminated by the optical signal emitted by the light source, vertically arranged stripes representing the high and low states of the transmitted signal or the transitions between the states.

[0063] The demodulation performance of the modulated data transmitted by the light source depends more specifically on the operating parameters of the image capture device used, i.e., on the sensitivity parameter defining the sensor sensitivity; the exposure time defining the "on" time of the sensor; and the sampling factor representing the ratio between the bright or dark stripes of the acquired image and the corresponding symbol digits. Advantageously, the calculation of multiple sampling factors is proposed, as described in more detail below.

[0064] The digital image acquired by the device 20 is transmitted to the device 23, which performs the calibration operation of the demodulation of the modulated data and, optionally, the demodulation and decoding operations to obtain, at the output, a set of decoded digital data D*.

[0065] In the absence of loss or error, the decoded digital data D* is identical to the digital data D.

[0066] The device 23 is an electronic computing device that implements the demodulation calibration method as described below according to its various embodiments.

[0067] The electronic computing device 23 includes a computing unit 22, such as one or more processors and an associated electronic storage unit 21. The electronic storage unit 21 particularly includes a memory, such as RAM, ROM, any type of non-volatile memory (e.g., EPROM, EEPROM, FLASH, NVRAM).

[0068] The image capture device 20, the memory unit 21, and the computing unit 22 are adapted to communicate via a communication bus.

[0069] 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 computer, or any other portable electronic device equipped with a camera and an electronic computing device.

[0070] The electronic computing device 23 is configured to implement a module 24: automatically adjust the sensitivity and exposure time parameters for placing the image capture device in an image acquisition mode suitable for decoding data transmitted via visible light communication. These parameters are implemented by the image capture device 20 at a given frequency (e.g., 30 frames per second) for digital image acquisition.

[0071] The capture device 20 supplies the acquired digital image to an electronic computing device. The electronic computing device 23 also implements:

[0072] - A module 26 for extracting a series of samples of the acquired digital image, each extracted sample taking one of two predetermined values, in fact a series of "1"s and "0"s;

[0073] - A module 28 for calculating and storing a plurality of sampling factors from the said series of samples, each sampling factor being associated with a predetermined pattern and indicating the number of samples of the same value representing that pattern, and

[0074] - A module 30 for applying the calculated sampling factors to demodulate and decode at least one transmitted data packet.

[0075] In one embodiment, the modules 24, 26, 28, 30 are implemented in the form of software instructions forming a computer program which, when implemented by a computer, implements the calibration method for demodulating data modulated by the amplitude modulation of an optical signal as described above.

[0076] A computer program comprising software instructions is also liable to be recorded on a non-transitory computer-readable medium. The computer-readable medium is for example a medium suitable for storing electronic instructions and coupled to the 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 (such as EPROM, EEPROM, FLASH, NVRAM), a magnetic card or an optical card.

[0077] In 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 an application-specific integrated circuit (such as an ASIC (Application-Specific Integrated Circuit)).

[0078] Figure 2 Is a flowchart of the main steps of an embodiment of a method for calibrating the demodulation of data modulated by the amplitude modulation of an optical signal emitted by a light source.

[0079] The method includes step 40: automatically adjusting (also called calibrating) the sensitivity and exposure time parameters for placing the image capture device in an image acquisition mode suitable for decoding data transmitted by visible light communication.

[0080] More specifically, the automatic self-adjustment for taking pictures is prohibited.

[0081] Preferably, the sensitivity is set as a percentage of the maximum sensitivity Gmax of the image capture device, for example 55% of Gmax.

[0082] Preferably, the exposure time is set to approximately 30 microseconds. Such an exposure time is a short time corresponding to a high sampling rate to facilitate the recovery of transmitted symbols.

[0083] Calculate and store the sampling factors to be applied, as explained in more detail below with reference to Figure 3 and 4 more detailed explanation.

[0084] After step 40 is step 42: Obtain a digital image obtained by an image capture device implementing fixed sensitivity and exposure time parameters.

[0085] After step 42 is step 44: Extract a series of samples from the obtained digital image, each extracted sample taking one of two predetermined values, such as "1" and "0". For example, it is a series of binary data extracted from the average value of each column by applying a threshold curve calculated according to the brightness of the obtained digital image.

[0086] In one embodiment, step 44 implements calculating the average brightness of each column of the obtained digital image, calculating a threshold curve based on the average value, and applying the threshold curve. Various methods for calculating such a threshold curve are known in the prior art and are applicable here.

[0087] A series of samples taking the value "1" or "0" according to whether the average value of the corresponding column is higher or lower than the corresponding value of the threshold curve. The sample series has as many samples as the number of columns in the digital image.

