WO1990007760A1

WO1990007760A1 - Electronic article surveillance system with improved differentiation

Info

Publication number: WO1990007760A1
Application number: PCT/US1989/005874
Authority: WO
Inventors: Phillip J. Lizzi; Richard A. Shandelman
Original assignee: Checkpoint Systems, Inc.
Priority date: 1989-01-09
Filing date: 1989-12-27
Publication date: 1990-07-12
Also published as: ES2020841A6; NO903912L; AU660708B2; AU4828790A; DD291654A5; EP0403632A4; NO180699B; JPH03503219A; AU631170B2; DK215790D0; NO903912D0; MX173773B; FI904415A0; CA2007310A1; AU2455192A; EP0403632A1; NO180699C; DK215790A

Abstract

An electronic article surveillance system (1) which is capable of identifying and discriminating between the different signatures of tags (2), improving the reliability of the system and even permitting the tags (2) to be classified by type, and separately addressed, includes a receiver (6) which incorporates improvements in its filtering and processing sections. A linear phase filter (9) is used to more effectively preserve the signal which is received, and thereby improve the signal which is ultimately delivered to the processor (11). The processor (11) is provided with a ''hysteresis-type'' threshold detector (70 and 71) which operates to preserve the original signal by improving the shape of the pulse which is ultimately delivered to the processor (11), and an adaptive processing routine which varies the subsequent processing of detected signals according to changes within the system (1) to improve the system's ability to discriminate between the different signals which are received.

Description

ELECTRONIC ARTICLE SURVEILLANCE SYSTEM WITH IMPROVED DIFFERENTIATION

APPENDIX

An appendix containing a total of 13 sheets pertaining to a computer listing is submitted with this specification.

The present invention generally relates to electronic security systems, and in particular, to an improved electronic article surveillance system.

A variety of electronic article surveillance systems have been proposed and implemented to restrict the unauthorized removal of articles from a particular premises. One common form of this is the electronic article surveillance system which has come to be placed near the exits of retail establishments, libraries and the like. However, electronic article surveillance systems are also used for purposes of process and

inventory controls, to track articles as they pass through a particular system, among other applications.

Irrespective of the application involved, such

electronic article surveillance systems generally operate upon a common principle. Articles to be monitored are provided with tags (of various different types) which contain a circuit (a resonant circuit) for reacting with an applied radio-frequency field. A transmitter and a transmitting antenna are provided to develop this applied field, and a receiver and a receiving antenna are

provided to detect disturbances in the applied field. If the resonant circuit of a tag is passed between the transmitting and receiving antennas (which are generally placed near the point of exit from a given premises), the applied field is affected in such fashion that a

detectable event is produced within the receiver. This is then used to produce an appropriate alarm. Systems of this general type are available from manufacturers such as Checkpoint Systems, Inc., of Thorofare, New Jersey, among others.

Although such systems have proven effective in both security as well as inventory and process management, it has been found that certain improvements to such systems would be desirable. Perhaps foremost is the ever-present desire to reduce to the extent possible any errors (e.g., false alarms) which are produced by such systems, particularly in terms of their discrimination between the presence of a tag (signifying the presence of a protected article) and other interference which may be present in the vicinity of the electronic article surveillance system. Any steps which can be taken to improve the accuracy of the system will tend to reduce such

undesirable results.

More recently, it has become of interest to provide an electronic article surveillance system with sufficient resolution to actually distinguish between different types of tags, resulting from differences in the resonant circuits which they contain. It has long been recognized that different types of tags have different "signatures" (responses) corresponding to the configuration of the resonant circuits which they contain. For example, the resonant circuit of a so-called "hard" tag will generally tend to produce a signal which is somewhat stronger than other types of tags, such as hang-tags and labels, resulting from differences in the size and configuration of the components which comprise these particular labeling devices. As a result, it becomes conceptually possible to differentiate between these various types of tags and labels by analyzing their signatures, by discriminating between the different signals which are possible. However, to date, available systems did not possess the sensitivity to detect these differences in a reliable fashion.

Summary of the Invention

It is therefore the primary object of the present invention to provide an electronic article surveillance system of improved accuracy and reliability.

It is also an object of the present invention to provide an electronic article surveillance system which can accurately and reliably react to an increased

proportion and diversity of labels or tags which it may encounter.

It is also an object of the present invention to provide an electronic article surveillance system which can reliably discriminate between the signal produced by a tag passing in the vicinity of the electronic article surveillance system, and potential sources of

interference.

It is also an object of the present invention to provide an electronic article surveillance system which can discriminate between different types of tags and labels.

It is also an object of the present invention to provide an electronic article surveillance system which can separately and adjustably address tags or labels according to desired operating parameters.

These and other objects are achieved in accordance with the present invention by providing the electronic article surveillance systems which were previously available with several different improvements which combine to achieve the above-stated goals.

For example, the transmitting antenna for the system now utilizes a "paired-lead" loop antenna configuration in place of the single-lead or single coaxial cable loop antennas of the prior art. The term "paired-lead" includes not only the twin-axial cable which is currently preferred for use but also other arrangements of two parallel leads, such as so-called "zip cords", paired coaxial cables and the like. Within each set of

paired-leads, one lead forms an "active" antenna loop, i.e. one which is driven by the transmitter circuitry, in the case of the transmitting antenna, and which drives the receiver circuitry in the case of the receiving antenna. The other lead forms a "passive" loop, i.e. one which is not driven or driving, but rather interacts with the respective active loop only through mutual coupling between them. The passive loop can then be appropriately passively loaded, and the combination of active and passive loop will then exhibit the desired flattened amplitude and linearized phase response. However, this beneficial effect will be obtained without substantially detracting from the efficiency of the antenna which is so configured. In addition, one of the paired leads, preferably the passive one, can supply energizing signals from the receiver circuitry to the alarm devices of the system (e.g., warning light or buzzer), whenever a tag is detected.

The receiver for the system is provided with improved means for detecting signals resulting from tags or labels passing in the vicinity of the receiving antenna,

including improvements in its filtering and processing sections. A linear phase (constant group delay) filter is used to more effectively preserve the signal which is received, and thereby improve the signal which is

ultimately delivered to the processor which follows. The processor is provided with a "hysteresis-type" threshold detector which operates to further preserve the original signal by improving the shape (width) of the pulse which is ultimately delivered to the processor following conversion from analog form, and an adaptive processing routine which varies the subsequent processing of

detected signals according to changes within the system (primarily resulting from changes and/or imperfections in the manner in which the tag or label is presented to the transmitting and receiving antennas), to improve the system's ability to discriminate between the different signals which are received by the unit.

These several improvements combine to provide an electronic article surveillance system which is capable of reliably identifying and discriminating between the different signatures of tags and labels which may come to pass in its vicinity, improving the reliability of the system and even permitting the tags and labels which may come to pass in the vicinity of the system to be

classified by type, and separately addressed. Further detail regarding an electronic article surveillance system having these capabilities may be had with

reference to the detailed description which is provided below, taken in conjunction with the following

illustrations.

Brief Description of the Drawings

Figure 1 is a block diagram of a conventional

electronic article surveillance system.

