Stephen J. Shellhammer

Novel signal-processing techniques in barcode scanning

Traditional barcode scanners use analog edge detectors to detect the boundaries of dark and light areas. As long as the edge strength exceeds a given threshold, the detected edge is considered to correspond to a bar/space boundary. Such a hard decision often leads to errors, especially in scanning barcodes that are poorly printed or scanned from a distance. An earlier study showed that much better results can be obtained by working directly with samples of the analog data. However, that method requires storing the entire sampled waveform of a scan line in memory, which is not cost effective. The method described in the article attempts to combine the performance advantages of processing the samples of the analog waveform with the low-cost advantage of the traditional methods. We still rely on edge-detection circuits, however, in addition to saving the edge time we also store information about the edge strength. In this way, we have been able to produce many of the benefits of the approach described in Joseph and Pavlidis (1993) with only a small increase in cost.

Selective sampling and edge enhancement in bar code laser scanning

This paper describes the basic design principles for a new series of bar code scanners from Symbol Technologies. Traditional bar code scanners include an edge detector which has several innate limitations. We propose replacing this edge detector with a selective sampling circuit. While the superiority of decoding the analog signal has been demonstrated, its implementation is too costly because of the need for considerable additional memory. Selective sampling achieves most of the advantages of analog decoding at a cost comparable to that of conventional decoders. Instead of sampling the signal periodically, it is only sampled when a certain event (e.g. an edge) is detected. At each edge two data values are produced: the edge time and the sampled value, often referred to as the edge strength. This strength value gives a measure of the intensity of the edge. Using selective sampling the new scanners can read poorly printed and noisy bar codes that cannot be read by traditional scanners. Another innate limitation of bar code laser scanners is the density of bar code that can be read. This limitation is due to the blurring of the bar code when scanned by a laser beam with a finite spot size. We propose the addition of an edge enhancement filter to the scanner, which compensates for the finite width of the optical beam. The proposed filter is designed to enhance the edges of the bar code so that for a given optical focusing it is possible to read higher density bar codes.

Distortion of Bar Code Signals in Laser Scanning

One of the most common methods of reading bar codes is to scan them with a laser beam and process the received optical signal. The received light is proportional to the integral of the beam profile multiplied by the region of the bar code covered by the beam. The finite spot size of the laser beam acts as a filter on the original bilevel bar code signal. This filtering can cause intersymbol interference between adjacent edges of the bar code signal. An edge detector attempts to reconstruct the original bilevel signal from this filtered signal. The edges detected in the filtered waveform are in different locations than those in the original bilevel bar code signal resulting in a distortion of the bar code signal. If this distortion is too large, it can prevent the bar code from being properly decoded. Thus, there is a limit on the spot size of the laser beam that can be used to scan a given density bar code.