[0088] In an optimized embodiment, step 44 implements:

[0089] - Calculate the average value of each column of the digital image and store the average value of each column associated with a column index, the column index being arranged between a first edge index corresponding to a first edge of the image and a second edge index corresponding to a second edge of the image;

[0090] - Determine the maximum value of the average values and calculate a threshold from the maximum value,

[0091] - Determine a first column index corresponding to a first average value greater than or equal to the threshold starting from the first edge index, and determine a second column index corresponding to a first average value greater than or equal to the threshold starting from the second edge index;

[0092] - Extract a subset of the average values between the first column index and the second column index, and

[0093] - Calculate a threshold curve for the subset of the average values and apply the calculated threshold curve to obtain a series of binary samples.

[0094] The method then includes step 46: Calculate a plurality of sampling factors.

[0095] Each sampling factor is associated with a predetermined pattern and represents the number of samples of the same value ("1" or "0") that represent that pattern in a series of samples.

[0096] In fact, when Manchester coding is applied and the synchronization word "1111" is used, the patterns to be considered are:

[0097] -M 1 = "1111"

[0098] -M 2 = "11"

[0099] -M 3 = "1"

[0100] -M 4 = "00"

[0101] -M 5 = "0"

[0102] In fact, when Manchester coding is applied and the synchronization word is "1111", the patterns correspond to a set of symbol groups of the same value that can be effectively modulated by the encoder and transmitter devices and transmitted in the transmission packet.

[0103] Sampling factor F i is associated with each pattern M i and F i is a number indicating the number of samples of the same value of the series of samples used to modulate the pattern.

[0104] Reference Figure 3 describes an embodiment of step 46 for calculating multiple sampling factors, while Figure 4 shows an example of the intermediate results obtained during the implementation of the calculation.

[0105] Thus, starting from a series of 45 samples taking binary values (or symbols) "1" or "0" respectively, an example of which is given in Figure 4 the method includes step 54: counting the consecutive sample numbers for each value, i.e., "1" or "0", and storing the sample numbers (sorted in descending order) in a list associated with that value. The result of step 54 is labeled 55.

[0106] As Figure 4 the example shows, at the end of step 54, a list L1 of consecutive "1" numbers in sequence 45 and a list L2 of consecutive "0" numbers in sequence 45 are obtained respectively.

[0107] The method then includes a filtering step 56, during which only the distinct numbers that occur more than or equal to 2 times are retained in each list. InFigure 4 The result of step 56 is shown in the example shown, labeled 57.

[0108] Thus, for example, in list L1, the numbers 18, 8, 4, and 3 are retained, while the number "1" corresponding to a single occurrence is removed.

[0109] As a result, advantageously, possible isolated errors are eliminated.

[0110] The method includes a grouping step 58, which includes, in the corresponding list, replacing the counted numbers that are at most a predetermined distance threshold apart with an updated number equal to the average of the counted numbers that are at most a predetermined distance threshold apart.

[0111] For example, the predetermined distance threshold is equal to 2.

[0112] In Figure 4 The result of step 58 is shown in the example shown, labeled 59. 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.

[0113] The numbers retained at the end of step 58 are called updated numbers.

[0114] As can be seen from the example, the updated numbers are real numbers, while the initially counted numbers are integers.

[0115] The updated numbers represent merged adjacent values.

[0116] Of course, variations for implementing the above steps can be envisioned within the capabilities of those skilled in the art.

[0117] Other computational variations can be envisioned, such as calculating the updated number by calculating the average of the numbers that are at most a predetermined distance threshold apart.

[0118] The method then includes step 60: detecting the presence of a predetermined synchronization word in the series.

[0119] In one embodiment, the synchronization word is "1111", which corresponds to pattern M 1 .

[0120] In one embodiment, step 60 implements the calculation of the ratio of the first two numbers of list L1 (the list related to the counted consecutive "1" numbers).

[0121] When the calculated ratio is within a given numerical range, for example, between 1.7 and 2.3, the detection is considered positive.

[0122] In the case of a positive detection in step 60, the method includes calculating a 62 sampling factor by multiplying each updated number by a predetermined uncertainty factor.

[0123] For example, a predetermined uncertainty factor is K = 1.3, which corresponds to an uncertainty margin of 30%. Other values of the uncertainty factor are possible, for example between 1.1 and 1.4, corresponding to an uncertainty margin varying between 10% and 40%.

[0124] Thereby, there is obtained Figure 4 Table 61 shown as an example in, which contains the calculated sampling factor F associated with each considered pattern.

[0125] Of course, other storage structures can be used to store the sampling factors associated with each considered pattern.

[0126] In one embodiment, the effective sampling factor applied to demodulation is an integer factor obtained from the calculated sampling factor. For example, each integer factor is chosen as the integer closest to the calculated sampling factor. For a plurality of P images, the steps of obtaining 42 digital images, extracting 44 a series of samples of the obtained digital images, and calculating 46 the respective sampling factors from the series of samples are carried out, where P is a predetermined number greater than or equal to 1, preferably greater than or equal to 2, for example equal to 25. The sampling factors for each pattern and each processed digital image are stored.

