Method and device for modulating and demodulating a light signal for symbol transmission

Abstract

The present invention relates to the modulation and demodulation of a light signal by a PWM control signal (100) in order to transmit a symbol represented by a pair of a first and a second frequency, wherein the first frequency and the second frequency of the pair of frequencies are determined such that the highest frequency, from among the first and the second frequencies, is at most equal to twice the lowest frequency, from among the first and the second frequencies; wherein the control signal (100) comprises a sequence of time cycles comprising at least a first time cycle having a duration defined on the basis of the first frequency, followed by at least a second time cycle having a duration defined on the basis of the second frequency.

Description

Method and device for modulating and demodulating a light signal for symbol transmission

[0001] The present invention relates to symbol transmission by modulation of light signals driven by a pulse-width modulated signal. Technological background

[0002] Light signal communication is a widely known technology. It is very often referred to as VLC (Visible Light Communication) or ILC (Infrared Light Communication).

[0003] VLC is a technology that uses electromagnetic waves in the visible light spectrum (frequency of 400 to 800 THz / wavelength of 780 to 375 nm) to transmit data. ILC is an alternative technology that uses electromagnetic waves in the infrared spectrum (frequencies of 3 THz to 384 THz / wavelength of 780 nm to 1 mm).

[0004] Unlike traditional wireless technologies such as Wi-Fi® or Bluetooth®, which use radio waves, VLC and ILC use light sources such as LEDs to transmit signals at up to 10 kbit / s over short distances.

[0005] Electronic devices can be configured to receive modulated light signals, such as those containing a photodiode. In certain situations, a mobile phone camera or a digital camera can also serve as a receiver for VLC-type modulated light signals. The image sensor in these devices consists of an array of photodiodes (pixels) that can provide either multiple channels (up to one pixel per channel) or spatial awareness of multiple light sources.

[0006] One of the main advantages of VLC and ILC is their enhanced security compared to wireless technologies, as light generally does not penetrate walls, significantly reducing the risk of unauthorized data interception. Furthermore, they can be used in environments where radio waves are prohibited, such as hospitals or airplanes.

[0007] VLC and ILC are also valued for their versatility. In addition to data transmission, they can be used for applications such as indoor positioning (localization inside buildings) and the provision of precise geolocation services.

[0008] However, VLC and ILC have some limitations. For example, VLC's efficiency is reduced in the presence of intense ambient light, which can affect data transmission quality. Furthermore, the range of VLC and ILC is limited compared to traditional wireless technologies.

[0009] Despite these limitations, VLC and ILC continue to attract increasing interest in various fields, including wireless communication, smart lighting, and navigation systems. With the ongoing development of LED technology and signal processing algorithms, VLC and ILC promise to play an increasingly important role in the modern communications landscape.

[0010] VLC and ILC are data communication methods that modulate a light signal using a control signal. This control signal is defined so that the modulated light signal flashes at a specific frequency. By varying the frequency, it is possible to transmit data in the form of symbols. This is known as modulation of the light signal by the control signal. The control signal is usually a PWM (Pulse Width Modulation) signal consisting of a series of pulses defining an alternation of high and low levels. Each high level corresponds to a high intensity level of the light signal (the "on" signal), and each low level corresponds to a low intensity level of the light signal (the "off" signal). The modulated light signal is then demodulated to recover the transmitted symbol(s).The modulation (and demodulation) is carried out according to a modulation rate which defines whether this modulation is carried out on part or all of the light power.

[0011] Modulating a light signal induces a flickering effect, which can be visible (when the light signal is in the visible spectrum) and therefore bothersome to the user. This flickering is regulated by standards such as IEEE 1789 (https: / / www.energy.gov / eere / ssl / articles / flicker-understanding-new-ieee-recommended-practice) or NF EN 12464-1. These standards recommend, in particular, using frequencies above 2 kHz for a modulation rate of 100% to avoid visible flickering. However, the inventor observed that flickering can be visible when the frequencies used are just above 2 kHz. To prevent this flickering from being visible, it is known to use frequencies well above 2 kHz, such as frequencies above 1 MHz.However, the use of this type of high frequency causes premature aging of lighting equipment because the high frequencies put a very high stress on this equipment, causing it to overheat.

[0012] One of the problems solved by the present invention is to modulate / demodulate a light signal (belonging to the visible or infrared spectrum) in such a way as to preserve lighting equipment which then operates in a nominal regime. Summary of the present invention

[0013] One object of the present invention is to solve at least one of the problems of the technological background described above.

[0014] According to a first aspect, the present invention relates to a method of modulating a light signal for the emission of a symbol, the method being controlled by a control signal comprising a succession of time cycles, each time cycle being defined by an alternation of a high level corresponding to a high level of light intensity of the modulated light signal maintained for a first duration and a low level corresponding to a low level of light intensity of the light signal maintained for a second duration, a total duration of each time cycle being defined from a frequency, the method is characterized in that it comprises the following steps:- determination of a pair of frequencies representing the symbol;- determination of the succession of time cycles comprising at least a first time cycle followed by at least a second time cycle;- each first time cycle having a total duration defined from the first frequency of the determined frequency pair; - each second time cycle having a total duration defined from the second frequency of the determined frequency pair; - the first and second frequencies of the determined frequency pair being determined so that the highest frequency, among the first and second frequencies, is at most equal to twice the lowest frequency, among said first and second frequencies.

[0015] According to a particular and non-limiting example of an embodiment, the succession of time cycles further comprises at least one third time cycle, said at least one third time cycle preceding the first of said at least one first time cycle of the succession of time cycles, each third time cycle being of a total duration defined from a symbol start frequency, the symbol start frequency being determined so that the highest frequency, among the symbol start frequency and the first frequency, is at most equal to twice the lowest frequency, among the first frequency and the symbol start frequency.

[0016] According to a particular and non-limiting embodiment, the sequence of cycles of the control signal further comprises at least one fourth time cycle, said at least one fourth time cycle being following the last of said at least second symbol, each fourth time cycle being of a total duration defined from a symbol end-of-emission frequency, the symbol end-of-emission frequency being determined so that the highest frequency, among the second frequency and the symbol end-of-emission frequency, is at most equal to twice the lowest frequency, among the second frequency and the symbol end-of-emission frequency.

[0017] According to a particular and non-limiting example of an embodiment, the first frequency, the second frequency, the symbol start frequency and the symbol end frequency are determined from a set of frequencies and the highest frequency in the set of frequencies is at most equal to twice the lowest frequency in the set of frequencies.

