Circuit for use with a light pen to reduce jitter

A discrimination circuit for determining whether input pulses from a photodetector are valid signals and for generating an output signal at a substantially predetermined time after the rise of a valid signal, said input pulses each having a rising edge, said circuit comprising:

• first comparator means for receiving said input pulses and for generating a first signal at substantially a first threshold crossing time whenever the instantaneous magnitude of an input pulse at its rising edge exceeds a first threshold level, wherein the generation of a first signal by the first comparator indicates that the input pulse is valid;
• second comparator means for receiving said input pulses and for generating a second signal when the instantaneous magnitude of an input pulse at its rising edge exceeds a second selected threshold level at substantially a second threshold crossing time, said second selected threshold level being lower than said first threshold level, so that the second threshold crossing times of any two valid pulses differ by an amount less than the difference between the first threshold crossing times of the two valid pulses;
• delay means connected to said second comparator means for delaying said second signal by a fixed time delay;
• and
• gate means operatively connected to said first comparator means and said delay means, said time delay being such that when the first comparator generates a first signal, the gate means receives the delayed second signal after the first threshold crossing time, said gate means generating an output pulse upon receiving the first signal and the delayed second signal, said output pulse being generated at substantially the arrival time of the delayed second signal, said second threshold level being selected such that the time of generation of the output signal is substantially the same for all valid signals and substantially independent of the slope of said valid signals.