Figures 2a and 2b are diagrammatic plan views showing an improved antenna system for use in conjunction with the transmitting and receiving portions of the electronic article surveillance system of Figure 1.

Figure 3 is a schematic diagram of an equivalent circuit for the antenna systems shown in Figures 2a and 2b.

Figure 4 is a graph which illustrates the frequency and phase response of the antenna systems shown in

Figures 2a and 2b.

Figure 5 is a schematic diagram of an improved

receiver used in conjunction with the electronic article surveillance system of Figure 1.

Figure 6 is a graph which illustrates the manner in which a received signal is processed by the receiver of Figure 5.

Figure 7 is a graph which illustrates the manner in which the analog signals shown in Figure 6 are converted to a digital representation, for presentation to the processor.

Figure 8 is a graph which illustrates the manner in which the processor operates to discriminate between the various digital signals which are received.

Figure 9 is a schematic representation of a security system which incorporates a plurality of surveillance devices and supporting equipment in a single interactive environment.

In the several views provided, like reference numerals denote similar elements. Detailed Description of the Preferred Embodiment

Figure 1 shows (in block diagram form) what generally constitutes the conventional components of an electronic article surveillance system 1 of the type manufactured by and available from Checkpoint Systems, Inc., of

Thorofare, New Jersey. This system 1 includes a tag 2 which can be applied to any of a variety of different articles in accordance with known techniques. For example, the tag 2 may take the form of a "hard" tag which is attachable to an article using the connecting pin with which this type of tag is generally provided. Alternatively, the tag 2 may take the form of a hang-tag which is appropriately tied to the article. The tag 2 may also take the form of a label adhesively affixed to the article. Any of a variety of types of tags and application techniques may be used to accomplish this general task.

Irrespective of the type of tag which is used, or its manner of attachment to the associated article, the tag 2 incorporates a resonant circuit (not shown) which is capable of reacting to applied fields of electromagnetic energy. A transmitting antenna 3 is provided which is capable of developing these applied fields responsive to the operation of associated transmitter circuitry 4. A receiving antenna 5 is provided for receiving

electromagnetic energy both from the transmitting antenna 3 and the resonant circuit of the tag 2 to develop a signal which is in turn applied to a receiver 6. The receiver 6 then operates upon this detected signal to determine that the tag 2 is present in the vicinity of the transmitting and receiving antennas 3, 5, and give an alarm if such is the case.

This is generally accomplished by applying the signal which is picked up by the receiving antenna 5 to an amplifier 7, which operates to improve this received signal. The amplified signal is then applied to a detector 8 which essentially operates to recover (or demodulate) the active (base band) component which is used to detect the presence of a tag 2 in the vicinity of the electronic article surveillance system 1 from the high frequency (carrier) component of the signal which is required for use in conjunction with the transmitting and receiving antennas 3, 5. The base band signal which is isolated by the detector 8 is then applied to a filter 9 which operates to further attenuate undesirable low and high frequency signal components, including noise and other interference inherent in the isolated signal. The filtered signal is then applied to a converter 10 which operates to convert the analog signal received from the filter 9 to a digital signal which is suitable for presentation to a digital processor 11. Operations are then performed within the processor 11 to interpret the signal which is received, and to determine whether this received signal indicates the presence of a tag 2 in the vicinity of the transmitting antenna 3 and the receiving antenna 5, thereby representing a detectable event.

As previously indicated, and in accordance with the present invention, this otherwise conventional

configuration is modified in various ways to improve the resolution of the resulting system, thereby improving its ability to differentiate between signals representative of a tag 2 passing near the transmitting antenna 3 and the receiving antenna 5, and other signals (noise, interference, etc.) which do not represent a properly detected event, and developing the ability to actually distinguish between different types of tags based upon differences in the signatures of the resonant circuits which they contain. This includes modifications to the transmitting antenna 3 and their receiving antenna 5, as well as modifications to the filter 9 and converter 10 which operate to provide signals to the processor 11, and the routine (software) which is employed to then process these received signals. Further detail regarding each of these improved components is provided below.

The transmitter circuitry 4 substantially corresponds in structure to the transmitters of prior electronic article surveillance systems of this general type.

However, where possible, steps are taken to reduce distortion within the unit.

Referring now to Figures 2a and 2b of the drawings, these show the manner in which antennas embodying the present invention may be configured and mounted.

Figure 2a shows this for the transmitting antenna 3, Figure 2b for the receiving antenna 5.

In each case, there is provided a housing 7. In its presently preferred embodiment, this housing 7 is made of a hollow synthetic plastic body, in whose interior all the other elements are positioned. Specifically, in the base portion 7a of Figure 2a, there is located the transmitter circuitry 4 (Figure 1) while, in the base portion 7a of Figure 2b, there is located the receiver circuitry 6 (Figure 1).

Each housing 7 has a pair of uprights 7b and 7c, which are connected by cross-members 7d and 7e. In each housing 7, the antenna loop 15 starts at the base portion 7a and extends upwardly on one side of the loop into upright portion 7b and on the other side into upright portion 7c. However, at cross-member 7d, these sides of the antenna loop 15 change places, i.e. the portion extending along upright 7b switches over to upright 7c and vice-versa. The antenna loop 15 is then completed within cross-member 7e.

This crossing over of the upper and lower portions of each antenna loop 15 is what creates far-field cancellation of the antenna patterns, as appropriate to satisfy FCC regulations, as well as to reduce

interference from remote sources of extraneous radio frequency energy. This technique of using one or more such cross-overs is known, and in itself, does not constitute an element of the present invention.

However, in accordance with the present invention, the antenna loop 15 is now formed of paired leads, which are preferably embodied in a twin-axial cable (a cable suitable for this purpose is available from Belden Wire and Cable Company, P.O. Box 1980, Richmond, Indiana

47375, under their product number 9271). Such a cable comprises an insulating sleeve, within which extends a pair of separate leads, surrounded by a conductive shield. A conductor for grounding the shield is also provided, and spacers are twisted in with the leads to maintain substantially uniform spacing of the elements within the outermost insulating sleeve.

It is also possible to make use of two discrete, generally parallel wires to form the antenna loop 15.

Paired coaxial cables may also be used. In any case, the individual leads are preferably uniformly spaced from one another throughout their lengths. Further, it is

preferable for the paired leads to.be uniformly twisted along their lengths since this reduces the effect of local irregularities.

When using a shielded set of paired leads, as in the case of the twin-axial cable previously discussed, it is appropriate to provide a break in that shield, to assist the leads inside the shield in performing their basic function as antenna elements. Such a break is

represented at 9a in Figure 2a, where the leads inside shield 9 become exposed. To maintain electrical

continuity for shield 9, the upper and lower portions separated by the

break are conductively connected by conductors 9b and 9c. Although not illustrated, the same break arrangement is preferably provided for the antenna 5 of Figure 2b.

In Figures 2a and 2b, the preferred twin-axial cable is represented somewhat diagrammatically by a tubular element 9 and by conductor pairs 17a, 17b and 18a, 18b, which are seen to emerge from the open lower ends of the element 9. Specifically, element 9 represents the conductive shield of the twin-axial cable; conductor pairs 17a, 17b and 18a, 18b represent the separate leads inside the cable, which become visible in Figures 2a and 2b where they emerge from the inside of shield 9, near the transmitter circuitry 4 and receiver circuitry 6, respectively.

