IE75351B1 - A method of encoding a security filament in particular for banknote paper - Google Patents

Abstract

The invention relates to a method of coding a security wire (F) for currency paper or the like, characterised in that it consists in applying to the wire a coding of a plurality of elementary information items constituted respectively by a plurality of segments having substantially the same length (L), the information in each segment being determined by the cyclic ratio of the length of an active or activatable area (for example magnetic or magnetisable) of the segment to the length of the said segment. Application to authentication.

Description

The present invention relates in general to encoding filaments such as magnetic filaments for authenticating security paper such as banknote paper.

The principle of encoding a magnetic filament incorporated in a bank note or the like has already been known for a long time, in particular from patent GB-A-1 127 043- Such encoding consists in organizing a suitable distribution of magnetic or magnetizable material along the filament so that reading means and processing means can derive information from said distribution.

A special encoding method for such a filament is also known, in particular from Document WO-A-9O/O8367. More precisely, that document teaches encoding binary digital information in which the presence of magnetic or magnetizable material on a given length of filament represents a first logic level, e.g. a 1, whereas the absence of magnetic or magnetizable material on another given length of the filament, typically identical to the first length, represents a second logic level, for example a 0.

Such encoding may turn out to be difficult to read, in particular when the code includes a long succession of 0 or of 1 bits, since it is then no longer possible for the reader means to resynchronize itself on the beginning of each bit.

In practice, encoding of this type can be used only with code reader machines in which the relative speed between the read head and the paper including the security filament is both fixed and welldetermined. In practice, it cannot be used with reading on the fly by means of a reader stylus having a magnetic head or the like.

In addition, coding of this known type can give rise to _ extended magnetic zones existing on the filament (a succession of 1 bits), which complicate manufacture and increase the risk of misreading, as explained below.

An encoding method according to the preamble of claim 1 is also known from Document WO-A-9O/O8367· However, that known method is not capable of resolving the problems of manufacture, and of reading reliability, as mentioned above.

The present invention seeks to mitigate these drawbacks of the prior art and in particular to provide an encoding method which provides good reliability on reading even under difficult conditions.

It enables information to be encoded that may be representative of the face value of a bank note, of its date of manufacture, of its serial number, etc.

To this end, in a first aspect, the present invention provides a method of encoding a security filament for banknote paper or the like having the characteristics of claim 1.

Preferred but non-limiting aspects of the method are exposed in sub-claims 2 to 7· The invention also provides a security filament for banknote paper or the like, obtained by the method as defined above, and security paper including such a filament.

Other aspects, objects, and advantages of the present invention appear more clearly on reading the following detailed description of a preferred implementation thereof, given by way of non-limiting example and made with reference to the accompanying drawings, in which: Figure 1 shows a first type of encoding in accordance with the present invention; Figure 2 shows a specific example of a security filament based on the encoding of Figure 1; Figure 3 shows another specific example of a security filament based on the encoding of Figure 1; Figure 4 shows an electrical signal that can be obtained on reading the filament of Figure 3; Figure 5 shows a second type of encoding in accordance with the invention; Figure 6 shows a third type of encoding in accordance with the invention; and Figure 7 shows a bank note incorporating a filament encoded by means of the invention.

With reference initially to Figure 1, there can be seen encoding in accordance with the invention using binary units. In this figure, and in all of the following figures, shaded areas represent zones where magnetic or magnetizable material is present, whereas white areas indicate the absence of magnetizable material.

In this encoding of a unit of information, each unit information segment occupies a specific length L of the magnetic filament F. A 0 is represented by absence of material along the entire length L, whereas a 1 is represented by the presence of material over a first third of the length L, while the second and third thirds have no material.

Thus, a 1 corresponds to a duty ratio of 33% between the length of the zone where the magnetic material is present and the30 total length of the segment, whereas a 0 corresponds to a duty ratio of zero. Naturally, the opposite convention could be adopted. It is specified here that throughout the present document the term duty ratio is used to designate the ratio of the length of the active zone of the code (in this case a zone including a substance that is magnetic or magnetizable) divided by the total length of the segment containing said active zone. A duty ratio can thus take any value lying in the range 0% to 100%.

Advantageously, a word constituted by a plurality of bits encoded in this way may be bracketed between a start segment and an end segment, or between appropriate delimiter segments. The essential purpose of such segments, in addition to delimiting words, is to enable reading to be synchronized on the bit-length L, regardless of the speed with which the paper moves relative to the reader system.

Figure 2 shows an example of how the face value of a bank note may be encoded.