[0018] According to a particular and non-limiting example of an embodiment, the succession of time cycles of the control signal further comprises at least one fifth time cycle following the last of said at least one first time cycle, at least one fifth time cycle following the last of said at least one second time cycle, at least one fifth time cycle following the last of said at least one third time cycle and at least one fifth time cycle following the last of said at least one fourth time cycle, each fifth time cycle being of a total duration defined from a cutoff frequency defined as the median value between the highest and lowest frequencies of the set of frequencies, the highest frequency of the set of frequencies being at most equal to four times the lowest frequency of the set of frequencies.

[0019] According to a particular and non-limiting example of implementation, the first, second, third, fourth and fifth time cycles are each repeated a defined number of times from a frequency change frequency.

[0020] According to a particular and non-limiting example of an embodiment, the frequency of frequency change varies according to lighting conditions of an environment in which the light signal is diffused.

[0021] According to a particular and non-limiting example of implementation, a duty cycle of each time cycle of the control signal is equal to the same value within a deviation around that value.

[0022] According to a second aspect, the present invention relates to a method for demodulating a light signal for the reception of a symbol, the method is characterized in that it comprises the following steps: - detection of a first frequency of a pair of frequencies from a frequency spectrum of the light signal determined from a first set of samples of the light signal; - detection of a second frequency of the pair of frequencies from a frequency spectrum of the light signal determined from a second set of samples of the light signal; - detection of the received symbol from the pair of frequencies; - the first frequency and the second frequency being detected such that the highest frequency, among the first and second frequencies, is at most equal to twice the lowest frequency, among said first and second frequencies.

[0023] According to a particular and non-limiting example of embodiment, the method further comprises a step of detecting a symbol reception start frequency from a frequency spectrum of the light signal determined from a third set of light signal samples, the detection of the symbol reception start frequency preceding the detection of the first frequency of the frequency pair.

[0024] According to a particular and non-limiting example of an embodiment, the method further comprises a step of detecting a symbol reception end frequency from a frequency spectrum of the light signal determined from a fourth set of light signal samples, the detection of the symbol reception end frequency following the detection of the second frequency of the frequency pair.

[0025] According to a particular and non-limiting embodiment example, the first frequency, the second frequency, the symbol reception start frequency and the symbol reception end frequency belong to a set of frequencies whose highest frequency is at most equal to twice the lowest frequency of the set of frequencies.

[0026] According to a particular and non-limiting example of an embodiment, the method further comprises a step of determining a cutoff frequency from a frequency spectrum of the light signal determined from a fifth set of samples of the light signal, the cutoff frequency is defined as a median value between the highest and lowest frequencies of the set of frequencies, the highest frequency of the set of frequencies being at most equal to four times the lowest frequency of the set of frequencies, the cutoff frequency being determined following the determination of the first frequency, following the determination of the second frequency, following the determination of the symbol reception start frequency and following the determination of the symbol reception end frequency.

[0027] According to a particular and non-limiting example of embodiment, the frequency spectrum is determined by spectral analysis of samples from the first, second, third, fourth or fifth set of samples of the light signal.

[0028] According to a particular and non-limiting example of an embodiment, the detection of the first frequency, the second frequency, the symbol reception start frequency, the symbol reception end frequency or the cutoff frequency from the frequency spectrum of the light signal determined from the samples respectively of the first, second, third, fourth or fifth set, comprises the following steps:- evaluation of scores associated with the frequencies of the set of frequencies from the amplitudes of the corresponding frequencies of the frequency spectrum determined from the samples respectively of the first, second, third, fourth or fifth set;- identification of the first frequency, the second frequency, the symbol reception start frequency, the symbol reception end frequency or the cutoff frequency among the frequencies in the set of frequencies from the scores associated with the frequencies in the set of frequencies.;

[0029] According to a particular and non-limiting embodiment example, the frequencies of the frequency set are less than 5kHz.

[0030] According to a particular and non-limiting example of implementation, the symbol is determined from a table associating a pair of frequencies and a symbol value.

[0031] According to a third aspect, the present invention relates to a light signal emitting device comprising a microcontroller configured for the implementation of the steps of the process according to the first aspect of the present invention.

[0032] According to a fourth aspect, the present invention relates to a light signal receiving device comprising a microcontroller configured for the implementation of the steps of the process according to the second aspect of the present invention.

[0033] According to a fifth aspect, the present invention relates to a computer program which includes instructions adapted for carrying out the steps of the process according to the first and / or the second aspect of the present invention, in particular when the computer program is executed by at least one processor.

[0034] Such a computer program can use any programming language, and be in the form of source code, object code, or an intermediate form between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0035] According to a sixth aspect, the present invention relates to a computer-readable recording medium on which is recorded a computer program comprising instructions for carrying out the steps of the process according to the first and / or second aspect of the present invention.

[0036] On the one hand, the recording medium can be any entity or device capable of storing the program. For example, the medium can include a storage means, such as ROM, RAM, CD-ROM or microelectronic circuit-type ROM, or a magnetic recording means or a hard drive.

[0037] On the other hand, this recording medium can also be a transmissible medium such as an electrical or optical signal, such a signal being able to be transmitted via an electrical or optical cable, by conventional or radio frequency, by self-directing laser beam, or by other means. The computer program according to the present invention can, in particular, be downloaded from a network such as the Internet.

[0038] Alternatively, the recording medium may be an integrated circuit in which the computer program is incorporated, the integrated circuit being adapted to execute or to be used in the execution of the process in question. Brief description of the figures

[0039] Other features and advantages of the present invention will become apparent from the description of the specific and non-limiting embodiments of the present invention below, with reference to the attached Figures 1 to 20, in which:

[0040] schematically illustrates an example of the time evolution of a control signal used to control the modulation of a light signal according to a particular and non-limiting embodiment of the present invention.

[0041] schematically illustrates an example of time evolution of a control signal used to control the modulation of a light signal when the first frequency F1 of the frequency pair is equal to twice the second frequency F2 of the frequency pair according to a particular and non-limiting embodiment of the present invention.

[0042] schematically illustrates an example of time evolution of a control signal used to control the modulation of a light signal when the first frequency F1 of the frequency pair is greater than the second frequency F2 of the frequency pair according to a particular and non-limiting embodiment of the present invention.

[0043] schematically illustrates an example of the time evolution of a control signal used to control the modulation of a light signal when the first frequency, replaced by a second frequency during the modulation of the light signal, is lower than the second frequency according to a particular and non-limiting embodiment of the present invention.

[0044] illustrates a flowchart of the different stages of a process for modulating a light signal for the emission of a symbol, according to a particular embodiment of the present invention.

[0045] illustrates a schematic example of the time evolution of a control signal used to control the modulation of a light signal according to a particular and non-limiting embodiment of the present invention.

[0046] illustrates a schematic example of the time evolution of a control signal used to control the modulation of a light signal according to a particular and non-limiting embodiment of the present invention.

[0047] illustrates a schematic example of the time evolution of a control signal used to control the modulation of a light signal according to a particular and non-limiting embodiment of the present invention.

