SIGNAL THEORETIC TECHNIQUES CAN BE USED TO OVERPRINT IN PROTECTIVE PATTERNS THAT ARE AUTOMATICALLY VERIFIABLE BUT AT THE SAME TIME PROVIDE VIRTUALLY THE SAME PROTECTION IN CREATING FORGERY-PROOF DOCUMENTS AS THE CONVENTIONAL LINE PATTERNS THAT ARE HUMANLY INSPECTED. PROPERTIES OF THE SIGNAL THEORETIC APPROACH ARE PATTERNS EXTENDING OVER THE ENTIRE AREA OF DOCUMENTS TO PROVIDE PROTECTION AGAINST MANIPULATIONS AND TO ALLOW VISUAL INSPECTIONS; PATTERNS CONTAINING INDIVIDUAL DATA OF DOCUMENTS IN CODED FORM TO COMPLICATE THE PROCESS OF FALSIFICATION; AND MACHINE-READABLE PROTECTIVE DATA PATTERNS ALLOWING AUTOMATIC VERIFICATION AND REVEALING POSSIBLE DATA MISMATCHES THAT MIGHT REMAIN UNDISCOVERED IN SIMPLE VISUAL INSPECTIONS. APPLICATIONS OF THE APPROACH CAN INCLUDE MONEY BILLS FOR AUTOMATIC CHANGING MACHINES AND PASSPORT PHOTOGRAPHS. PROTECTIVE DATA PATTERNS CAN INCLUDE BINARY-CODED TEXT AND HIDING INFORMATION IN A PATTERN LAYOUT FEATURING BITS OF DATA SCATTERED OVER THE DOCUMENT SURFACE. THE DOCUMENT IS READ BY AN AUTOMATIC DEVICE CONSISTING OF AN OPTICAL SCANNER, AN ANALOG-TO-DIGITAL CONVERTER, A MICROPROCESSOR, AND AN ALPHA-NUMERIC DISPLAY. DATA SIGNALS EXTRACTED FROM THE DOCUMENTS ARE ARRANGED ON A TWO-DIMENSIONAL TRANSFORM SPECTRA TO SEPARATE THE CODE FROM THE DOCUMENT BACKGROUND. THE OPTIMUM DATA PATTERN WILL ACHIEVE THE LOWEST ERROR PROBABILITY AND WILL BE COMPOSED OF KARHUNEN-LOEVE BASIS FUNCTIONS DETERMINED INDIVIDUALLY FOR EACH TYPE OF DOCUMENT. MATHEMATICAL EQUATIONS, ILLUSTRATIONS, PHOTOGRAPHS, AND REFERENCES ARE PROVIDED. (DEP)