A calibration system for circuit inspection tools in which analog threshold levels are automatically set by digital signals. The inspection too includes an optical detector system that provides video signals to channels each containing an analog processor. In the analog processor an analog reference signal, an analog video inspection signal which may contain a defect indication and a digital threshold signal are employed. The digital threshold signal is converted into an analog threshold signal which is summed with the analog reference signal that has been inverted. The inverted, thresholded reference signal is then summed with the analog video signal. The summed analog video signal and thresholded reference signal is compared with a zero level. If the sum is greater than zero a defect is indicated.