[0048] illustrates a schematic example of the time evolution of a control signal used to control the modulation of a light signal according to a particular and non-limiting embodiment of the present invention.

[0049] illustrates a schematic example of the time evolution of a control signal used to control the modulation of a light signal according to a particular and non-limiting embodiment of the present invention.

[0050] illustrates examples of PWM signals with duty cycles ranging from 0% (zero light intensity level) to 100% (maximum light intensity level).

[0051] illustrates an example of a PWM signal with a constant duty cycle of 50%.

[0052] illustrates a flowchart of the different stages of a process for demodulating a light signal for the reception of a symbol, according to a particular embodiment of the present invention.

[0053] illustrates an example of steps 310, 320, 340 and 360 of method 300 of demodulating a light signal for the reception of a symbol, according to a particular embodiment of the present invention.

[0054] illustrates an example of a frequency spectrum obtained from a discrete Fourier transform spectral analysis of a set of samples.

[0055] illustrates an example of updating the scores associated with the frequencies of the frequency set from three spectra determined from three consecutive sets of samples, according to a particular and non-limiting embodiment of the present invention.

[0056] illustrates an example of a base 64 table that groups 64 associations between symbols and pairs of frequencies, according to a particular and non-limiting embodiment of the present invention.

[0057] schematically illustrates a device 1 configured to emit and / or receive at least one symbol carried by a light signal modulated by a control signal, according to a particular and non-limiting embodiment of the present invention.

[0058] schematically illustrates an implementation of the device 2 for emitting light signals configured to emit at least one symbol carried by a light signal modulated by the control signal according to a particular and non-limiting example of the present invention.

[0059] schematically illustrates an implementation of the light signal reception device 3 configured to receive at least one symbol carried by a light signal modulated by the control signal 100 according to a particular and non-limiting example of the present invention. Description of examples of achievements

[0060] A method and device for modulating and demodulating a light signal, the transmission of a symbol, will now be described in what follows with joint reference to figures 1 to 20. The same elements are identified with the same reference signs throughout the description that follows.

[0061] The terms "first(s)", "second(s)" (or "first(s)", "second(s)"), etc. are used in this document by arbitrary convention to allow identification and distinction of different elements (such as operations, means, frequency pairs, etc.) implemented in the embodiments described below.

[0062] Subsequently, a light signal is a signal that can belong to the visible spectrum or the infrared spectrum.

[0063] Infrared light (the infrared spectrum) is classified into three groups: - Near Infrared (NIR): This band covers wavelengths from 0.75 to 1.4 micrometers (750 to 1400 nanometers). Typical applications include remote sensing, fiber optic communication, and certain forms of medical imaging; - Mid Infrared (MIR): Wavelengths in this band range from 1.4 to 3 micrometers (1400 to 3000 nanometers). The MIR is used in various applications such as spectroscopy for chemical analysis and gas detection, as well as for certain heating technologies.- Far Infrared (FIR): This band extends from 3 micrometers to about 1000 micrometers (3000 nanometers to 1 millimeter), although the precise classification may vary.FIR is primarily used for heating applications and certain types of remote sensing and thermal imaging.

[0064] For example, light signals may belong to the near-infrared frequency band, in the 750-950 nm range, with a peak at 850 nm.

[0065] The present invention relates to a method of modulating a light signal modulated by a PWM type control signal for the emission of a symbol and a method of demodulating the modulated light signal for the reception of said symbol.

[0066] The modulation and demodulation methods according to the present invention make it possible to transmit a light signal between a light signal emitting device and a light signal receiving device without generating visible flickering (when these light signals belong to the visible spectrum) for a user and without prematurely aging the emitting and receiving devices.

[0067] The modulation and demodulation processes according to the present invention make it possible to optimize the energy efficiency and lifespan of light signal emitting and receiving devices.

[0068] The modulation and demodulation methods according to the present invention use low frequencies (well below 1 MHz and close to 2 kHz) which optimize the efficiency of both the light signal transmitter and receiver devices, allowing lighting equipment to operate at its nominal capacity and thus preserving its lifespan. The transmission duration of the light signals modulated by these low frequencies improves the probability of correctly reading each frequency used for modulating the emitted light signals.

[0069] Laillustre schematically illustrates an example of the time evolution of a control signal 100 used to control the modulation of the light signal according to a particular and non-limiting embodiment of the present invention.

[0070] The control signal 100 is of the PWM type. A PWM signal is a gate signal consisting of a succession of time cycles. Each time cycle is defined by an alternation of a high level, corresponding to a high intensity level of the light signal (the "on" light signal), maintained for a first duration 101, and a low level, corresponding to a low intensity level of the light signal (the "off" light signal), maintained for a second duration 102. A total duration 103 of each time cycle is defined from a frequency F i The total duration of a time cycle is equal to the inverse of a frequency.

[0071] The light signal is modulated by the control signal 100 (step 130). For this purpose, the light signal is at a high level of light intensity (“on”) when the control signal 100 is at a high level and at a low level of light intensity (“off”) when the control signal 100 is at a low level.

[0072] According to a particular and non-limiting embodiment of the present invention, the time cycle of duration 10³ can be repeated several times. The number of repetitions is defined by the inverse of a frequency change rate.

[0073] For example, the frequency change frequency is at least equal to 100Hz to ensure a frequency change speed of less than 10 ms and thus avoid the appearance of visible flickering of the light signal (when this light signal belongs to the visible spectrum).

[0074] The repetition of the time cycle increases the reliability of demodulation of the modulated light signal.

[0075] Furthermore, excessively slow frequency changes can impact the lighting level. Additionally, a change frequency below 100 Hz induces visible flickering of the modulated light signal (when this light signal belongs to the visible spectrum).

[0076] According to a particular and non-limiting embodiment of the present invention, the frequency of change can vary depending on the lighting conditions of an environment in which the light signal is emitted.

[0077] For example, the lower the lighting conditions, e.g., the lighting conditions in the immediate environment of the light signal emitting device indicate low brightness, and the higher the number of repetitions, i.e., a low frequency of frequency change.

[0078] For example, the frequency of frequency changes varies proportionally to the ambient brightness of the light signal emitting device.

[0079] For example, if the frequency change is 1 / 3, then the time cycle, with a total duration of 10³, is repeated three times. The light signal modulated by the control signal 100 flashes according to the frequency F. i for a duration of 104. Another frequency F j can then replace the frequency F i so that the light signal flashes according to the frequency F j The frequencies F i and F j can be equal or different.

[0080] According to a particular and non-limiting embodiment of the present invention, the control signal 100 can be determined from a pair of frequencies (F1, F2) chosen to represent a symbol. The first frequency F1 and the second frequency F2 of the chosen frequency pair (F1, F2) are determined such that the higher of the two frequencies is at most twice the lower of the two frequencies.

