Edge detection method and apparatus for shot noise limited signals

• US 6,129,279 A
• Filed: 12/21/1999
• Issued: 10/10/2000
• Est. Priority Date: 11/01/1996
• Status: Expired due to Term

• Current Assignee: PSC Scanning, Inc. (Datalogic ADC Incorporated)
• Original Assignee: PSC Scanning, Inc. (Datalogic ADC Incorporated)
• Inventors: Olmstead, Bryan L., Colley, James E.
• Primary Examiner(s): EVANS, FANNIE L
• Application Number:
  US09/469,946
• Time in Patent Office: 294 Days
• Field of Search: 235/462.16, 235/462.17, 235/462.18, 235/462.19
• US Class Current: 235/462.16
• CPC Class Codes: G06K 7/14 using light without selection of wavelength, e.g. sensing reflected white light G06K7/10831 - G06K7/1097 take precedence
• CPC Class Codes: G06K 7/1486 Setting the threshold-width for bar codes to be decoded

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.

Claims

1. In a barcode scanner system, a second derivative signal processor for processing an input signal from which the relative widths of the bars and spaces of a scanned barcode label are to be determined comprising:

first input capture means for storing the times of successive assertions of an STV signal corresponding to a light-to-dark transition;

second input capture means for detecting a first assertion of an RTV signal corresponding to a dark-to-light transition following the successive STV signal assertions; and

,

means for determining the relative widths of the bars and spaces in the input signal using the last STV signal assertion stored and the first RTV signal assertion.

2. In a barcode scanner system, a second derivative signal processing method for determining relative widths of bars and spaces in an input signal being processed comprising the steps of:

(a) detecting successive assertions of an STV signal corresponding to light-to-dark transitions as determined from second derivative zero crossing with respect to the input signal and wherein all but the last of such successive second derivative zero crossings result from noise in the input signal;

(b) detecting a first RTV signal corresponding to a dark-to-light transition with respect to the input signal;

(c) determining the relative widths of bars and spaces in the input signal using the last STV signal assertion detected and the first RTV signal detected; and

,

(d) repeating steps (a) to (c) until the entire input signal has been processed.