Patent US 3,986,000

Bar-code data is read in the presence of noise. An analog output signal from a bar-code sensor has amplitude variations dependent upon the presence and absence of bars as well as upon differences in backgrounds upon which the bars are printed. The output signal is normalized to minimize variations therein, and the normalized signal in turn is converted to a digital signal at a sample interval small compared to the period of the known waveform. An expected response to each bar is of a known waveform. A digital reference set is stored which represents products formed by multiplying each quantization level of the digital signal by each sample of the known waveform taken at the sample interval. For each sample interval along the output signal, members of the reference set are selected dependent upon the sample interval of the known waveform and the corresponding amplitude of the digital signal, and summed to form one point of a correlation output function. The correlation output function in turn is continuously sensed for the presence of predetermined bar dependent characteristics.