[0081] This constraint on the frequencies F1 and F2 of the determined frequency pair (F1, F2) ensures that during a frequency change in the modulation of the light signal, the ratio of the lower of the two frequencies F1 and F2 to the higher of the two frequencies F1 and F2 is at least 50%. For example, if the first frequency F1 is higher than the second frequency F2, then the ratio F2 / F1 is at least 50%. If the second frequency F2 is higher than the first frequency F1, then the ratio F1 / F2 is at least 50%.

[0082] For example, F1 = 2kHz and F2 = 3.5kHz to comply with this constraint.

[0083] Respecting the constraint on the frequencies F1 and F2 avoids the appearance of a visible flickering effect of the light signal (when it belongs to the visible spectrum).

[0084] Laillustrate schematically an example of time evolution of a control signal used to control the modulation of a light signal when the first frequency F1 of the frequency pair is equal to twice the second frequency F2 of the frequency pair according to a particular and non-limiting embodiment of the present invention.

[0085] The control signal 100 is described here with a frequency change frequency of 1 / 3, i.e. a time cycle of duration 1031 defined by the first frequency F1 is repeated three times (total duration 1041) and a time cycle of duration 1032 defined by the second frequency F2 is repeated three times (duration 1042).

[0086] According to this example, the first frequency F1 of the frequency pair is equal to twice the second frequency F2, i.e. F1=2*F2, and the duration 1032 is equal to twice the duration 1031. The duration 1042 is equal to twice the duration 1041. The ratio F2 / F1=50%, which verifies the constraint on the frequencies F1 and F2 that prevents visible flickering of the light signal (when this light signal belongs to the visible spectrum).

[0087] Laillustrate schematically an example of time evolution of a control signal used to control the modulation of a light signal when the first frequency F1 of the frequency pair is greater than the second frequency F2 of the frequency pair according to a particular and non-limiting embodiment of the present invention.

[0088] The control signal 100 is described here with a frequency change frequency of 1 / 3, that is to say that a time cycle of duration 1031 defined by the first frequency F1 is repeated three times (total duration 1041) and a time cycle of duration 1032 defined by the second frequency F2 is repeated three times (duration 1042).

[0089] According to this example, for the constraint on the frequencies F1 and F2 of the determined frequency pair (F1, F2) to be verified, the first frequency F1 of the frequency pair must be less than twice the second frequency F2, i.e. F1 < 2 * F2, and the duration 1032 is then less than twice the duration 1031. The duration 1042 is less than twice the duration 1041. The ratio F2 / F1 > 50%, which verifies the constraint on the frequencies F1 and F2, preventing visible flickering of the light signal (when this light signal belongs to the visible spectrum).

[0090] Laillustrate schematically an example of time evolution of a control signal used to control the modulation of a light signal when the first frequency F1 of the frequency pair is less than the second frequency F2 of the frequency pair according to a particular and non-limiting embodiment of the present invention.

[0091] The control signal 100 is described here with a frequency change frequency of 1 / 3, that is to say that a time cycle of duration 1031 defined by the first frequency F1 is repeated three times (total duration 1041) and a time cycle of duration 1032 defined by the second frequency F2 is repeated three times (duration 1042).

[0092] According to this example, for the constraint on the frequencies F1 and F2 of the determined frequency pair (F1, F2) to be verified, the second frequency F2 of the frequency pair must be less than twice the first frequency F1, i.e. F2 < 2 * F1, and the duration 1031 is then less than twice the duration 1032. The duration 1041 is less than twice the duration 1042. The ratio F1 / F2 > 50%, which verifies the constraint on the frequencies F1 and F2, which prevents visible flickering of the light signal (when this light signal belongs to the visible spectrum).

[0093] Laillustre un flowchart des différents étapes d’un processus 200 de modulation d’un signal lumière pour le emission d’un symbole, selon une exemple d’incarnation particulier de la présent invention.

[0094] In a first step 210, a pair of frequencies (F1, F2) is determined to represent the symbol. The first frequency F1 and the second frequency F2 of the determined pair of frequencies (F1, F2) are determined such that the higher frequency, among the first and second frequencies, is at most equal to twice the lower frequency, among said first and second frequencies.

[0095] The control signal 100 comprises a succession of time cycles.

[0096] In a second step 220, the sequence of time cycles of the control signal 100 is determined. The sequence of time cycles comprises at least one first time cycle s1 followed by at least one second time cycle s2 as illustrated in Figure 1. Each first time cycle s1 has a total duration of 10³¹, defined from the first frequency F1 of the determined frequency pair (F1, F2). Each second time cycle s2 has a total duration of 10³², defined from the second frequency F2 of the determined frequency pair (F1, F2). The first frequency F1 and the second frequency F2 of the determined frequency pair (F1, F2) are determined such that the higher of the two frequencies is at most twice the lower of the two frequencies.

[0097] According to an example embodiment, illustrated in Figure 1, the succession of time cycles of the control signal 100 may further include at least one third time cycle s3. This at least one third time cycle s3 precedes the first of this at least one first time cycle s1 of the succession of time cycles of the control signal 100. Each third time cycle s3 has a total duration 1033 defined from a starting frequency F s symbol emission start frequency. The symbol emission start frequency is determined so that the highest frequency, between the first frequency (F1) and the symbol emission start frequency, is at most equal to twice the lowest frequency, between the first frequency (F1) and the symbol emission start frequency.

[0098] The number of the third time cycle s3 is defined from the frequency of frequency change.

[0099] The time sequence of frequencies used for modulating the light signal is then given by [F s ][F 1sy ][F 2sy ].

[0100] According to an example embodiment, illustrated here, the succession of time cycles of the control signal 100 may further include at least a fourth time cycle s4. Said at least a fourth time cycle s4 following the last of said at least a second time cycle s2. Each fourth time cycle s4 has a total duration 1034 defined from an end frequency F e symbol emission frequency. The symbol emission end frequency is determined so that the highest frequency, between the second frequency and the symbol emission end frequency, is at most equal to twice the lowest frequency, between the second frequency and the symbol emission end frequency.

[0101] The number of the fourth time cycle s4 is defined from the frequency of frequency change.

[0102] The time sequence of frequencies used for modulating the light signal is then given by [F 1sy ][F 2sy ][F e ].

[0103] Laillustre un example de exemplement schematicement un example de temps évolution d'un signal de contrôle servir pour contrôle de modulation d'un signal lumière selon une exemplaire particulière et non limite de l'présent invention.

[0104] In this example, the control signal 100 comprises the first, second, third, and fourth time cycles described above. The control signal 100 is described here with a frequency change of 1 / 3, i.e., each time cycle is repeated three times.

