A multi-gap magnetic character reader is provided with the ability to automatically detect double documents. Where only weak signals are sensed, all signals are combined and compared to a reference. Where the weak signals are due to a piggyback document, the combined signals will be large with respect...http://www.google.com.hk/patents/US4510615?utm_source=gb-gplus-share專利 US4510615 - Magnetic character reader with double document detection
