FIELD OF THE INVENTION The present invention relates to a method for authenticating a timepiece, in particular a watch.

BACKGROUND OF THE INVENTION Counterfeit consumer goods, commonly called knock-offs, are counterfeit or imitation products offered for sale. The spread of counterfeit goods has become global in recent years and the range of goods subject to infringement has increased significantly. Expensive watches (and spare parts for watches) are vulnerable to counterfeiting, and have been counterfeited for decades. A counterfeit watch is an illegal copy of a part or all of an authentic watch. According to estimates by the Swiss Customs Service, there are some 30 to 40 million counterfeit watches put into circulation each year. It is a common cliche that any visitor to New York City will be approached on a street corner by a vendor with a dozen such counterfeit watches inside his coat, offered at bargain prices. Extremely authentic looking, but very poor quality watches fakes with self-winding mechanisms and fully working movements can sell for as little as twenty dollars. The problem is becoming more and more serious, with the quality of the counterfeits constantly increasing. For example, some fakes' movements and materials are of remarkably passable quality and may look good to the untrained eye and work well for some years, a possible consequence of increasing competition within the counterfeiting community. Counterfeit watches cause an estimated $1 Billion loss per year to the watch industry.

Authentication solutions that have been used for protection of consumer goods from counterfeiting are often based on marking the item with a specific material, code, or marking, engraving, etc. However, these methods modify the nature and the appearance of the object, and this is often not acceptable in the watch (and other luxury items) industry, where the design of the object and its visual appearance is of paramount importance. Also, these methods require an active intervention at the time of manufacturing and, correspondingly an important change of the production process.

Counterfeiters often focus on the outer appearance of the watch and fit a cheap movement inside, because the potential buyer will focus more on the appearance of the piece, and because good movements are expensive. Even when a good quality movement is used, it is very difficult and expensive to make an exact copy and the counterfeit will prefer to use one that is easier to get or to manufacture. It is therefore desirable, to asses the authenticity of a timepiece, to have as much information as possible not only on its outer appearance but also on its inner content. It is furthermore desirable not to have to open the piece , as the operation requires specialized equipment and procedures, it may have an impact on the performances of the piece (e.g. water tightness), and may invalidate the manufacturer's warranty.

It is therefore desirable to authenticate a timepiece in a manner that is as non-invasive as possible and as reliable as possible without having to open the timepiece.

SUMMARY OF THE INVENTION An object of the invention is to provide a method for authenticating a timepiece that is non-invasive and reliable.

This object is solved by the subject matter of the independent claims. Preferred embodiments are subject matter of the dependent claims.

An embodiment of the invention provides a method for authenticating a timepiece comprising measuring acoustic vibrations emitted by said timepiece to obtain an electrical signal, said electrical signal indicating a variation of a magnitude of said measured acoustic vibrations as a function of time, wherein said electrical signal comprises a plurality of acoustic events associated with mechanical shocks taking place in said timepiece, extracting in said electrical signal or in a representation of said electrical signal in a time, frequency or time-frequency domain at least one of a magnitude information on a magnitude of one of said plurality of acoustic events, a time information on said one of said plurality of acoustic events and a frequency information on a frequency of said one of said plurality of acoustic events, comparing said extracted at least one of a magnitude information, time information and frequency information with at least one of a reference magnitude information, reference time information and reference frequency information, and deriving an information on an authenticity of said timepiece based on the comparison result.

According to an embodiment of the invention, said extracting step comprises extracting, in a time sequence of said electrical signal corresponding to one of said plurality of acoustic events, an amplitude information on an amplitude of a first acoustic sub-event of said one of said plurality of acoustic events.

According to an embodiment, the extracting step comprises separating a series of consecutive events E; with i=i...n into different classes and analyzing each class separately. As an example, one class may correspond to odd events (1=1,3,5,...) and another to even events (1=2,4,6,...), which amounts to separating ticks and tocks. More generally, classes may contain events with the same value of (i modulo p), where (ϊ modulo p) is the remainder of integer division of ί byp andp is an integer number. For example, whenp is equal to twice the number of teeth of the escapement wheel, each class contains the events (ticks or tocks) associated with one specific escapement wheel tooth.

According to an embodiment of the invention, said extracting step comprises extracting, in a time sequence of said electrical signal corresponding to one of said plurality of acoustic events, a time delay information on a time delay between a first acoustic sub-event of said one of said plurality of acoustic events and a second acoustic sub-event of said one of said plurality of acoustic events. According to an embodiment of the invention, said method further comprises performing a transform of said electrical signal into a frequency domain to obtain a frequency-domain power spectrum indicating a variation of a power of said electrical signal as a function of frequency, wherein said extracting step comprises extracting at least one frequency information on a frequency associated with a peak of said frequency-domain power spectrum.