[0105] According to a particular and non-limiting embodiment of the present invention, the first frequency F1 and the second frequency F2 can be determined from a set F of frequencies and the highest frequency of the set F of frequencies is at most equal to twice the lowest frequency of the set F of frequencies.

[0106] According to a particular and non-limiting embodiment of the present invention, the first frequency F1, the second frequency F2 and the starting frequency F s can be determined from a set F of frequencies and the highest frequency of the set F of frequencies is at most equal to twice the lowest frequency of the set F of frequencies.

[0107] According to a particular and non-limiting embodiment of the present invention, the first frequency F1, the second frequency F2 and the final frequency F ecan be determined from a set F of frequencies and the highest frequency of the set F of frequencies is at most equal to twice the lowest frequency of the set F of frequencies.

[0108] According to a particular and non-limiting embodiment of the present invention, the first frequency F1, the second frequency F2, the starting frequency F s and the end frequency F e can be determined from a set F of frequencies and the highest frequency of the set F of frequencies is at most equal to twice the lowest frequency of the set F of frequencies.

[0109] According to a particular and non-limiting embodiment of the present invention, the succession of time cycles of the control signal 100 may further comprise at least one fifth time cycle s5 following the last of said at least one first time cycle s1 and at least one fifth time cycle s5 following the last of said at least one second time cycle s2, each fifth time cycle s5 having a total duration 1035 defined from a cutoff frequency F c defined as the median value between the highest and lowest frequencies of the set of frequencies, the highest frequency of the set of frequencies being at most equal to four times the lowest frequency of the set of frequencies.

[0110] If the control signal 100 also includes at least a third time cycle s 3,the control signal 100 may further include at least one fifth time cycle s5, said at least one fifth time cycle s5 being following the last of said at least one third time cycle s3.

[0111] If the control signal 100 further includes at least a fourth time cycle s 4, the control signal 100 may further include at least one fifth time cycle s5, said at least one fifth time cycle s5 being following the last of said at least one fourth time cycle s4.

[0112] According to a particular and non-limiting embodiment of the present invention, the first, second, third, fourth and fifth time cycle can each be repeated a number of times defined from the frequency of frequency change.

[0113] Laillustre un example de exemplement schematicement un example de temps évolution d un signal de contrôle servir pour contrôle de modulation d signale lumière selon un example d’incarnation particulier et non limite de la présent invention.

[0114] According to this embodiment, the frequency change is 1, meaning that the sequence of time cycles of the control signal 100 comprises a single time cycle per frequency. The control signal consists of a first cycle s1, a second cycle s2, a third time cycle s3, and a fourth time cycle s4. The first, second, third, and fourth frequencies from which the durations of the first, second, third, and fourth time cycles can be arbitrary; that is, the lowest and highest frequencies of the set F of predetermined frequencies can be arbitrary. Replacing the first frequency F1 with the frequency F2, for example, can then produce a flickering of the light signal (when this light signal belongs to the visible spectrum).To prevent visible blinking from occurring when the first frequency F1 changes to the second frequency F2, a fifth time cycle s5 is added between the first time cycle s1 and the second time cycle s2 if the frequency change frequency is equal to 1, or between the last of the first time cycles s1 and the first of the second time cycles s2 if the frequency change frequency is greater than 1. The duration of this fifth time cycle s5 is defined from a cutoff frequency defined as the median value between the highest and lowest frequencies of the set F of frequencies.Thus, replacing the first frequency F1 with the cutoff frequency and then replacing the cutoff frequency with the second frequency F2 satisfies the frequency constraint and prevents the light signal from flickering, which would occur if the first frequency F1 were replaced by the second frequency F2 without passing through the cutoff frequency. The same applies to any other frequency change between two frequencies, particularly between the symbol's start frequency and the first frequency F1, or between the second frequency F2 and the symbol's end frequency.

[0115] The light intensity of a PWM-modulated light signal varies according to a duty cycle defined by the ratio of the duration of a high level of the PWM signal to the duration of a low level of the PWM signal.

[0116] The illustration shows examples of PWM signals with a period of pPWM and duty cycles (DC) ranging from 0% (light intensity level read) to 100% (maximum light intensity level). The dotted line on the illustration represents a resulting light intensity level.

[0117] According to a particular and non-limiting embodiment of the present invention, the duty cycle can be at most equal to 85%.

[0118] This particular embodiment prevents the equipment from being subjected to excessive stress and the risk of overheating. For example, if the highest frequency in the set F of frequencies is 5 kHz, the duration of each time cycle will not exceed 30 ms, thus preventing the light signal emitter and receiver devices implementing the present invention from overheating.

[0119] According to a particular and non-limiting embodiment of the present invention, the duty cycle can be at least equal to 15%.

[0120] This example is advantageous because it guarantees that the light signal receiving device receives enough light to demodulate that light signal.

[0121] According to a particular and non-limiting embodiment of the present invention, the duty cycle of each time cycle of the control signal 100 can be equal to the same value within a deviation around this value, i.e. that this duty cycle is quasi-constant.

[0122] This illustrates an example of a PWM signal with a constant duty cycle of 50%.

[0123] Maintaining a constant duty cycle avoids effects of variation in light intensity, and therefore visible flickering (when this light signal belongs to the visible spectrum).

[0124] According to a particular and non-limiting embodiment of the present invention, the frequency change can be triggered synchronously with a rising or falling edge of the control signal 100.

[0125] This example allows the duty cycle to remain constant.

[0126] Laillustre un flowchart des différents étapes d’un processus 300 demodulation de un signal lumière pour la réception d’un symbole, selon une exemple d’incarnation particulier de la présent invention.

[0127] The light intensity of the light signal is modulated by a control signal 100 consisting of a succession of time cycles. Each time cycle is defined by an alternation between a high level, corresponding to a high level of light intensity of the light signal (the "on" signal), maintained for a first duration, and a low level, corresponding to a low level of light intensity of the light signal (the "off" signal), maintained for a second duration. The total duration of each time cycle is defined from a frequency within a set F of frequencies. For example, the modulation of the light signal is controlled by the control signal 100 generated by the process of...

[0128] In a step 310, a first frequency F1 of a pair of frequencies is detected from a frequency spectrum of the light signal determined from a first set E1 of samples of the light signal.

[0129] In a step 320, a second frequency F2 of the frequency pair is detected from a frequency spectrum of the light signal determined from a second set E2 of samples of the light signal.

[0130] In step 330, the symbol is determined from the pair of detected frequencies.

[0131] The first frequency F1 and the second frequency F2 of the frequency pair are detected so that the highest frequency, among the first and second frequencies, is at most equal to twice the lowest frequency, among said first and second frequencies.

[0132] According to a particular and non-limiting embodiment of the present invention, in a step 340, a starting frequency F sThe reception of the symbol can be detected from a frequency spectrum of the light signal determined from a third set E3 of light signal samples, the detection of the starting frequency F s reception of the symbol preceding the detection of the first frequency of the frequency pair.

