Signal processing circuit of a bar code reader

Claim 1

A signal processing circuit of a bar code reader, comprising: signal input means for optically detecting a bar code provided on a surface of goods and outputting a first signal after optical-to-electrical conversion; differential means connected to the signal input means, for amplifying the first signal, differentiating the first signal, filtering the first signal after differentiation to eliminate high frequency noise, and outputting a second signal after filtering; comparison means connected to the differential means, for integrating the second signal and outputting a third signal after integration, and comparing the second signal with the third signal and outputting a fourth signal when the third signal is larger than the second signal; slice level generation means connected to the differential means, for holding a peak voltage of the second signal, suppressing the peak voltage to a predetermined upper limitation voltage, discharging the suppressed peak voltage in accordance with a predeter­mined time constant, and dividing the suppressed peak voltage into a predetermined voltage rate to obtain a positive slice voltage and a negative slice voltage; gate signal output means connected to the differential means and the slice level generation means, for comparing the second signal with the positive slice voltage and the negative slice voltage; and signal output means connected to the comparison means and the gate signal output means, for inputting the fourth signal and the positive slice voltage, inputting the inverted fourth signal and the negative slice voltage, and outputting a gate signal to obtain a binary-coded signal.