More specifically, conductors 17a and 17b represents the so-emerging opposite ends of the same one of the two separate leads inside shield 9; conductors 18a and 18b represent the opposite ends of the second one of the two separate leads inside shield 9.

As shown in Figure 2a, transmitter circuitry 4 is connected to that one lead whose emerging ends are designated by reference numerals 17a, 17b in Figure 2a. This transmitting circuitry thus constitutes an "active" load for this lead and the loop which that lead forms inside shield 16 constitutes the "active" loop of the transmitting antenna.

In Figure 2b, it is the receiver circuitry 6 which is connected to that one lead whose emerging ends are similarly designated by reference numerals 17a, 17b in Figure 2b. Accordingly, in Figure 2b, it is the

receiving circuitry which constitutes and "active" loop of the receiving antenna.

Turning now to the other lead inside each shield 9, the emerging ends of that lead, which are designated by reference numerals 18a, 18b in each of Figures 2a and 2b, are not connected to the respective active loads (namely to transmitter or receiver circuitry 4, 6). Rather the emerging portions 18a, 18b of these leads are connected in each of Figures 2a and 2b to a "passive" load 20 and the loop which each of these leads forms inside its shield 9 thus constitutes the "passive" loop of the respective antenna.

Each of these passive loops is in turn coupled to the active loop inside the same shield 9 by means of the mutual coupling which exists between two closely adjacent leads.

The impedance of passive load 20 is so chosen that, when it is reflected back into the respective active load through the above-mentioned mutual coupling, the overall effect will be to impart to each antenna loop 15 a much flatter amplitude response and a much more linear phase response than could otherwise have been obtained, without substantially reducing the antenna efficiency.

Because of the distributed nature of the mutual coupling between the leads inside each shield 9, it is difficult to provide a precise equivalent circuit for the arrangement. An approximation of such an equivalent circuit for the transmitter portion of the system is shown in Figure 3 within the broken line rectangle designated by reference numeral 19.

As illustrated in Figure 4, to which reference may now be made, the use of a second lead in the manner embodying the present invention changes the antenna amplitude response from one which is generally similar to that shown at 21 in Figure 4, to one which is generally similar to that shown at 22, i.e. to one which is

significantly more uniform throughout the operative frequency band. Also illustrated in Figure 4 is a corresponding improvement in the antenna's phase

response, from a response generally like that shown at 23, to a comparatively more linear response such as shown at 24

By so flattening the antennas' amplitude response and linearizing their phase response, it becomes possible to effectively detect tag signals over a wider range of frequencies, without creating more false alarms. This is important because the resonant circuit which is part of each tag 2 tends to vary in resonant frequency from one tag to another. Because of this, conventional practice requires a swept frequency to be utilized by the system (e.g., 8.2 MHz ± 800 KHz) so as to effectively interact with such tags despite their variation in resonant frequency. Even then, some tags had to be rejected following their manufacture because they could not satisfy the tolerance requirements for the electronic article surveillance system with which they were to be used. By making it possible to effectively detect a broader range of frequencies, the electronic article surveillance system 1 of the present invention will operate to detect a wider range of resonant tags, in turn permitting a significantly reduced number of tags to be rejected in the course of their manufacture.

Using a twin-axial cable as the receiving antenna 5 provides an additional advantage for the system 1. It is the principal function of the receiver 6 to activate an appropriate alarm when the presence of a tag 2 is

detected between the transmitting antenna 3 and the receiving antenna 5. To that end, there may be mounted inside the upper cross member 7e of housing 7 in Figure 2b a conventional warning light arrangement

diagrammatically represented by rectangle 25. In order to energize this warning light when required, a d-c connection needs to be provided between it and the receiver 6 located in the base 7a of the housing 7. The passive lead (the one whose emerging ends are designated by reference numerals 18a and 18b in Figure 2b) may be used for that purpose. Specifically, d-c output from receiver 6 may be applied to that lead via a connection which is diagrammatically represented by lead 26 in

Figure 2b. At the top of the loop formed by the

twin-axial cable, a connection is made to the same passive lead near the warning light arrangement 25, as diagrammatically represented by connecting lead 27 in Figure 2b. As a result, there is no need for a separate, additional lead between receiver 6 and warning light 25. Potential adverse effects on antenna performance,

resulting from the presence of such an additional lead, are thereby averted.

The result is a highly effective transmitting antenna 3 and receiving antenna 5 which are more uniformly responsive to signals received in the operating frequency range for the system. In addition to the effect of reducing the number of tags which must be rejected for being out of specification (thereby reducing waste), this has the further advantage of providing a relatively

"clean" (distortion-free) signal to the improved receiver 6' of the present invention, which is more fully

illustrated in Figure 5 of the drawings, for further processing as follows.

Referring now to Figure 6, the signal 28 which is received at the antenna 5 (Figure 6a) will primarily constitute a base band signal (e.g., 20 KHz) modulated upon the system's operating frequency (e.g., 8.2 MHz) and contained within an "envelope" corresponding to the intensity (amplitude) of the field which is then being received. The operative frequency (8.2 MHz) is

preferably swept (± 800 KHz approximately 82 times each second) to account for variations in the resonant

circuits of the tags 2. When the tag 2 is caused to pass between the transmitting antenna 3 and the receiving antenna 5, a small deflection 29 will develop in this envelope, which must then be detected by the receiver 6' to provide an appropriate alarm signal. To be noted is that this deflection will occur in both phase and

amplitude, but will be very small in magnitude (generally 1/1000 to 1/10000) in relation to the carrier signal. Careful detection techniques must therefore be used to isolate this signal, and then identify it, as follows, with reference to both Figure 5 and Figure 6 of the drawings.

The received wave form is first amplified (amplifier 7) and then introduced to the detector 8. This

amplification may include a pre-filtering (at 30) and/or post-filtering (at 31) step, if desired. The detector 8 essentially operates to recover (demodulate) the base band (0-20 KHz) signal from its swept carrier (swept about a nominal 8.2 MHz) frequency. The resulting wave form (Figure 6b) will therefore substantially correspond to the isolated base band signal 32, with an added perturbation 33 which corresponds to the deflection 29 (change in amplitude and phase) produced by the presence of the tag 2 between the transmitting antenna 3 and the receiving antenna 5. To be noted is that this signal will tend to vary depending upon the location and orientation of the tag 2 relative to the antennas 3, 5, including variations in both the base band signal 32 and the detected perturbation 33. The resulting signal is preferably then amplified (amplifier 34) prior to introduction to the filter 9.

The filter 9 then operates to isolate the detected signal 32 from other signals which may come to be received by the antenna 5, such as the basic (8.2 MHz) carrier signal, other interfering signal (including signals received from the transmitter 4), and noise outside of the useful band. Preferably used for this purpose is a series combination of a high-pass filter 35 for eliminating undesired lower frequency components followed by a low-pass filter 36 for eliminating

undesired higher frequency components.