[0133] Detection of the starting frequency F s The reception of the symbol indicates that the next two frequencies that will be detected will indicate the received symbol.

[0134] According to a particular and non-limiting embodiment of the present invention, in a step 350, a final frequency F e The reception of the symbol can be detected from a frequency spectrum of the light signal determined from a fourth set E4 of light signal samples, the detection of the end frequency F ereception of the symbol following the detection of the second frequency of the frequency pair.

[0135] The detection of the end frequency F e The reception of the symbol indicates the end of the light signal demodulation process for symbol reception.

[0136] According to a particular and non-limiting embodiment of the present invention, the first frequency, the second frequency, the starting frequency F s symbol reception and termination frequency F e reception of the symbol may belong to the set F of frequencies whose highest frequency is at most equal to twice the lowest frequency of the set of frequencies.

[0137] For example, the frequency set F includes frequencies ranging from 2kHz to 4kHz.

[0138] According to a particular and non-limiting embodiment of the present invention, in a sixth step 360, a cutoff frequency F c can be detected from a frequency spectrum of the light signal determined from a fifth set E5 of light signal samples, the cutoff frequency F c can be defined as a median value between the highest and lowest frequencies of the entire frequency range, the highest frequency of the entire frequency range being at most equal to four times the lowest frequency of the entire frequency range, the cutoff frequency F c being determined following the detection of the first frequency and following the detection of the second frequency.

[0139] According to one variant, the cutoff frequency F c can also be detected following the detection of the starting frequency F s .

[0140] According to one variant, the cutoff frequency F c can also be detected following the detection of the end frequency F e .

[0141] The use of the cutoff frequency F c is advantageous because it allows increasing the amplitude of the entire frequency range F compared to the previous implementation example which does not use a cutoff frequency F c .

[0142] For example, the frequency set F includes frequencies ranging from 2kHz to 8kHz.

[0143] Furthermore, the use of the cutoff frequency F cThis allows the received light signal to be demodulated without needing to know a parameter of the modulation process, because the stoppage of the score update, described later, is determined by the time interval between the detection of two consecutive cutoff frequencies. This time interval can vary depending on, for example, the lighting conditions in the environment of the light signal emitter.

[0144] According to a particular and non-limiting embodiment of the present invention, the frequencies of the set F of frequencies are less than 5kHz.

[0145] For example, the F frequency set includes frequencies ranging from 2kHz to 5kHz and the FM median frequency is equal to 3.5kHz.

[0146] According to a particular and non-limiting embodiment of the present invention, the frequency spectrum can be determined by spectral analysis of samples from the first, second, third, fourth or fifth set of samples of the light signal.

[0147] According to a particular and non-limiting embodiment of the present invention, the detection of the first frequency (step 310), the second frequency (step 320), the starting frequency F s symbol reception (step 340), end frequency F e symbol reception (step 350) or cutoff frequency F c (step 360) from the frequency spectrum of the light signal determined from the samples respectively of the first, second, third, fourth or fifth set, may include steps 370 and 380.

[0148] Laillustre an example of steps 310, 320, 340, 350 and 360 of process 300 of demodulating a light signal for the reception of a symbol, according to a particular embodiment of the present invention.

[0149] Steps 310, 320, 340, and 360 each detect a frequency F r (first frequency F1, second frequency F2, starting frequency F) s symbol reception frequency, end frequency F e symbol reception, cutoff frequency F c ) from a set of samples E i (i=1 to 5).

[0150] In step 370, SCi scores associated with the frequencies of the frequency set F are evaluated from the amplitudes of the corresponding frequencies of the frequency spectrum determined on the sample set E i .

[0151] According to a particular and non-limiting embodiment of the present invention, step 370 comprises steps 371 to 373.

[0152] In step 371, a frequency spectrum SPi of the light signal is determined by spectral analysis of continuously received light signal samples. This means that the spectral analysis is performed over a sliding time window so that the frequency spectrum is updated as light signal samples become available. Thus, the first set Ei (i=1) of available samples is formed, and a frequency spectrum of the light signal is determined. At least one new sample becomes available, and the second set of samples Ei (i=2) is formed from this at least one new sample and, possibly, from at least one sample from the first set E1. At least one new sample becomes available, and the third set Ei (i=3) of samples is formed from this at least one new sample and, possibly, from at least one sample from the second set E2, and so on.

[0153] For example, the received light signal is sampled at a frequency of 44kHz and each set of samples comprises 128 samples (N=128).

[0154] According to a particular and non-limiting embodiment of the present invention, the frequency spectrum of the light signal is determined by applying a discrete Fourier transform to samples of a set of samples of the received light signal.

[0155] To do this, N samples x(n) (n=0,…, N-1) of the received light signal are defined on the sliding time window of duration T=N*Te with Fe=1 / Te. N points S(k) of the frequency spectrum are then determined on this time window of duration T by applying the discrete Fourier transform, each of these points S(k) corresponding to a particular frequency and amplitude of a frequency component of the frequency spectrum:

[0156]

[0157] Figure 1 illustrates an example of a frequency spectrum SPi obtained from a discrete Fourier transform spectral analysis of the sample set Ei. The frequency spectrum is represented in a two-dimensional space where the x-axis represents the frequencies f composing the spectrum and the y-axis represents the amplitudes a corresponding to these frequencies. In this example, the frequency spectrum is determined from N=11 samples.

[0158] In step 372, the SCi scores associated with the frequencies of the F set of frequencies are initialized, for example to 0.

[0159] In step 373, the SCi score of each frequency in the set F of frequencies is updated from the amplitudes of the corresponding frequencies in the frequency spectrum determined during an evaluation period.

[0160] This illustrates an example of updating the SCi scores associated with the frequencies of the frequency set F from three SPi spectra determined on two sets E i of consecutive samples according to a particular and non-limiting embodiment of the present invention.

[0161] According to the example, the set F of frequencies comprises three frequencies F1, F C and F2. At a), the SC1 score associated with the frequency F1 is updated from the amplitude a1 of the first of the three frequency spectra, the SC score c of the cutoff frequency F c is updated from the amplitude a c The SC2 score for frequency F2 is updated from the amplitude a2 of the first of the three frequency spectra. In lab), the SC1 score associated with frequency F1 is updated from the amplitude corresponding to frequency F1 of the second of the three frequency spectra.c of the cutoff frequency F c is updated from the amplitude a c corresponding to the cutoff frequency F c The second of the three frequency spectra and the SC2 score of frequency F2 is updated from the amplitude a2 corresponding to frequency F2 of the second of the three frequency spectra. At a), the SC1 score associated with frequency F1 is updated from the amplitude corresponding to frequency F1 of the third of the three frequency spectra, the SC score c of the cutoff frequency F c is updated from the amplitude a c corresponding to the cutoff frequency F c of the third of the three frequency spectra and the SC2 score of the frequency F2 is updated from the amplitude a2 corresponding to the frequency F2 of the third of the three frequency spectra.

