Universal self-clocking code reading method and apparatus

DETAILED DESCRIPTION The technique of the present invention generally comprises steps of measuring a reference length of at least one bar and space sequence (or pair) placed in an initial location ahead of the first data bit of a code stream, regardless of the code format being employed.
This may be done by detecting the pulses at the leading and trailing edges of bar codes, (the transition points) for example.
Alternatively, a fixed reference length may be provided to the decoder apparatus for use without measuring any other reference.
Then, all the possible valid locations for the next leading edge pulse, a signal in the signal stream, and the next trailing edge pulse in the signal stream for the immediately following data bit are predicted.
These possible valid pulse locations are all of the various valid locations permitted in a given code format, and are predicted, or projected in time, utilizing predicting algorithms.
The algorithms have been developed to account for accelerative or other types of distortion or for constant velocity scanner operation.
Then a search function or gating function is conducted segmenting the data stream about each possible predicted valid leading and trailing edge location for the next data bit.
The leading and trailing edge pulses are found, each pulse being in one of the various possible locations therefor.
Finally, the data content of the bit cell in question is defined according to the locations in which the possible valid edge pulses were found.
The basic method may be repeated utilizing the width of the data cell just measured and rescaling it to make a new reference for the following data cell.
If a leading edge pulse is not found within one of its various valid locations, or a trailing edge pulse is not found within one of its various valid edge locations, an error is instantly indicated at the bit level.