According to an embodiment of the invention, said transform of said electrical signal into a frequency domain is a Fourier transform, preferably a Fast Fourier transform.

According to an embodiment of the invention, said method further comprises performing a transform of said electrical signal into a time- frequency representation indicating a frequency information of said electrical signal as a function of time, wherein said extracting step comprises extracting at least one of a frequency information and time information in said time-frequency representation of said electrical signal.

According to an embodiment of the invention, said transform of said electrical signal into a time-frequency representation is one of a short- time Fourier transform, a Gabor transform, a Wigner transform and a wavelet transform.

According to an embodiment of the invention, said method further comprises separating every other acoustic event in said electrical signal and performing said method steps on an electrical signal comprising only every other acoustic events.

According to an embodiment of the invention, said method further comprises encoding said extracted at least one of a magnitude information, time information and frequency information to create a unique identifier for said timepiece, said unique identifier for said timepiece being used as said at least one of a reference magnitude information, reference time information and reference frequency information. Another embodiment of the invention provides a computer readable medium for storing instructions, which, upon being executed by a processor of a computer device, cause the processor to execute the steps of measuring acoustic vibrations emitted by a timepiece to obtain an electrical signal, said electrical signal indicating a variation of a magnitude of said measured acoustic vibrations as a function of time, wherein said electrical signal comprises a plurality of acoustic events associated with mechanical shocks taking place in said timepiece, extracting in said electrical signal or in a representation of said electrical signal in a time, frequency or time-frequency domain at least one of a magnitude information on a magnitude of one of said plurality of acoustic events, a time information on said one of said plurality of acoustic events and a frequency information on a frequency of said one of said plurality of acoustic events, comparing said extracted at least one of a magnitude information, time information and frequency

information with at least one of a reference magnitude information, reference time information and reference frequency information, and deriving an information on an authenticity of said timepiece based on the comparison result. The invention is not necessarily limited to the analysis of ticks alone or tocks alone, it could also be a combination of tick and tock thereof, can be used.

BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a schematic representation of an escapement in a timepiece.

Fig. 2 is a representation of acoustic vibrations in a timepiece as a function of time. Fig. 3 is a close-up view on two events in the time sequence represented in Fig. 2.

Fig. 4 is a close-up view on the first event represented in Fig. 3. Fig. 5 illustrates a first embodiment of. a method for authenticating a timepiece according to the invention.

Fig. 6 illustrates a second embodiment of a method for authenticating a timepiece according to the invention.

Fig. 7 illustrates a third embodiment of a method for authenticating a timepiece according to the invention. Fig. 8 is a time-frequency representation of the acoustic vibrations of a timepiece according to a first model.

Fig. 9 is a time-frequency representation of the acoustic vibrations of a timepiece according to a second model.

Fig. 10 is a time-frequency representation of the acoustic vibrations of a timepiece according to a third model.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, the various embodiments of the present invention will be described with respect to the enclosed drawings.

A timepiece, such as a watch, comprises a mechanical movement which produces a characteristic noise, which is commonly referred to as tick- tock. This tick-tock sound, which is characteristic of a timepiece, is due to the impacts happening between the various mechanical pieces of the escapement of the timepiece, which is a device transferring energy to the time-keeping element, the so-called impulse action, and allowing the number of its oscillations to be counted, the locking action. The ticking sound is the sound of the gear train stopping at the escapement locks.

Fig. 1 shows a representation of the main parts of an escapement. An escapement comprises a balance wheel 11, a pallet fork 12 and an escape wheel 13. The balance wheel 11 comprises an impulse pin 14, which strikes against the pallet fork 12. Further, the escape wheel 13 comprises teeth which strike an entry pallet jewel 15 and an exit pallet jewel 16 of the pallet fork 12.

According to an embodiment of a method for authenticating a timepiece according to the invention, the acoustic vibrations of a timepiece to be authenticated are measured, for instance using a microphone, preferably a contact piezoelectric microphone. The acoustic vibrations emitted by the timepiece are measured and an electrical signal is obtained, which indicates a variation of the magnitude of the measured acoustic vibrations as a function of time. Such an electrical signal is represented in Figs. 2 to 4.