It is a particular goal of the electronic article surveillance system 1 of the present invention to

preserve those wave forms which are being processed through the system 1 responsive to a detected tag 2, to the extent possible. Filtering inherently tends to adversely affect such signals, not only in terms of their amplitude, but also by imparting time-delay distortion to the signals which are being processed. The amplitude of the resulting signal is preferably restored in an

amplifier 40 which follows the filter 9. However, preservation of the original wave form remains

compromised as a result of the encountered time-delay distortion.

Previously, and referring now to Figure 6c, such distortion had been compensated for by operating upon not only the primary signal 41 produced by a tag passing between the transmitting and receiving antennas of the system, but also one or more of the distortion products 42 produced by the filtering step. In accordance with the present intention, the filter 9 is presently

configured as a linear phase (constant group delay) filter to avoid the adverse effects of time-delay

distortion. Any of a variety of known linear phase filter configurations may be used for this purpose. The result is a filtered signal 43 (Figure 6d) which as closely as possible corresponds to the initial signal produced by the transmitter circuitry 4 and isolated by the detector 8 (Figure 6b). As will be further addressed below, this has significant advantages in connection with the subsequent processing which is to take place,

contributing to the various improvements which are provided in accordance with the present invention. A smoothing filter 44 preferably follows the amplifier 40 to further remove noise components within the operating base band.

What is more, such filtering permits the received signal to be more effectively distinguished from that of the transmitter within a significantly lower frequency band, when the detected signal resulting from the

presence of the tag 2 is exhibiting an increased

magnitude from previously available systems. By way of explanation, and referring now to Figures 6e and 6f, the receiver 6' will operate to detect both a signal 45 from the transmitter 4 and a signal 46 from the tag 2

(including the signals and their harmonics). As shown in Figure 6e, the tag signal 46 will not be easily

distinguished from the transmitter signal 45 (which are of the same general type) until the frequency band 47 is reached. However, referring now to Figure 6f, it is seen that the above-described filtering causes the transmitter signal 45' to roll off more rapidly than the tag signal 46, allowing the tag signal 46' to be differentiated from the transmitter signal 45' within the frequency band 48, where the tag signal 46' exhibits an increased magnitude. This operates to preserve more of the

available tag signal 46' for further processing.

Referring now to Figure 7, the filtered signal 50 shown in Figure 7a (including responses 51 representing detected tags and responses 52 representing interfering signals) is then applied to the converter 10 to be converted from the analog signal which is received from the filter 9 to a digital signal which is appropriate for presentation to the processor 11. As with prior

processors of this general type, the received analog signal is digitized to a one-bit resolution (a "one" or a "zero") since this has been found to provide sufficient resolution for interpretation by the processor 11. To be noted is that while this is presently preferred in view of its simplicity, it would be equally possible for higher resolution conversions to be used in conjunction with a multi-bit processor, if desired.

Referring now to Figure 7b, such conversion was previously accomplished using a threshold detector which operated to detect levels exceeding certain selected thresholds 55, 56 centered about a pre-selected level 57, to produce desired transitions (forming pulses) according to variations in the level of the applied analog signal (developing a positive pulse for both positive-going and negative-going signals), in this case the tag signal of Figure 6c. This in turn developed a series of positive pulses 58, 59, 60, 61 having pulse widths which would vary according to the analog signal which was then received from the filter 9.

The widths of these resulting pulses defined the

"signature" for a particular tag 2 detected between the transmitting antenna 3 and the receiving antenna 5.

Other pulses would also be developed resulting from other signals, particularly interference in the vicinity of the electronic article surveillance system. However, since these additional pulses had characteristics (widths) which differed from the signature of the tag 2 which was being searched for, it was possible for the processor of the system to determine whether a particular series of pulses corresponded to the signature (pattern) of a tag 2, or an interfering signal.

As previously indicated, a broader range of signals for enabling this determination to proceed will be made available by the transmitter and receiver components which have earlier been described, as well as the

associated transmitting antenna 3 and receiving antenna 5, which cooperate to better preserve the signals which are to be operated upon. However, even with these improvements, it was found that the techniques which were employed by previous processors to make such a

determination were still generally insufficient to distinguish between these various pulses with sufficient particularity for the processor 11 to be able to

discriminate between different signatures corresponding to different types of tags, in addition to its primary function of distinguishing between tag signatures and interfering signals.

The primary reason for this arises from certain considerations relating to the tag 2 which is then being passed between the transmitting antenna 3 and receiving antenna 5. As is the case with any tag, and particularly in connection with an unauthorized removal of an article it can be expected that the tag 2 will not always be placed in an optimum position relative to the

transmitting antenna 3 and the receiving antenna 5 to produce a maximized signal at the receiving antenna

(i.e., generally parallel to the plane of the

transmitting antenna 3 and the receiving antenna 5).

Rather, it can be expected that the tags will come to be placed at different angles relative to the antennas 3, 5.

As a result, signals of different quality will often come to be applied to the converter 10, producing widely different signals for interpretation by the processor 11. For example, and referring now to Figure 7c (somewhat expanded in scale for illustrative purposes), a signal 65 of relative strength will tend to cross the selected threshold 55 rather quickly, and will return to that selected threshold rather late, developing a relatively wide pulse 66. However, a signal 67 of reduced strength will more rapidly reach and return to the selected threshold 55, producing a pulse 68 of significantly reduced width. This has been found to complicate, and often compromise the signal processing steps which are to follow.

The technique which is generally used to distinguish between pulses which correspond to the signature of a tag and pulses which correspond to an interfering signal is to determine whether the received pulse has a duration (width) which falls within a predefined "window". This window is established (set) within the processor 11 and must be broadly defined to accommodate not only the variety of different tag configurations which can be anticipated, but also the broad spectrum of detected pulses which might correspond to an interfering signal. As a result, it was not possible for such systems to distinguish between different types of tags (and their signatures), and it was not uncommon for these systems to fail to distinguish a valid pulse of reduced width (i.e., the pulse 68) from a source of interference, failing to detect the presence of a tag 2 between the antennas 3, 5. Broadening the defined window would help the system to recognize a greater number of tags. However, this has the corresponding disadvantage of also identifying and accepting a greater number of interfering signals as the presence of a tag, leading to an increased number of false alarms. This generally necessitated the striking of a balance which was at times less than optimum.

In accordance with the present invention, various steps are taken within the converter 10 and the processor 11 to improve the overall detection process, and to more carefully distinguish between the signature of a tag and other signals which may come to be received in the course of operating the electronic article surveillance system 1.

The first of these improvements forms part of the converter 10, and relates to the manner in which the initial threshold comparisons are made. Specifically, a "hysteresis-type" threshold comparison is made, making use of two different thresholds (developed by the two different comparator circuits 70, 71 of Figure 5) which are selected to define (detect) the leading and trailing edges of the converted pulse, respectively. Referring now to Figure 7d, by properly selecting the two different thresholds 72, 73, the same initial signals 65, 67 which are shown in Figure 7c will result in pulses 74, 75 which are significantly closer in proportion to one another than were the pulses 66, 68. As a result, the pulses 74, 75 constitute a more accurate representation of the initial signal. This applies not only to the stronger signals, but also to the signals of reduced strength, which operates to significantly expand upon the range of signals which are effectively detectable by the converter 10, for subsequent processing.