In this example each word, e.g. a word having eight segments of length L, comprises a first portion constituted by a succession of M Is and a second portion constituted by a succession of N 0s, where M + N = 8. In this case, not only does the encoding of individual segments have a varying duty ratio, so also does the encoding of an entire word. Each word can thus take up eight different states, with a duty ratio between the number of 1 segments and the total number of segments in the word varying in this case over the range 0% to 100% in steps of 12-5%.

Such encoding in accordance with the present invention is particularly advantageous in that decoding in a processor unit situated downstream from reader means can be performed in a manner that is extremely simple by using suitable integrator means. In addition, given the very structure of each word, there is no need for a start segment or for an end segment or for a delimiter segment. Furthermore, reading can be performed equally well in either direction.

Naturally, any other form of word may be used.

In particular, the encoding of unit segments as described above may be used for encoding information in a magnetic filament using a standardized code such as ASCII, which allows a wide range of information to be encoded. It is also possible to encode information using the format of some bar codes, and in particular the so-called MSI bar code which is based on variations in a duty ratio between solid zones and empty zones. The code selected is preferably a code that includes automatic error correction.

Another variant of the way in which the above-described segment encoding can be used is shown in Figure 3· In this example, there are special delimiter segments that bracket each end of a word which is characterized not by a special distribution of binary or other information within the word, but by the number of individual information units the word contains all having the same value or the same meaning. In this example, there are P information segments, where P may vary, for example, as a function of the face value of bank notes, with these segments all being constituted as described above by one-third of a length encoded with magnetic substance followed by two-thirds of a length free of such substance.

In this example, the delimiter segments include magnetic zones that are longer. In particular, it is possible to provide either delimiter segments of length L' where L' is greater than the length L of information segments (as shown), or else delimiter segments having the same length L, but in which the duty ratio is greater than the duty ratio of information segments.

Such a solution is particularly advantageous in that it makes it possible to use processing means that are particularly simple downstream from the means for reading the magnetic filament. Thus, if an integrator stage having a suitable time constant is provided in said processor means, an output signal is obtained therefrom as shown in Figure 4. It will be understood that by providing two threshold detectors downstream from the integrator and having two different threshold levels SI and S2, the first detector can be used for detecting delimiter segments while the second detector having a lower threshold can be used for detecting information segments which provide a lower peak value at the output from the integrator. Thus, a simple counter connected downstream from the second detector and suitable for being reset to zero by the first detector serves to deliver digital information equal to the number P of information segments in the word under consideration.

Although the above description relates to binary digital encoding (two values 0 or 1), the invention may also be used for different forms of digital encoding, for example a ternary form of encoding. Under such circumstances, three different duty ratios are used for encoding the three possible values of a unit of information, with the first duty ratio preferably being zero.

Another type of encoding using the principles of the present invention is now described with reference to Figure 5· In this case, each unit segment is of fixed length L, but the duty ratio of the magnetic or magnetizable zone divided by the total length of a segment is variable either continuously or in steps.

Such an approach can enable completely arbitrary analog information to be encoded, in particular by using pulse coded modulation (a technique known as PCM) based on varying the width of the pulses.

In the example shown, a plurality of successive unit segments have respective pulse widths that are representative of the amplitude of an analog signal (i.e. amplitudes are represented by the lengths of corresponding magnetized or magnetizable zones, which lengths are proportional to the duty ratio). The example shows how a sinewave of fixed period T is encoded.

To discriminate between different types of bank note, sinewave signals of different periods T depending on the type of the note and/or having different waveforms depending on the type of the note may be encoded. Example waveforms include sawtooth waveforms, triangular waveforms, and rectangular waveforms.

In addition, analog signals may be encoded in which the period, waveform, amplitude, etc., may all vary continuously or in steps along the length of the security filament, thereby enriching the information content of the filament. In order to make it possible for the filament to be read in either direction, it is preferable to use waveforms that are symmetrical.

Another approach of the present invention is shown in Figure 6. It consists in encoding an analog signal using the principle of delta modulation. In this case, the security filament includes a _ succession of same-length segments capable of having two different duty ratio values, e.g. 0% and 33%. corresponding respectively to a drop in the encoded analog signal and to a rise therein.

Encoding of this type is advantageous in that it is well adapted to data compression, thereby making it possible to incorporate increased information density in the security filament for given length and magnetic ink printing resolution.

In a variant, a derivative of delta modulation can be used in which each unit of information can take up three values, in a manner analogous to the ternary encoding mentioned above.

A particular advantage of the forms of encoding described with reference to Figures 5 and 6 lies in good noise immunity and also in good selectivity. More precisely, interference signals can easily be eliminated by appropriate signal processing downstream from an appropriate demodulation circuit, and in particular by smoothing of a type that is known per se. In practice, this is no longer a case of encoding discrete digital or alphanumerical information, but of encoding a special signature for the security paper together with the corollary that error sensitivity can be small.