Fig. 2 represents the acoustic vibrations emitted by a timepiece as a function of time. The represented signal has a frequency of 3 Hz, i.e. six beats take place every single second. The signal alternates between tick events and tock events.

Fig. 3 represents a closer view on the start of the sequence of tick events and tock events shown in Fig. 2. Fig. 3 shows a first event 1 and a second event 2 of the sequence of ticks and tocks of Fig. 2. The first event 1 spreads in a time range comprised between about o and 15 ms, while the second event 2 spreads in a time range comprised between about 165 ms and 185 ms. As can be seen from Fig. 3, each one of the first event 1 and second event 2 is itself a sequence of several sub-events, which are illustrated in more detail in Fig. 4.

Fig 4 shows a close-up view on the first event 1 in the representation of Fig. 3. The first event 1 comprises a first sub-event 11, a second sub-event 12 and a third sub-event 13. The first sub-event 11 takes place in a time range comprised between about o and 3 ms, the second sub-event 12 takes place in a time range comprised between about 3.5 ms and about 10,5 ms. The third sub-event 13 takes place in a time range comprised between about 10.5 ms and about 18 ms. The first sub-event 11, second sub-event 12 and third sub-event 13 therefore make up the first event 1 shown in Fig. 3, which corresponds to one acoustic event of the timepiece.

Fig. 5 illustrates a first embodiment of a method for authenticating a timepiece according to the present invention. Fig. 5 is a representation of the instantaneous power of the acoustic vibrations emitted by a timepiece to be authenticated as a function of time. According to a first embodiment of a method for authenticating a timepiece according to the invention, the acoustic vibrations emitted by the timepiece are measured and an electrical signal is obtained. The electrical signal indicates a variation of the magnitude of the measured acoustic vibrations as a function of time. In the first embodiment illustrated with respect to Fig. 5, this electrical signal is the representation of the instantaneous power of the acoustic vibrations as a function of time.

According to the first embodiment of the present invention, an amplitude information of one or more events of a series of events is extracted from the representation of the instantaneous power of the measured acoustic vibrations. In particular, an amplitude of a sub-event within one event is extracted. The extracted amplitude information could be peak amplitude or average amplitude. The extracted amplitude information is preferably a relative amplitude, since it depends on how the signal has been normalized.

Fig. 5 shows a first sub-event 101 and a second sub-event 102. The first sub-event 101 takes place in a time range comprised between about 3.5 ms and 4.5 ms, while the second sub-event 102 takes place in a time range comprised between about 11 ms and about 13 ms. The extracted amplitude is a beat-to-beat variation of a sub-event, e.g. the first sub- event 101. Further, an amplitude of the second sub-event 102 may be extracted. The extracted amplitude information is then compared with a reference amplitude information. This reference amplitude information has been previously measured and stored for the timepiece model, which is to be authenticated. By comparing the extracted amplitude information obtained for the timepiece to be authenticated with the reference amplitude information, an information on an authenticity of the timepiece to be authenticated can be derived.

In particular, from the average amplitudes Ai...A _n of a series of events lto n, information on the number of teeth of the escapement wheel can be obtained, as well as the number of teeth on the escapement wheel pinion and on further wheels down the gear train. This information can be used for authentication purposes.

According to a second possibility of the first embodiment of the present invention, instead of an amplitude information, a time-delay

information may be extracted from the time sequence of the measured acoustic vibrations of the timepiece. For instance, one or more time delay(s) Δ between the highest peak of the first sub-event 101 and the highest peak of the second sub-event 102 may be extracted. This time delay Δ obtained for the timepiece to be authenticated can then be compared with a reference time delay which has been previously stored for the timepiece model to be authenticated. The time delay may be an absolute time delay or a relative time delay. For example, referring to Fig. 4, (t2-ti)/(ti-to) is a relative time delay. The ratio of (ti-to) in event i to (ti-to) in event j is also a relative time delay. This information can also be used for authentication purposes.

According to a preferred embodiment of the invention, which may apply to the first embodiment of the invention but also to the further embodiments, which will be outlined in the following description, the measurements of the acoustic vibrations of the timepiece are carried out on every other acoustic event in the obtained electrical signal. This means that every other acoustic event in the electrical signal is separated, i.e. only the "ticks" or the "tocks" of the electrical signal are separated, and the steps of the method for authenticating a timepiece according to an embodiment of the present invention are performed on an electrical signal comprising only every other acoustic event, i.e. only the "ticks" or the "tocks". More generally, the acoustic events may be separated according to any subset, not only every other acoustic event, but every n event, where n is equal to 2, 3, 4, 5, etc. Separating every other acoustic event corresponds to the case of n equal to 2 and represents a preferred embodiment of the present invention.