Selection of the two different thresholds 72, 73 is made according to the particular signature

(characteristics) of the tag 2 which is to be operated upon, as well as the anticipated environment for the system. Consequently, these levels are preferably made adjustable to accommodate different applications. This may include both adjustments in relative level (i.e., upper and lower thresholds varied as a pair) as well as adjustments in the difference between the two selected thresholds, as desired. It is even possible to adjust the thresholds 72, 73 so that one is positive while the other is negative, should this be indicated for a

particular application.

Referring now to Figure 8 of the drawings, this improved signal is in turn applied to the processor 11, which incorporates additional improvements for further discriminating between tag signatures and interference, as follows. As is conventional, following the detection of a leading edge 82 of a first pulse 81 resulting from a detected signal 80 (either a tag signature as illustrated, or an interfering signal), steps are taken to determine whether that pulse's trailing edge 83 falls within a predefined window 85 established for the anticipated pulse width of a desired tag signature. If so, steps are then taken to analyze the next pulse 90 in the detected series 80.

Previously, this was accomplished by similarly

comparing the width of the second pulse 90 with a preestablished (fixed) window for that pulse. However, in accordance with the present invention, this prior

technique is replaced with an analysis of the second pulse 90 according to a variable window 91 which is

"redefined" (computed and adjusted) according to a routine established within the processor 11. The

computational adjustment which is made is based upon the analysis of the first pulse 81 in the series 80, and certain assumptions which are made regarding the

anticipated characteristics of the second pulse 90 which is to follow. If the second pulse 90 is then determined to constitute the signature of a tag 2, a counter

(conventionally provided in software within the processor 11) is incremented as before. However, to be noted is that this incrementing is performed after only two pulses 81, 90 have been successfully analyzed, as distinguished from the prior systems which would generally require a third pulse 95 of the detected signal 80 to be analyzed before this determination could be made.

As previously indicated, electronic article

surveillance systems of this general type are configured to repeatedly sweep about the nominal operating frequency of the system, thereby developing repeated signals corresponding to the presence of a tag 2 between the antennas 3, 5. This in turn produces plural signatures which must then be detected by the processor 11, in similar fashion. In addition to making a determination as to whether or not a subsequently received signal corresponds to the signature of a tag 2 or some other signal (i.e, interference), as described above, steps are also taken to determine whether or not the detected signal corresponds in time to a scheduled sweep by the transmitter circuitry 4. If an identified signature is detected during a scheduled sweep of the system, steps are again taken to increment the system's counter.

Otherwise, a spurious signal is deemed to exist and that signal is ignored.

In prior systems, this continued until the counter reached a selected number (e.g., six or seven counts), when a tag 2 would be deemed to be present and an alarm sounded. However, when a tag 2 passes through the electromagnetic field which is produced by the system, it is often the case that the relationship between the field (flux) which is produced and the resonant circuit of the tag 2 which is moving through that field will vary. This would in turn cause variations in the tag signals

(primarily in magnitude) which were detected responsive to successive sweeps of the transmitter circuitry, which at times prevented an effective recognition of a tag signature by the processor 11. The improvements

described in connection with the electronic article surveillance system 1 of the present invention operate to improve the reliability of this detection process.

However, it is still possible for tag signatures to go undetected. It is for this reason that there is yet another improvement which is incorporated into the processor 11.

Specifically, it was previously the practice to reset the counter to zero if an anticipated tag signature was not detected during a scheduled sweep of the system, prior to reaching the designated count. This was done to avoid false alarms and the like, but could also result in the failure to detect a tag 2. In accordance with the present invention, this technique is replaced with an up/down counter (within the processor 11) which operates to track both successfully detected signatures, and other events, responsive to periodic sweeps of the transmitter. To this end, if a tag signature is detected, and if the detected signature occurs following a scheduled sweep (within a defined window), the counter is incremented. Detected events occurring outside of the windows defined for the swept signal are ignored. If no tag signature is detected within the prescribed window, the counter is decremented. This continues until such time as the counter either reaches a prescribed threshold (e.g., five counts) or returns to zero (no tag present),

significantly diminishing the effects of undetected signatures. To be noted is that a variety of different counts may be selected for use in this regard. For example, it is possible for an increment to result in an increase of one, or more than one. Similarly, a

decrement may correspond to one, or some greater number. The count established for an increment may be the same as that established for a decrement (i.e., one to one), or different counts may be used, as desired in a particular application.

Referring again to Figure 5, a system for providing these functions generally comprises a processor 11 which receives its primary signal 100 from the dual threshold detectors 70, 71, and appropriate controlling signals from an external signal detector 101 which precedes the linear phase filter 9 (which provides a logic level for timing purposes), and is provided with the computer program listing which follows this specifications

(Appendix). If desired, the processor 11 is additionally controllable (programmable) at 102 to vary the window which is used to analyze the first pulse of a received signal (subsequent pulses are analyzed according to computationally adjusted windows as previously

described).

To be noted is that the processor 11 can also be controlled, at 103, to change the sweep rate of the electronic article surveillance system 1 from the previously described rate of 82 Hz to a different sweep rate if desired. This permits the electronic article surveillance system 1 to separately address tags using different sweep rates, for reasons which are best illustrated with reference to Figure 9.

In practice, it is not uncommon for a complete security system 105 to employ a plurality of electronic article surveillance devices 106, 107, 108, in addition to other support equipment such as tag deactivators 109, 110 and the like. In many cases, these structures must be positioned relatively close to one another, which can give rise to interference between these various devices. Such interference results from operating each of the several units at the same basic frequency. Small differences in these operating frequencies (resulting from design tolerances and the like), or their

sychronization, can produce beat patterns which at times generate false alarms and other spurious signals.

Previously, this was accommodated by sychronizing the several units employed to one master unit (e.g., synchronizing the devices 106, 107 and deactivators 109, 110 to the device 108), thereby avoiding interference between the various units employed. However, this often complicated the installation of such systems, in view of the wires which needed to be run between the several units, and could also at times produce unacceptable interference on such connecting wires (which would themselves tend to act as antennas producing interfering signals). In any event, when initially installing a security system of this general type, it was necessary to very carefully adjust (tune) the various components of that system to reduce the foregoing problems to the extent possible. At times, it was even necessary to readjust the various components of the system to maintain this careful balance.

In accordance with the present invention, the need for such special measures is eliminated by causing each of the several components which comprise the installed system to operate at different sweep rates, thus avoiding the potential for interference between these respective components. For example, the devices 106, 107, 108 could be operated at three different sweep rates, with the deactivators 109, 110 operating at a fourth and different sweep rate (it is not necessary for the deactivators to operate at different rates so long as their rate of operation differs from those of the accompanying

electronic article surveillance devices). Due to the programmability of the processor 11, this improvement in system operation is achieved in a straightforward manner which can be tailored to particular applications, as desired.

To be noted is that the different sweep rates which are used can be selected, as desired, although it is presently considered important to maintain the selected sweep rates above 70 Hz and below 90 Hz to avoid

impairment of the system's overall function, and to separate the selected sweep rates by at least 3 Hz to permit the system to distinguish between the sweep rates which are available.

These above-described adjustments can either be incorporated into the system by pre-established

programming of the processor 11, if desired, or by switchably selecting between them according to the particular application which is needed. This would include both the selection of basic sweep rate for the system, as well as the selection of window parameters for detecting tag signatures.

