METHOD OF AND APPARATUS FOR PROCESSING ANALOG SCAN DATA SIGNALS DERIVED BY SCANNING BAR CODE SYMBOLS USING A LASER BEAM, WHEREIN A REAL-TIME BAR CODE ELEMENT DETECTOR IS USED TO CONTROL THE DETECTION OF ZERO-CROSSINGS OCCURRING IN THE SECOND DERIVATIVE OF SAID ANALOG SCAN DATA SIGNALS

摘要

A scan data signal processor is disclosed for use with a laser scanning system having a plurality of different focal zones within which a laser scanning beam is scanned across one or more bar code symbols and an analog scan data signal is automatically produced. The scan data signal processor comprises a first and second derivative signal generation stages, a second-derivative zero-crossing detector, a real-time bar code element detector, and a control logic circuitry. The first derivative signal generation stage derives a first derivative signal from the analog scan data signal. The second derivative signal generation stage derives a second derivative signal from the first derivative signal. The second-derivative zero-crossing detector detects zero crossings in said second derivative signal. The real-time bar code element detector automatically detects the presence and absence of bar code element data encoded within said analog scan data signal. The control logic circuitry automatically activates the second-derivative zero-crossing detector in response to the detection of the presence of bar code element data encoded within said analog scan data signal, and automatically deactivates the second-derivative zero-crossing detector, in response to the detection of the absence of bar code element data encoded within the analog scan data signal. By virtue of the present invention, it is now possible to enable the detection of zero-crossings occurring in the second derivative of the analog scan data signals only when bar code element data has been detected, indicative that a bar code symbol has been scanned. This results in more accurate generation of digital output signals from the scan data signal processor.