Edge detection method and apparatus for shot noise limited signals

Abstract

A method and apparatus for improving the accuracy of bar and space measurements in a barcode scanner system utilizing second derivative signal processing is provided wherein edges are more accurately detected under low signal-to-noise conditions, such as when shot noise is the major source of noise in an input signal and the larger noise levels present during the light portions of the input signal may cause premature triggering of a light-to-dark transition in the output signal. In a preferred embodiment, an input capture circuit captures and stores the time of successive assertions of an STV signal corresponding to light-to-dark transitions. Upon detection by a second input capture circuit of a first assertion, following the STV assertions, of an RTV signal corresponding to dark-to-light transitions, the last STV signal time is stored and used with a previously stored RTV time to determine a space width. The new RTV signal time and the last STV signal time are then used to determine a bar width. Thus, light-to-dark transitions in the output signal are triggered on the last second derivative zero crossing which occurs within a comparator gate instead of the first zero crossing. Subsequent assertions of an RTV signal are preferably ignored until after one or more additional STV signal assertions occur. The entire process repeats for each light and dark position until the entire signal has been processed.