Accordingly, it is seen that a variety of improvements are combined in accordance with the present invention to significantly reduce distortions within the system, to better preserve the basic signals which are developed responsive to the presence of a tag, and to more

effectively interpret the signals which result. This includes not only the careful design of various

components to reduce distortion, but also the specific improvements of the present invention including the improved configurations for the transmitting antenna 3 and the receiving antenna 5, the improved configuration for the filter 9 and the converter 10, and the improved processing routines which are performed within the processor 11. The result is a system which not only improves the differentiation of tag signals from other interfering signals, but which is sufficiently sensitive to even permit a discrimination between different tag signatures.

Such improved discrimination gives rise to

capabilities which were not achievable with previously available electronic article surveillance systems. For example, it now becomes possible to actually discriminate between different types of tags, permitting a

classification of tag groups according to their signature (characteristics). This can be used to better match the electronic article surveillance system 1 to the

particular tag which is to be used, to achieve a more error-free result, or to distinguish between different types of tags used with the electronic article

surveillance system 1. This can also be used to change the sweep rate used in conjunction with operation of the electronic article surveillance system 1, to avoid interference with adjacent components. What is more, these functions are easily varied by adjusting

(programming) the parameters to be used within the processor 11, as previously described.

It will therefore be understood that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.

************************************************************************** PROGRAM FOR 63(A/B)01/637(A/B)01 VOC MICROPROCESSOR IN ALPHA RECEIVER

( 6 MHZ Resonator)

(Fixed sweep rate selects)

( Panasonic smoothing filter )

***********************************************************************

********************** VER.1.2A ******************************

********************** PORT DEFINITIONS *****************************

MICRO IS SET UP IN MODE 7 (SINGLE CHIP)

PORT 1

P17 P16 P15 P14 P13 P12 P11 P10

MSB Pgm Sel LSB Aim Time Alm Src (+) (-) J1a J1b

in in in in in- in- swpsel in

PORT 2

P27 P26 P25 P24 P23 P22 P21 P20

N / u Video En ICR(Video)

Mode Set

out- in in in-

PORT4

P47 P46 P45 P44 P43 P42 P41 P40

TP5 TP4 P.S. PS En Ext lnh Aim Almflg - - - out out in- in in out out-

**************************************************************************

EQUATE TABLE

************************************************************************** p1ddr equ 00h

p2ddr equ 01h

p1data equ 02h

p2data equ 03h

p4ddr equ 05h

p4data equ 07h

stacktop equ 0ffh

romstart equ 0f000h ramstart equ 40h

ramstop equ 0ffh

pstmax equ 45

tcsr equ 08h

ocr equ 0bh

beeptime equ 0c000h

icr equ 0dh

timer equ 09h

twoffset equ 1200

onesec equ 22 ; one sec. timer

fivsec equ 112 ; five sec. timer

maskon equ 02h

swp78 equ 19230

swp82 equ 18290

swp86 equ 17440

swp90 equ 16670

swpmin78 equ 19480

swpmin82 equ 18520

swpmin86 equ 17650

swpmin90 equ 16850

swpmax78 equ 18990

swpmax82 equ 18070

swpmax86 equ 17240

swpmax90 equ 16480

swpadj78 equ 20

swpadj82 equ 0

swpadj86 equ -20

swpadj90 equ -35

***************************************************************************

MICRO SETUP

The 6301 is configured in a mode 7 status as follows:

1. Internal RAM from 40h to FFh

2. Internal ROM from F000h to FFFFh

3. NMI tied to ground

4. IRQ line tied to beat note detection ckt- 5. Output (P41) used for Alarm Level

6. Timer input (P20) used for +thresh ored -thresh input

7. P42 is output used for Sonalert and lamp driver

8. P10/P11 (J1) select sweep rate parameters

************************************************************************ status ds 1

signflg ds 1

tlead ds 2

tfall ds 2

tend ds 2

temp ds 2

tlimits ds 4

pent ds 1

vflag ds 1

almenflg ds 1

pstimflg ds 1

pstimer ds 1

afmcntr ds 1

wϊndowl ds 2

window2 ds 2

tpcnt ds l

time95 ds 2

almflg ds 1

tmark ds 2

tagcnt ds 1

srchflg ds 1

vent ds 1

atimer ds 1

atimflg ds 1

rtclk ds 1

tripcnt ds 1

tdetlim ds 4

tdrop ds 2

tbuff ds 2

bniflg ds 1 inhflg ds 1

swptime ds 2

swpmin ds 2

swpmax ds 2

baslimit ds 4

*************************************************************************

Initialization Procedure

************************************************************************* org romstart

reset Idaa #00000000b ;init i/o port 1

staa plddr

Idaa #00000000b

staa p1 data

Idaa #00001000b ;init i/o port 2

staa p2ddr

Idaa #00001000b ;disable composite video

staa p2data

Idaa #11000110b ;init i/o port 4

staa p4ddr

Idaa #00000000b

staa p4data

Ids #stacktop ;init top of stack

ramchk Idx #ramstart ;write / read ram

Idaa #0aah

ramlp1 staa 0,x

cmpa 0,x

bne ramerror

inx

cpx #ramstop+1

bne ramlpl

Idx #ramstart

Idaa #55h

ramlp2 staa 0,x

cmpa 0,x

bne ramerror inx

cpx #ramstop+1

bne ramlp2

bra ramok

ramerror Idaa #01 h ;set bit 0 of status staa status

bra romchk

ramok clr status ;reset bit 0 romchk Idx #romstart ;compare rom chksum clra

romlp adda 0,x

inx

bne romlp

tsta

beq romok

Idaa #02h ;set bit 1 of status oraa status

bra romexit

romok Idaa #0fdh ;clear bit 1

anda status

romexit staa status

iochk Idaa p2data ;get levels on port 2 anda #00001111b

cmpa #00001111b ;normal reset levels bne ioerror

Idaa p4data ;get port4 levels anda #11000110b

cmpa #00000000b

bne ioerror

bra io_ok

ioerror Idaa status ;update status byte oraa #04h ;set bit 2 of status bra ioexit

