Electronic article surveillance (EAS) systems are often used to prevent unauthorized removal of articles from a protected area, such as a library or retail store. An EAS system usually includes an interrogation zone or corridor located near the exit of the protected area and markers or tags attached to the articles to be protected. EAS systems have been based on magnetic, RF, microwave and magneto-strictive technologies. Regardless of the particular technology involved, the EAS systems are designed such that the tag will produce some characteristic response when exposed to an interrogating signal in the corridor. Detection of this characteristic response indicates the presence of a sensitized tag in the corridor. The EAS system then initiates some appropriate security action, such as sounding an audible alarm, locking an exit gate, etc. To allow authorized removal of articles from the protected area, tags that are either permanently or reversibly deactivatable (i.e., dual status tags) are often used.
In the ideal case, the EAS system initiates an alarm sequence only when a sensitized tag is present in the corridor. However, EAS systems are sensitive to electromagnetic interference in their operating environment which can interfere with detection of a sensitized tag or can cause false alarms. The degree of sensitivity to interference depends on a variety of factors, such as the type of EAS system, the operating bandwidth of the system, the bandwidth and statistical characteristics of the interference, and the system receiver design. Many EAS systems operate at a frequency of approximately 10 to 40 KHz. This frequency band may contain significant asynchronous interference in a library environment, principally from CRTs and TVs. Depending on their distance from the EAS system, these sources of interference can impair or disable detection ability.
Synchronous interference can be synchronous with either the power line signal or with the EAS system itself. Interrogation synchronous interference occurs when the drive field signal generated during an interrogation activates other objects in the environment, such as metal door frames, metal wall studs, metal gates or other metal objects. These objects then emit a signal which is often similar to the characteristic response of a magnetic tag. Power line synchronous interference is noise that tends to occur during the same point relative to the phase of the power line signal. Both interrogation and power line synchronous interference can reduce the ability of an EAS system to detect a sensitized tag or can cause false alarms.
When noise is spectrally overlapping, as with the types of interference described above, it is very difficult to suppress using conventional linear filtering methods. Because the spectral signature of the magnetic tag is broadband, any in-band filtering of the received signal to remove interference will distort the signal of interest. In a linear filtering scheme, a trade-off exists between filtering the noise and distorting the signal of interest. Thus, a linear filtering scheme alone may not increase the reliability of an EAS system.