[0127] The method includes, when the number P is greater than or equal to 2, after a predetermined number P of images have been reached (verification 70), a step 72 of calculating the average sampling factor for each pattern, which average sampling factor is equal to the average of the sampling factors associated with said pattern calculated for each obtained digital image.

[0128] The method then includes step 74: applying the sampling factor to recover at least one data packet.

[0129] Preferably, the sampling factor is applied to recover data packets from a plurality of P' obtained digital images, where P' is a predetermined number greater than or equal to 2, for example 10.

[0130] The verification of the recovered data is then carried out by applying an error detection code, and the percentage of verified demodulation is calculated.

[0131] More generally, an effectiveness indicator is calculated from all P' processed images.

[0132] The effectiveness indicator is compared with a predetermined effectiveness threshold, for example equal to 80% when the indicator is the percentage of verified demodulation.

[0133] More generally, when the indicator is the percentage of verified demodulation, the effectiveness threshold is for example between 60% and 100%.

[0134] If the validity threshold is reached or exceeded (test in step 76), the average sampling factor is stored for the image capture device used during storage step 78.

[0135] Subsequently, the stored sampling factor is used to demodulate and decode the transmitted data.

[0136] If the validity threshold is not reached, steps 42 to 76 are iterated for subsequent digital images, to which a denoising or correction filter, for example, is applied.

[0137] Advantageously, in one embodiment, the sampling factor is calculated for a plurality of acquired digital images, which results in a good statistical representation of the operation of the image capture device.

[0138] Advantageously, the method automatically provides a fine adaptation to any particular non - linearities of the image capture device used. This enables any image capture device already installed in electronic device models already on the market to be applied.

Claims

1. A method for calibrating the demodulation of data modulated by amplitude modulation of an optical signal emitted by a light source of an encoder, the modulated data being encapsulated in a formatted transmission packet, the method being implemented by a demodulation calibration device comprising a digital image capture device having associated sensitivity and exposure time parameters, and an electronic computing device configured to receive a digital image acquired by said image capture device, the method being characterized in that it comprises the following steps: -A) automatically adjusting (40) sensitivity and exposure time parameters for placing the image capture device in an image acquisition mode suitable for decoding data transmitted via visible light communication, -B) acquiring (42) a digital image by the digital image capture device, the acquired digital image comprising an area including an area having light stripes and dark stripes, the light stripes and the dark stripes corresponding to a high state and a low state of the received light signal, respectively, -C) extracting (44) a series of samples from the acquired digital image, each extracted sample taking one of two predetermined values, -D) calculating (46) a plurality of sampling factors from said series of samples, each sampling factor being associated with a predetermined pattern and indicating a number of samples of the same value representing said pattern, and for each predetermined pattern storing the calculated sampling factor associated with said pattern.

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

3. The method of claim 2, comprising, after recovering at least one transmitted data packet, verifying said data by applying an error detection code.

4. The method according to any one of claims 1 to 3 comprises applying steps B) to D) to a predetermined number P of acquired digital images so as to obtain a plurality of sampling factors for each acquired digital image, and calculating (72) an average sampling factor for each pattern, which is equal to the average of the sampling factors associated with the pattern for each acquired digital image.

5. The method of claim 4 further comprising applying the average sampling factor to recover transmission data for a plurality of digital images, at least one transmission data packet for each digital image, verifying the recovered data by applying an error detection code, and calculating a validity indicator.

6. The method according to claim 5, wherein: If the validity indicator (76) is less than a validity threshold, steps B) to D) are iterated for the predetermined number of subsequently acquired digital images.

7. The method according to any one of claims 1 to 6, wherein: The step D) of calculating a plurality of sampling factors from the series of samples comprises: - For each sample value, count (54) the consecutive sample numbers of said value and arrange the counted numbers in descending order in the list associated with said sample value.

8. The method according to claim 7, comprising, for each list, filtering (56) for retaining only the distinct numbers which occur a number of times greater than or equal to 2.

9. The method according to claim 8 includes grouping (58) count numbers that are at most a predetermined distance threshold apart, the grouping comprising replacing the count numbers that are at most a predetermined distance threshold apart in a corresponding list with an update number that is equal to the average value of the count numbers that are at most a predetermined distance threshold apart.

10. The method according to claim 9, further comprising detecting (60) the presence of an update number corresponding to a predetermined synchronization word in one of the lists.

11. The method according to claim 10, comprising, in case of positive detection, calculating (62) the sampling factor by multiplying each update number by a predetermined uncertainty coefficient.

12. A method according to any one of the preceding claims, wherein: The sensitivity is adjusted to 55% of the maximum sensitivity of the image capture device.

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

14. The demodulation calibration device according to claim 13, wherein the device is a mobile phone or an electronic tablet.

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