io_ok Idaa status ;clear bit 2

anda #0fbh ioexit staa status

beepcode Idaa status ;get status code anda #07h

beq nofault

bita #01 h ;test ram bit bne rambeep

bita #02h ;test rom bit bne rombeep

bita #04h ;test i/o bit bne iobeep

nofault Idab #1

bra beep

rambeep Idab #2

bra beep

rombeep Idab #3

bra beep

iobeep Idab #4

bra beep

beep Idaa p4data ;alarms on oraa #06h

staa p4data

Idaa #2

bsr bdelay

Idaa p4data ;alarm off anda #0f9h

staa p4data

Idaa #3

bsr bdelay

decb

bne beep

bra rnext

bdelay Idx #0c000h

blpl dex

bne blp1

deca bne bdelay

rts

rnext Idaa #2 ;init tdet counter

staa pent

clr pstϊmflg

clr vent

clr vflag

clr almenflg

clr srchflg

clr tagcnt

clr aimfig

clr pstimer

clr almcntr

clr rtclk

clr atimflg

clr atimer

clr bniflg

clr inhflg

Idd timer ;init valid reply timers

std tend

std tmark

***********************************************************************

Read target type selector switches

*********************************************************************** option Idaa pldata ;read option jumper

anda #0c0h

bne j1

Idx #t1 limits ;get t1 pulse limits

Idd 0,x

std baslimit

Idd 2,x

std baslimit+2

Idaa #5

staa tripcnt

bra rlast J1 cmpa #40h

bne j2

Idx #t3limits ;get t1 pulse limits Idd 0,x

std baslimit

Idd 2,x

std baslimit+2

Idaa #5

staa tripcnt

bra rlast

J2 cmpa #80h

bne J3

Idx #t2limits ;get t1 pulse limits Idd 0,x

std baslimit

Idd 2,x

std baslimit+2

Idaa #5

staa tripcnt

bra rlast

J3 Idx #t4limits ;get t1 pulse limits

Idd 0,x

std baslimit

Idd 2,x

std baslimit+2

Idaa #5

staa tripcnt

bra rlast

rlast Idd baslimit initialize time windows std tdetlim

Idd basllimit+2

std tdetlim+2

Idx #swptbl ;init sweep rate windows Idd 0,x

std swptime Idab #8

abx

Idd 0,x

std swpmin

Idab #8

abx

Idd 0,x

std swpmax

Idaa tcsr ;enable oci, iedg (-)

anda #11111101b

oraa #00011000b

staa tcsr

Idaa p2data ;enable composite video

anda #0f7h

staa p2data

cli

jmp main

**********************************************************************

Main Routine

********************************************************************** main Idx #swptbl ;get J1 position and

Idab pldata ;convert to sweep rate data

andb #03h

aslb

abx

Idd 0,x ;get sweep period

std swptime

Idab #8 ;get min. sweep time

abx

Idd 0,x

std swpmin

Idab #8 ;get max. sweep time

abx

Idd 0,x

std swpmax *********************************************************************

Adjust timing windows for selected sweep rate

********************************************************************* Idab #8 ;make sweep / window adjustments abx

Idd 0,x

addd baslimit

std tlimits

Idd 0,x

addd baslimit+2

std tlimits+2

*********************************************************************

Check external flags and sensors

********************************************************************* Idaa p4data ;check external inhibit

bita #08h

beq no_inh

Idaa #0ffh ;set external inhibit flag

staa inhflg

Idaa p2data ; disable video input

oraa #08h

staa p2data

bra alarmen

no_inh clr inhflg ;clear external inhibit flag

Idaa p2data ;enable video input

anda #0f7h

staa p2data

alarmen Idab #0ffh ;set alarm enable flag

stab almenflg

Idaa p4data

bita #00010000b ;test people sensor enable

bne ptimstat ; sensor enabled

clr pstimflg

bra valchk

ptimstat tst pstimflg ; people sensor timer running? beq psense

Idaa #pstmax ;check for people sensor timeout cmpa pstimer

bhi valchk

ptjnh clr almenflg

clr pstimflg

bra valchk

psense bita #00100000b ;sensor active?

bne ptjnh

clr pstimer ;start people sensor timer

Idaa #0ffh

staa pstimflg

**********************************************************************

Check and time sort valid target responses

********************************************************************** valchk tst vflag ;valid reply?

beq timechk

clr vflag

Idd tmark ;save last valid time

std tbuff

Idd tend ;get new valid time

std tmark

tst srchflg ;aquire mode (clear)

beq pulsel

Idd tmark ;reply in window?

subd time95

bmi early ;reply too early

vnext1 subd swpmin

addd swpmax

bpl failsrch ; reply too late

Idd tmark ;update next window

addd swpmax

std time95

*********************************************************************

Adjust up / down alarm threshold counter ********************************************************************* inc tagcnt

Idaa tagcnt ;alarm condition?

tsta ;valid tag threshold det.

bpl thresh

clr tagcnt ;zero negative count

jmp noalarm

thresh cmpa #10 ;limit counter

bit cntest

Idaa #10

staa tagcnt

cntest Idaa tagcnt ;test for thresh, count

cmpa tripcnt

bio noalarm

Idaa #0ffh ;set alarm flag

staa almflg

clr vent

bra vexit

timechk Idd tmark ;check for timeout

addd swpmin

subd timer

bmi failsrch

bra vexit

failsrch clra ;reset flags

staa almflg

staa vent

dec tagcnt

bgt fnext

staa srchflg ;lose aquisition

staa tagcnt

fnext Idd tmark ;update time slots

addd swptime

std tmark

addd swpmax

std time95 bra vexit

pulset Idd tmark ;init. search effort

addd swpmax

std time95

clra ;adjust counters / timers staa vent

staa almflg

coma

staa srchflg

inc tagcnt

bra vexit

early Idd tbuff ; restore original valid time std tmark

inc vent ;c.b. inhibit function Idaa #5

cmpa vent

bgt vexit

clra ;too many replies staa armflg

staa srchflg

staa tagcnt

staa vent

bra vexit

noalarm clr vent

clr almflg

vexit Idaa p4data ;tp4 low

anda #01111111 b

staa p4data

alarm tst almflg ;check alarm status bne setime

tst atimflg ;timer running? beq alarmoff

tst atimer ;timeout? bgt a_enbl

clr atimflg bra alarmoff

setime Idaa pldata ;read alarm time sel bita #00100000b

beq five

Idaa #onesec

staa atimer

bra aflag

five Idaa #fivsec

staa atimer

aflag Idaa #0ffh ;set alarm timer flag staa atimflg

a_enbl tst almenflg ; alarm enabled? beq alarmoff

Idaa p4data ;turn alarms on oraa #02h

staa p4data

Idaa p1 data ;read alarm select bita #10h

bne beeper

Idaa p4data ;steady alarm oraa #04h

staa p4data

bra next

alarmoff Idaa p4data ;turn off alarms anda #0f9h

staa p4data

bra next

beeper Idab #maskon ;pulsed alarm

andb rtclk

bne hipulse

Idaa p4data ; reset pulse output anda #0fbh

staa p4data

bra next

hipulse Idaa p4data ;set pulse output oraa #04h

staa p4data

next jmp main

**********************************************************************

Internal time clock routine (interrupt)

**********************************************************************timeclk Idaa tcsr ;clear ocr flag

Idd ocr ;reset interrupt flag

std ocr

cli

inc rtclk

inc pstimer

dec atimer

rti

**********************************************************************

Composite video processing routine

********************************************************************** tdet Idaa pldata ;determine + / - threshold

anda #00001100b

cmpa #0ch ;false level detect

beq tfaultl

bita #00000100b ;+ threshold?