[0162] As an example, an SCi score associated with a frequency from the F set of frequencies is updated by adding the amplitude of that frequency from the frequency spectrum.

[0163] In step 380, at the end of the evaluation period, the frequency Fr is detected among the frequencies of the set F of frequencies from the SCi scores associated with the frequencies of the set F of frequencies.

[0164] For example, the frequency associated with the highest score is the detected frequency Fr.

[0165] According to a particular and non-limiting embodiment of the present invention, the symbol is represented by a pair of frequencies (F1, F2) whose association is stored in a reference frequency table.

[0166] Laillum illustrates an example of a base-64 table that groups 64 associations between symbols and frequency pairs, according to a particular embodiment of the present invention. In this example, the symbols A, B, etc., are represented by frequency pairs expressed in binary form, in which each frequency is identified by three bits. For example, the symbol 'E' is associated with the binary word 000100, where 000 identifies the first frequency of the frequency pair and 100 identifies the second frequency of the frequency pair.

[0167] The diagram schematically illustrates a device 1 configured to modulate and / or demodulate a light signal from a control signal, according to a particular and non-limiting embodiment of the present invention.

[0168] Device 1 is, for example, configured to carry out the operations described opposite the steps of at least one of the processes described opposite Figures 5, 13, and 14. Examples of such a device 3 include, but are not limited to, electronic equipment used, for example, for lighting an enclosed environment. The elements of device 1, individually or in combination, can be integrated into a single integrated circuit, into several integrated circuits, and / or into discrete components. Device 1 can be implemented as electronic circuits or software (or computer) modules, or a combination of electronic circuits and software modules.

[0169] Device 1 comprises one (or more) microprocessor(s) 10 configured to execute instructions for carrying out the steps of at least one of the processes described opposite Figures 5, 13, and 14. The microprocessor 10 may include integrated memory, an input / output interface, and various circuits known to those skilled in the art. Device 1 may further comprise at least one memory 11, for example, volatile and / or non-volatile memory, and / or include a memory storage device that may comprise volatile and / or non-volatile memory, such as EEPROM, ROM, PROM, RAM, DRAM, SRAM, flash, magnetic disk, or optical disk.

[0170] The computer code comprising the instructions corresponding to the steps of at least one of the processes described opposite figures 5, 13 and 14 which it is necessary to load and execute by the microprocessor 10 is for example stored on memory 11.

[0171] According to various specific and non-limiting embodiment examples, device 1 can be configured to be coupled in communication with other similar devices or systems and / or with external devices such as, for example, remote servers equipped with means of communication.

[0172] According to a particular and non-limiting embodiment, device 1 may include a block 12 of interface elements for communicating with these external devices. The interface elements of block 12 may include one or more of the following interfaces: - radio frequency (RF) interface, for example of the Wi-Fi® type (according to IEEE 802.11), for example in the 2.4 or 5 GHz frequency bands, or of the Bluetooth® type (according to IEEE 802.15).1), in the 2.4 GHz frequency band, or Sigfox type using UBN (Ultra Narrow Band) radio technology, or LoRa in the 868 MHz frequency band, LTE (Long-Term Evolution), LTE-Advanced; USB interface (Universal Serial Bus); HDMI interface (High Definition Multimedia Interface); LIN interface (Local Interconnect Network).

[0173] According to a particular and non-limiting embodiment, device 1 can provide output signals to one or more external devices, such as a display screen 15, touch or non-touch, one or more speakers 16 and / or other peripherals 17 via output interfaces 18, 19, 20 respectively. According to a variant, one or more of the external devices is integrated into device 1.

[0174] The diagram schematically illustrates an implementation of the device 2 for receiving light signals configured to modulate a light signal from the control signal 100 according to a particular and non-limiting example of the present invention.

[0175] Device 2 includes a driver 180 (driver in English) which provides electrical power to the other electrical elements of device 1. For example, driver 180 converts an alternating voltage of 110-220v into a direct voltage of 7-50V.

[0176] Device 2 also includes a pilot signal generator 181 and a light signal generator 182 driven by the pilot signal at the output of generator 181.

[0177] For example, the light signal generator 182 is a light-emitting diode driven by the pilot signal at the output of the generator 181.

[0178] The generator 181 includes a microcontroller 183 configured to generate the control signal 100 according to the method described opposite and a DC voltage modulation means 184 driven by the control signal 100 to provide the signal which drives the voltage across the terminals of the light signal generator 182. For example the DC voltage modulation means 184 is a MOSFET type transistor.

[0179] For example, the MOSFET transistor is suitable to support frequencies of at least 100KHz.

[0180] According to one variant, the generator 181 may include a voltage converter 185 which adapts the output voltage of the driver 180 to the supply voltage of the microcontroller 183.

[0181] According to this device 2, the DC voltage modulation means 184 is controlled by the control signal 100 generated at the output of the microcontroller 183. The signal at the output of the DC voltage modulation means 184 controls the power supply of the light signal generator 182 to reproduce the variations in light intensity of the control signal 100. A symbol can then be transmitted by the light signal thus modulated which is produced by the light signal generator 182.

[0182] For example, the elements of the light signal generator 182 can be grouped on the same printed circuit board (PCB for Printed Circuit Board).

[0183] The diagram schematically illustrates an implementation of the device 3 light signal receiver configured to demodulate a light signal according to a particular and non-limiting example of the present invention.

[0184] Device 3 is configured to identify blinking frequencies of a continuous light signal (when this light signal belongs to the visible spectrum), i.e., as samples of this light signal become available.

[0185] For example, blinking frequencies can range from 2kHz to 5kHz.

[0186] Device 3 is also configured to minimize its electrical energy consumption.

[0187] The device 190 includes a light signal reader 191 such as a single-crystal photosensitive unit.

[0188] Device 3 further includes a filter 192 and an amplifier 193 associated with a means for varying the amplification level 194. The amplifier 193 amplifies the filtered signal according to the light intensity of the signal, using an amplification level adjusted by the means 194. The amplifier thus amplifies the filtered light signal to its maximum (before saturation) to optimize demodulation performance. Device 1 is then configured to demodulate a light signal under varying lighting conditions ranging from 10 lux to 10,000 lux.

[0189] Device 3 further includes a microcontroller 195 configured to demodulate the filtered and amplified light signal according to a process described opposite Figures 13 and 14 and to obtain the symbol represented by the light signal.

[0190] Of course, the present invention is not limited to the embodiments described above but extends to a method for modulating and demodulating a light signal that would include secondary steps without falling outside the scope of the present invention. The same would apply to a device configured for implementing such a method.