Fig. 6 illustrates a second embodiment of a method for authenticating a timepiece according to the present invention. Fig. 6 is a representation of the power spectrum of the measured acoustic vibrations emitted by a timepiece to be authenticated as a function of frequency. According to the second embodiment of the invention, the acoustic vibrations emitted by a timepiece to be authenticated are measured and an electrical signal is obtained, which indicates a variation of a magnitude of the measured acoustic vibrations as a function of time. This electrical signal is transformed into a frequency domain, so as to obtain a frequency- domain power spectrum indicating a variation of a power of the electrical signal as a function of frequency. The frequency-domain transform to be used according to this embodiment may be one of the usual frequency-domain transforms, such as a Fourier transform, in particular a Fast Fourier transform.

The frequency-power spectrum of the measured acoustic vibrations of the timepiece to be authenticated reveals several peaks in the power spectrum representation at several frequencies. In the particular example represented in Fig. 6, eleven peaks can be identified in the power spectrum, the power spectrum value of which is larger than 100 on the logarithmic scale of Fig. 6. These peaks in the power spectrum can be identified at frequencies f to fo, which are comprised in the range between o and 40 kHz. It must be noted that these values are given for illustrative purposes only and are not limiting. In particular, even though the particular example of a threshold set at loo for identifying peaks in the power spectrum has been given, the person skilled in the art will immediately understand that another threshold may be set, depending on the amount of frequency peaks desired as frequency information. For instance, the threshold could be set at ιοοο, so that only a few peaks can be identified.

This frequency information, i.e. the respective frequencies f ₀ - to fi ₀ in the example of Fig. 6 corresponding to peaks in the frequency-domain power spectrum of the measured acoustic vibrations of the timepiece to be authenticated, is extracted from the frequency-domain power spectrum and compared with a reference frequency information, which has been previously stored for the timepiece model. This comparison enables to derive an information on an authenticity of the timepiece to be authenticated by simply comparing the frequency information obtained for the timepiece to be authenticated with the reference frequency information for the timepiece model to be authenticated.

According to an embodiment of the present invention, information on the width of the spectral peak can also be used for authentication or identification purposes.

According to another embodiment of the present invention, the spectrum is preferably the average of several spectra. It can be either the average of a number of consecutive events or the average of a number of events from the same class.

In the frequency-domain power spectrum representation of the measured acoustic vibrations emitted by the timepiece to be

authenticated, the dominant contribution within the power spectrum comes from the loudest portions within the measured acoustic vibrations emitted by the timepiece to be authenticated. These loudest portions of the acoustic vibrations correspond to the events and sub-events, as the ones represented in Figs. 3 and 4. Fig. 7 illustrates a third embodiment of a method for authenticating a timepiece according to the present invention. Fig. 7 is a time-frequency representation of the acoustic vibrations emitted by the timepiece to be authenticated. Fig 7 characterizes the electrical signal obtained by measuring acoustic vibrations emitted by the timepiece to be authenticated both in the time domain and frequency domain. Unlike a transform into a frequency domain, which only gives information on the frequencies that are present in the transformed signal, a time-frequency representation gives information on which frequencies are present at which time. It can therefore be used to associate specific frequencies with specific events taking place in the time domain.

According to the third embodiment of a method for authenticating a timepiece according to the present invention, the time-frequency transform to be used may be one among the several time-frequency transforms available and known to the person skilled in the art. In particular, only to cite a few possible transforms, the transform into a time-frequency representation may be one of the short-time Fourier transform, a Gabor transform, a Wigner transform and a wavelet transform.

Fig. 7 shows a time-frequency representation of the measured acoustic vibrations of a timepiece to be authenticated, which has been obtained by using a continuous wavelet transform. The wavelet transform is described, for example, in C. Torrence and G.P. Compo, Bulletin of the American Meteorological Society, 79, 1998. The use of a wavelet transform represents a preferred embodiment of the present invention, since the wavelet transform is a convenient tool for time-frequency analysis, with a number of interesting features, such as the possibility to adapt the time-frequency resolution to the problem under investigation, as well as the good mathematical properties. The continuous wavelet transform takes a time-domain signal s(t), the electrical signal of the measured acoustic vibrations emitted by the timepiece to be authenticated, the electrical signal indicating a variation of the magnitude of the measured acoustic vibrations as a function of time, and transforms this time-domain signal into a time-frequency representation W(f, t), which is defined by the folio wing formula:

where

ψ is called the wavelet function (there are several types to choose from) and

c is a constant which depends on the chosen wavelet function

The exemplary time-frequency representation shown in Fig. 7, which is also referred to as spectrogram, represents the values of | W(f,t)\ ² , which has been obtained using a Morlet wavelet:

with: ω = 40 and

m ^ ^ _m ²

As already mentioned above, according to a preferred embodiment of the invention, the measurements of the acoustic vibrations of the timepiece are carried out on every other acoustic event in the obtained electrical signal. This means that every other acoustic event in the electrical signal is separated, i.e. only the "ticks" or the "tocks" of the electrical signal are separated, and the steps of the method for authenticating a timepiece according to an embodiment of the present invention are performed on an electrical signal comprising only every other acoustic event, i.e. only the "ticks" or the "tocks". In the context of the third embodiment, the continuous wavelet transform is applied to this signal of the separated events, and an average is then performed on a predetermined number of acoustic events. According to a preferred embodiment of the invention, the average is performed over at least 10 acoustic events, preferably at least 20 acoustic events.

As already mentioned above, Fig. 7 is a time-frequency representation of the measured acoustic vibrations of the timepiece to be authenticated, which has been obtained by performing a continuous wavelet transform of the time-domain signal obtained by measuring the acoustic vibrations emitted by the timepiece. In Fig. 7, it can be seen that the spectrogram reveals a first sub-event 201 in a time span comprised between about o ms and about 2 ms. A second sub-event is also visible in a time span comprised between about 3 ms and 5 ms. Finally, a third sub-event 203 can be identified in a time span comprised between about 10 ms and 14 ms. Further to the time information that can be obtained from the spectrogram represented in Fig. 7, frequency information can also be obtained for each of the sub-events identified. Indeed, the frequency values of harmonics leading to peaks in a frequency-domain

representation of the electrical signal obtained by measuring the acoustic vibrations of the timepiece to be authenticated can be easily obtained from the time-frequency representation of Fig. 7 with the additional time information being directly accessible. For instance, as far as the third sub-event 203 is concerned, spots or areas can be identified for the approximate coordinates (11 ms, 32 kHz), (11 ms, 16 kHz). Further, stripes can also be identified, for instance between about 11 and 13 ms, for a frequency of about 8 kHz. As far as the second sub-event 202 is concerned, a spot could also be identified for the approximate coordinate (3-5 ms , 32 kHz). By using this time-frequency information, which is obtained from a time-frequency representation of the electrical signal obtained by measuring acoustic vibrations emitted by the timepiece to be

authenticated, information on an authenticity of the timepiece can be derived. In order to do so, the time-frequency information is extracted from the time-frequency representation and compared with reference time-frequency information, which has been previously stored for the timepiece model. By comparing the time-frequency information extracted for the timepiece to be authenticated with the reference time- information for the timepiece model, it can be derived whether the timepiece is authentic or not.

It has been observed by the inventors of the present invention that the reliability and degree of precision of the invention are such that it is possible to even identify differences between the timepieces of an identical model. Indeed, because of manufacturing tolerances, even two timepieces of an identical model differ from each other. When applying the principles underlined in the present invention to different timepieces from the same series and the same manufacturer, it can be seen that the corresponding acoustic measurements are different and the extracted relevant respective pieces of frequency information, which characterize the fingerprint of the respective timepiece, are different. Hence, an identifier can be defined for a timepiece without having to open the timepiece.

Fig. 8 shows an exemplary spectrogram obtained for a timepiece according to a first model. Fig. 9 represents a spectrogram for a timepiece according to a second model. Fig. 10 represents a spectrogram for a timepiece according to a third model. These spectrograms show that each timepiece model can be associated with a characteristic time- frequency representation. Consequently, by comparing the time- frequency representation of a timepiece to be authenticated with a reference time-frequency representation, which is expected for this particular timepiece model, authenticity information on the timepiece to be authenticated can be derived. Hence, it can be derived whether a timepiece to be authenticated is an authentic product or a counterfeited product. Even though the present invention has been described with respect to the particular case of mechanical shocks within the timepiece being the primary source of vibrations, the person skilled in the art will immediately recognize that the principles outlined in the present application can be applied to another source of vibrations. For instance, it could be emdsaged to apply the principles according to the embodiments of the present invention to an external source of vibrations.