bne tminusl

clr sϊgnflg

bra window

tminus1 Idaa #0ffh

staa signflg

window Idd icr ;get th , tw1

std tread

addd #twoffset ;compute end time window std tdrop

Idx #tdetiϊm ;delta time table

nslope Idaa tcsr ;arm falling edge (+)

oraa #02h

staa tcsr Idaa icr ; clear icf flag

icflpl Idaa tcsr ;wait for icflag

bita #80h

bne ftimesav

Idd tdrop

subd timer

bmi tfault1

bra icflp1

tfault1 jmp texit

ftimesav Idaa tcsr ;arm leading edge (-)

anda #0fdh

staa tcsr

Idd icr ;get tf(n)

std tfall

subd tlead ;delt(n) = tf(n) - t1(n)

std temp

subd 0,x ;delt > delt(min)

bmi texit

Idab #2 ;update limit addr. pointer

abx

Idd temp ;delt < delt(max)

subd 0,x

bpl texit

***********************************************************************

Next pulse adaptive computation

*********************************************************************** Idd temp ;update pulse limits

addd #180

std 0,x

Idx #tdetlim

Idd temp

addd #2

std 0,x

dec pent

beq valid icflp2 Idaa tcsr ;wait for icflag(-)

bita #80h

bne signchk

Idd tdrop ;check for window overflow

subd timer

bmi texit

bra icflρ2

***********************************************************************

Video sign alternation check

*********************************************************************** signchk Idaa pldata ;get threshold sign

anda #00001100b

cmpa #0ch ;check for false levels

beq texit

bita #00000100b ;+ threshold?

bne tminus2

tst signffg ;check for sign change

beq texit

clr signflg

bra tnext

tminus2 tst signflg ; -threshold

bne texit

com signflg

tnext Idd icr ;gett(n)

std tlead

bra nslope

valid Idaa #0ffh ;set valid reply flag

staa vflag

Idd tdrop ;update valid time

std tend .

Idaa p4data ;valid reply ind. on t.p.5

anda #10111111b

oraa #01000000b

staa p4data

Idab #25 ;set valid pulse width (50us) tpdly decb

bne tpdly

anda #10111111b

staa p4data

Idaa tcsr ; clear tag ringing edge

Idaa icr

texit Idaa tcsr ; rearm negative slope

anda #Ofdh

staa tcsr

Idaa icr ;clear icf flag

Idaa #2 ;restore counter / pointer

staa pent

Idd tlimits ;restore t1 pulse limits

std tdetlim

Idd tlimits+2

std tdetlim+2

rti

*********************************************************************

External signal adaptive inhibit function

********************************************************************* bni tst bniflg ;already in this routine?

bne xrestore ; restore index register on stack

Idaa #0ffh ;set bni active flag

staa bniflg

Idaa p2data ;inhibit video for a time

oraa #08h

staa p2data

Idx #320 ;delay time = (20 + 2.5 * N) us.

bnidly cli

dex

bne bnidly

sei

tst inhflg ;check external inhibit flag

bne bnext

anda #0f7h ;enable video line staa p2dat

bnext Idaa tcsr ;clear possible icr flag

Idaa icr ;when video is enabled

clr bniflg

rti

xrestore tsx ;form pointer to stacked xreg

Idd #500 ; restore original delay value

std 3,x

rti

********************************************************************

Initial pulse window table

******************************************************************** t1limiis dw 320,500 ;window times (0.5us/cnt)

t2iimits dw 360,500

t3limits dw 400,500

t4limits dw 420,500

********************************************************************

Sweep rate select parameter table

******************************************************************** swptbl dw swp78

dw swp90

dw swp86

dw swp82

dw swpmin78

dw swpmin90

dw swpmin86

dw swpmin82

dw swpmax78

dw swpmax90

dw swpmax86

dw swpmax82

dw swpadj78

dw swpadjθO

dw swpadj86

dw swpadj82 org 0ff00h ;version i.d. string

db 'ALPHA RECEIVER VER. 1.2A 10/10/88' org 0ffeeh

dw reset ;lost processor recovery trap org 0fff4h

dw timeclk ;coarse real time clk using

;ocr overflow

org 0fff6h

dw tdet ;threshhold detector (timer) interrupt org 0fff8h ;beat note detection interrupt dw bni

org 0fffeh

dw reset ;start of reset section

end reset

Claims

CLAIMS What is claimed is:

1. An electronic article surveillance system comprising a transmitter for providing a signal to a

transmitting antenna, to develop an electromagnetic field, and a receiving antenna for receiving signals including signals produced by a resonant circuit forming part of a first tag means associated with an article to be protected, and for providing said received signals to a receiver having means for identifying said tag signals, and means for

discriminating between the tag signals produced by the resonant circuit of said first tag means and tag signals produced by the resonant circuit of a second tag means different from said first tag means.

2. The system of claim 1 wherein said receiver includes a filter for separating said tag signals from other signals received by said receiver, and wherein said filter is a linear phase filter.

3. The systemp of claim 1 wherein said tag signals are analog signals, wherein said receiver includes means for converting said analog signals to digital signals, and wherein said converting means operates responsive to two different threshold levels.

4. The system of claim 3 wherein one of said two

different threshold levels operates to define a leading edge of a digital pulse, and another of said two different threshold levels operates to define a trailing edge of said digital pulse.

5. The system of claim 1 wherein said tag signals are in the form of a series of pulses, wherein said receiver includes processor means for identifying said tag signals, and wherein said identifying means includes means for determining if a first pulse in said series of pulses has a duration which falls within a

selected window, and means for determining if a second pulse in said series of pulses has a duration which falls within a window which varies responsive to the duration of said first pulse.

6. The system of claim 5 wherein said selected window is adjustable according to the tag means which is to be detected.

7. The system of claim 5 wherein said receiver includes a counter for counting tag signals identified by said processor means, and wherein said counter is

incremented when said tag signals are identified within a prescribed time period, and decremented when said tag signals are not identified within said prescribed time period.

8. The system of claim 1 wherein said transmitter

produces a primary signal which is periodically swept about said primary signal at a defined rate, and wherein said rate is adjustable.

9. An electronic article surveillance system comprising a transmitter for providing a signal to a

transmitting antenna, to develop an electromagnetic field, and a receiving antenna for receiving signals including signals produced by a resonant circuit forming part of a tag means associated with an article to be protected, and for providing said received signals to a receiver having means for identifying said tag signals, wherein said tag signals are analog signals, wherein said receiver includes means for converting said analog signals to digital signals, and wherein said converting means operates responsive to two different threshold levels.

10. The system of claim 9 wherein one of said two

11. An electronic article surveillance system comprising a transmitter for providing a signal to a

transmitting antenna, to develop an electromagnetic field, and a receiving antenna for receiving signals including signals produced by a resonant circuit forming part of a tag means associated with an article to be protected, and for providing said received signals to a receiver having means for identifying said tag signals, wherein said tag signals are in the form of a series of pulses, wherein said receiver includes processor means for identifying said tag signals, and wherein said identifying means includes means for determining if a first pulse in said series of pulses has a duration which falls within a selected window, and means for determining if a second pulse in said series of pulses has a duration which falls within a window which varies responsive to the duration of said first pulse.

12. The system of claim 11 wherein said selected window is adjustable according to the tag means which is to be detected.

13. The system of claim 11 wherein said receiver includes a counter for counting tag signals identified by said processor means, and wherein said counter is

14. An electronic article surveillance system comprising a transmitter for providing a signal to a

transmitting antenna, to develop an electromagnetic field, and a receiving antenna for receiving signals including signals produced by a resonant circuit forming part of a tag means associated with an article to be protected, and for providing said received signals to a receiver having means for identifying said tag signals, wherein said

transmitter produces a primary signal which is periodically swept about said primary signal at a defined rate, and wherein said rate is adjustable.