Both when using pulse width modulation and when using delta modulation, the reader device for the security filament includes a decoding module enabling the analog signal to be restored. This signal, possibly after additional processing, filtering, and time scale compression, may be restored acoustically. Under such circumstances, the authenticity of the paper can be verified by an operator merely by listening to the signal.

An advantage that is common to all implementations of the invention may be mentioned at this point. It has been observed experimentally that when a security filament or film has been coated with magnetic or magnetizable ink over an extended zone, there is a large risk of the ink breaking or cracking once dry, accompanied by a risk of the ink coming off and thus of the security filament being read wrongly. In contrast, given that the encoding of the present invention never requires extensive magnetic zones to be provided, this risk is minimized.

Whatever form of encoding is used, making discontinuous magnetized filaments and incorporating them in banknote paper requires the use of techniques that are themselves known.

For example, the magnetic zones may be obtained by printing using a magnetic ink on a film that is not magnetic. A material that is particularly suitable for the film is polyester which may be colored or otherwise depending on whether or not it is desired to mask the printing. In a first case, the film may be cut up into very narrow strips that can be considered as being filaments and they can be embedded in the thickness of the paper. Alternatively, strips cut from the film may be transferred onto the surface of the paper, e.g. using a conventional hot transfer technique. It is also possible to use a detachable varnish which can be transferred using the same technique.

Printing is advantageously performed using the photogravure or the silkscreen technique. In practice, using these techniques, the length of unit information segments may advantageously lie in the range 1 mm to a few mm. Naturally, shorter lengths may also be obtained.

In order to ensure that each document, and in particular each bank note, includes at least one entire word together with its delimiters, if any, either continuously or when using encoding by means of an analog signature with at least one or more full waves thereof being encoded, it is advantageous to provide a length of filament incorporated in the document that is equal to or greater than twice the length of the encoded word or wave, as the case may be. Thus, Figure 7 shows a bank note B which is rectangular in shape and which includes, parallel to and in the vicinity of one of its shorter edges, a filament or tape which is encoded by means of the invention and for which, regardless of the offset between the position of the information on the filament and the edges of the bank note, it is certain that there will always be at least one full word on the bank note.

The filament integrated in banknote paper is preferably read by initially magnetizing the magnetizable zones by passing close to a magnet, and then by passing beneath a sensor such as a coil sensor, a magnetoresistive sensor, or a Hall effect sensor.

Naturally, the present invention is not limited in any way to the embodiment described above and shown in the drawings, and the person skilled in the art will be able to provide variants or modifications.

Thus, although the entire description above has been given with reference to encoding by magnetization or by absence of magnetization, the invention is advantageously applicable to encoding using zones having any kind of active or activatable substance and zones that do not include such a substance. If the substance is activatable, activation may be performed in any suitable manner, for example by applying a field of electromagnetic waves.

For example, substances may be used that have special behavior when subjected to infrared or to ultraviolet radiation, or else metallization substances may be used.

The invention may be applied to authenticating all kinds of security paper, in particular bank notes, checks, etc., and also all documents made of plastic, such as credit cards or the like.

Claims (10)

CLAIMS:

1/ A method of encoding a security filament or tape (F) for banknote paper or the like, in which an encoded word is applied to the filament in the form of a plurality of information units respectively constituted by a plurality of segments all of substantially the same length, each word including a fixed number of segments, the method being characterized in that the information in each segment is determined by the duty ratio of the length of an active or activatable zone of the segment to the total length of said segment, and in that the fixed number of segments comprise a first succession (M) of segments having a first information value and a second succession (N) of segments having a second information value.

2. / A method according to claim 1, characterized in that the active or activatable zone of each segment is a magnetic or magnetizable zone.

3. / A method according to claim 1 or 2, characterized in that a binary code is used, with two information values corresponding to two different duty ratios.

4. / A method according to claim 3. characterized in that one of the duty ratios has the value zero.

5. / A method according to claim 1, characterized in that an analog signal is encoded.

6. / A method according to claim 5. characterized in that the encoding is performed using pulse width modulation.

7. / A method according to claim 5. characterized in that the encoding is performed using delta modulation.

8. / A method of encoding a security filament or tape for banknote paper or the like, substantially as herein described with reference to the accompanying drawings.

9. / A security filament for banknote paper or the like, obtained by a method according to any preceding claim.

10. / Banknote paper, in particular for use in bank notes, and including a security filament according to claim 9.