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Peak Detector and Threshold Circuit for Bar Half Bar Readers

IP.com Disclosure Number: IPCOM000079398D
Original Publication Date: 1973-Jun-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Etiz, F: AUTHOR [+2]

Abstract

A bar-half bar code is a self-clocking code which uses a full bar for a logical "1" and a half bar for a logical "0". The code pattern is usually decoded by using the space between the bars to separate the data bits. If a multiple sensor scanner is used, sensing the lower half of the bars generates timing strobes and sensing the upper half of the bars generates the data -- i.e., if the upper half bar is detected, the bit is a 1, and otherwise a 0 is decoded.

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Peak Detector and Threshold Circuit for Bar Half Bar Readers

A bar-half bar code is a self-clocking code which uses a full bar for a logical "1" and a half bar for a logical "0". The code pattern is usually decoded by using the space between the bars to separate the data bits. If a multiple sensor scanner is used, sensing the lower half of the bars generates timing strobes and sensing the upper half of the bars generates the data -- i.e., if the upper half bar is detected, the bit is a 1, and otherwise a 0 is decoded.

If a single channel scanner is used, the code pattern can be decoded by measuring the signal amplitude. A full bar gives greater modulation than a half bar, and the logical decision can be made using two thresholds to generate the timing pulses and decision level of a 0 or 1. However, it is still necessary to detect the spaces between bars to determine when the next data bit is detected. If the space between bars is crowded, the signal may not drop below the half-bar level, and the next data bit will be lost. This is illustrated in Fig. 1.

This method uses a peak detector circuit to generate the timing signals for sampling the data line. A sampling pulse is generated each time the signal from the scanner has a peak. Thus, the last bit in the pattern in Fig. 2 is detected, even though the signal between the bars only showed a small dip. The threshold circuit for the data line to detect the full bars is the same as in the conventional technique.

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