Claims

Method for modulating a light signal for the emission of a symbol, the method being controlled by a control signal (100) comprising a succession of time cycles, each time cycle being defined by an alternation of a high level corresponding to a high level of light intensity of the modulated light signal maintained for a first duration and a low level corresponding to a low level of light intensity of the light signal maintained for a second duration, a total duration of each time cycle being defined from a frequency, the method is characterized in that it comprises the following steps:- determination (210) of a pair of frequencies (F1, F2) representing the symbol;- determination (220) of the succession of time cycles comprising at least a first time cycle (s1) followed by at least a second time cycle (s2);- each first time cycle (s1) having a total duration (1031) defined from the first frequency (F1) of the determined frequency pair (F1, F2); - each second time cycle (s2) having a total duration (1032) defined from the second frequency (F2) of the determined frequency pair (F1, F2); - the first frequency (F1) and the second frequency (F2) of the determined frequency pair (F1, F2) being determined so that the highest frequency, among the first and second frequencies, is at most equal to twice the lowest frequency, among said first and second frequencies. A method according to claim 1, wherein the succession of time cycles further comprises at least one third time cycle (s3), said at least one third time cycle preceding the first of said at least one first time cycle (s1) of the succession of time cycles, each third time cycle (s3) being of a total duration (1033) defined from a symbol emission start frequency, the symbol emission start frequency being determined so that the highest frequency, among the symbol emission start frequency and the first frequency, is at most equal to twice the lowest frequency, among the first frequency and the symbol emission start frequency. A method according to claim 1 or 2, wherein the succession of cycles of the control signal further comprises at least one fourth time cycle (s4), said at least one fourth time cycle being following the last of said at least second symbol (s2), each fourth time cycle (s4) being of a total duration (1034) defined from a symbol end-of-emission frequency, the symbol end-of-emission frequency being determined so that the highest frequency, among the second frequency and the symbol end-of-emission frequency, is at most equal to twice the lowest frequency, among the second frequency and the symbol end-of-emission frequency. A method according to claim 3, wherein the first frequency (F1), the second frequency (F2), the starting frequency (F s ) of the symbol emission and the termination frequency (F e) emission frequencies of the symbol are determined from a set of frequencies and the highest frequency of the set of frequencies is at most equal to twice the lowest frequency of the set of frequencies. A method according to claim 3, wherein the succession of time cycles of the control signal further comprises at least one fifth time cycle (s5) following the last of said at least one first time cycle (s1), at least one fifth time cycle (s5) following the last of said at least one second time cycle (s2), at least one fifth time cycle (s5) following the last of said at least one third time cycle (s3), and at least one fifth time cycle (s5) following the last of said at least one fourth time cycle (s4), each fifth time cycle (s5) having a total duration (1035) defined from a cutoff frequency (F c) defined as the median value between the highest and lowest frequencies of the set of frequencies, the highest frequency of the set of frequencies being at most equal to four times the lowest frequency of the set of frequencies. A method according to claim 3, wherein the first, second, third, fourth and fifth time cycles are each repeated a defined number of times from a frequency change frequency. A method according to claim 6, wherein the frequency of frequency change varies according to lighting conditions of an environment in which the light signal is diffused. A method according to any one of claims 1 to 7, wherein a duty cycle of each time cycle of the control signal is equal to the same value within a deviation around that value. A method for demodulating a light signal for the reception of a symbol, the method is characterized in that it comprises the following steps: - detection (310) of a first frequency (F1) of a pair of frequencies from a frequency spectrum of the light signal determined from a first set of samples of the light signal; - detection (320) of a second frequency (F2) of the pair of frequencies from a frequency spectrum of the light signal determined from a second set of samples of the light signal; - detection (330) of the received symbol from the pair of frequencies; - the first frequency (F1) and the second frequency (F2) being detected such that the highest frequency, among the first and second frequencies, is at most equal to twice the lowest frequency, among said first and second frequencies. Method according to claim 7, further comprising a step of detecting (340) a symbol reception start frequency from a frequency spectrum of the light signal determined from a third set of light signal samples, the detection of the symbol reception start frequency preceding the detection of the first frequency of the frequency pair. Method according to claim 7 or 8, further comprising a step of detecting (350) a symbol end-reception frequency from a frequency spectrum of the light signal determined from a fourth set of light signal samples, the detection of the symbol end-reception frequency following the detection of the second frequency of the frequency pair. A method according to claim 11, wherein the first frequency, the second frequency, the symbol reception start frequency and the symbol reception end frequency belong to a set of frequencies of which the highest frequency is at most equal to twice the lowest frequency of the set of frequencies. A method according to claim 12, further comprising a step of determining (360) a cutoff frequency from a frequency spectrum of the light signal determined from a fifth set of samples of the light signal, the cutoff frequency being defined as a median value between the highest and lowest frequencies of the set of frequencies, the highest frequency of the set of frequencies being at most equal to four times the lowest frequency of the set of frequencies, the cutoff frequency being determined following the determination of the first frequency, following the determination of the second frequency, following the determination of the symbol reception start frequency and following the determination of the symbol reception end frequency. Method according to claim 11, wherein the frequency spectrum is determined by spectral analysis of samples from the first, second, third, fourth or fifth set of samples of the light signal. Method according to claim 12, wherein the detection of the first frequency (310), the second frequency (320), the symbol reception start frequency (340), the symbol reception end frequency (350) or the cutoff frequency (360) from the frequency spectrum of the light signal determined from the samples respectively of the first, second, third, fourth or fifth set, comprises the following steps:- evaluation (370) of scores associated with the frequencies of the set of frequencies from the amplitudes of the corresponding frequencies of the frequency spectrum determined from the samples respectively of the first, second, third, fourth or fifth set;- identification (380) of the first frequency (step 310), of the second frequency (step 320), of the symbol reception start frequency (step 340), of the symbol reception end frequency (step 350) or of the cutoff frequency (step 360) among the frequencies in the set of frequencies from the scores associated with the frequencies in the set of frequencies.; A method according to claim 5 or 11, wherein the frequencies of the frequency set are less than 5kHz. A method according to one of the claims, wherein the symbol is determined from a table associating a pair of frequencies and a symbol value. Light signal emitting device comprising a microcontroller configured for the implementation of the process steps according to any one of claims 1 to 6, 14 to 17. Light signal receiving device comprising a microprocessor configured for carrying out the steps of the process according to any one of claims 7 to 17. Computer program that includes instructions adapted for carrying out the steps of the process according to any one of claims 1 to 17 when the computer program is executed by at least one processor. Computer-readable recording medium on which is recorded a computer program comprising instructions for carrying out the steps of the process according to any one of claims 1 to 17.

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