Analog scan data signal processor employing a time-domain substrate noise filter for non-linearly processing analog scan data signals and reducing substrate noise signal levels therein prior to first derivative signal generation and processing operations carried out within said analog scan data signal processor

A scan data signal processor for use in a laser scanning system, and capable of receiving and processing an analog scan data signal in response to the scanning of a bar code symbol by a laser scanning beam, said scan data signal processor comprising:

• a non-linear time-domain substrate noise filtering circuit for processing said analog scan data signal and producing a time-domain filtered signal in which substrate noise in said analog scan data signal is smoothed or filtered out primarily during signal levels corresponding to the scanning of substrates and bar code spaces realized on the substrate by said laser scanning beam;
• a first derivative signal generation circuit for deriving a first derivative signal from said time-domain filtered signal, said first derivative signal generation circuit including a plurality of focal-zone controlled first derivative signal pass-band filters and amplifiers integrated therewith;
• a second derivative signal generation circuit for deriving a second derivative signal from said time-domain filtered signal, said second derivative signal generation circuit including a plurality of focal-zone controlled second derivative signal pass-band filters and amplifiers integrated therewith;
• a first derivative signal threshold-level generation circuit for generating upper and lower first derivative signal threshold levels for use in determining when said first derivative signal has attained its peak positive and negative values during signal processing operations;
• and
• a binary A/D signal conversion circuit including
• a second derivative zero-crossing detector for detecting zero-crossings in said second derivative signal,
• a pair of first derivative signal comparators for comparing said first derivative signal with said upper and lower first derivative signal threshold levels so as to detect positive and negative peak values in said first derivative signal, and in response thereto, producing gating signals at the time of occurrence of said detected positive and negative peak values;
• a digital output signal generating circuit for receiving detected second derivative zero-crossings provided thereto as input, and producing a digital output signal for subsequent signal processing, and
• a second derivative zero-crossing gating circuit, responsive to said gating signals, for gating to said digital output signal generating circuit, only detected second derivative zero-crossings occurring between alternating positive and negative peaks in said first derivative signal detected by said pair of first derivative signal comparators;
• wherein the second derivative signal generation circuit further comprises
• a first scan-range controlled second derivative signal pass-band filter and amplifier arranged along a first channel (A) of said second derivative signal generation circuit corresponding to the first scan range of said laser scanning system, and
• a second scan-range controlled second derivative signal pass-band filter and amplifier arranged along a second channel (B) of said second derivative signal generation circuit corresponding to the second scan range of said laser scanning system.