| 摘要 |
Improvements in theft-detection or surveillance systems of the type in which an alternating electromagnetic field is established across a doorway or other portal and is monitored to detect the presence within the field of a marker or tag member comprising a small strip of permalloy or like material of high permeability hidden or otherwise carried on merchandise or other articles and objects to thereby "mark" such merchandise or objects, i.e., to make them readily detectable even though hidden from view. More particularly regarded, the improvements in accordance herewith reside in signal-processing electronic circuitry which increases detection sensitivity and accuracy while at the same time reducing erroneous detection results. In particular, the circuitry utilizes summing and differencing techniques to improve signal-to-noise ratios and eliminate previously-unsuspected sources of error, and facilitates use of the concept of frequency spectrum-content ratios as a determinant in distinguishing between apparent detection of true markers from other objects or structures whose response to the alternating interrogation field closely resembles that of the true markers and would normally produce erroneous detection indications. Basically this is accomplished by using the aforementioned summing and differencing techniques to produce at least one summed signal resultant and one differenced signal resultant which have different frequency content spectrums, and subsequently processing such signal resultants by use of sampling techniques representative of both marker-presence and marker-absence, comparison of these samples through summing, differencing and peak-integrating techniques, as well as by other more particular approaches.
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