This invention provides a zero signal state detecting circuit for detecting zero signal state of the signal in an optical receiver or optical repeater. The zero signal state detecting circuit in accordance with the present invention comprises a level shifting circuit, a smoothing circuit, a first comparator, a peak detecting circuit and a second comparator. The output signal from the timing circuit to produce two output signals is shifted by the level shifting circuit. One of the two outputs of the level shifting circuit is smoothed by the smoothing circuit. The output of the smoothing circuit and the other one of the outputs of the level shifting circuit are compared by the first comparator and the output is detected by the peak detecting circuit. The output of the peak detecting circuit is compared by the second comparator against a reference voltage, and the zero signal state of the optical receiving signal is detected.

A reader for a magnetically coded card with a magnetodiode as the magnetic field sensor. A circuit for compensating for changes in magnetodiode resistance due to ambient temperature change. First and second amplifiers connected in a self balancing nulling system and having the magnetodiode signal as an input, and providing a null output signal for no magnetic field at the diode, with third and fourth amplifiers connected as comparitors and providing high and low output signals for magnetic fields of opposite polarity.

In a facsimile system, for example, a document is scanned along successive scan lines and is analyzed by a photosensitive reader to produce an analog image analysis signal (V.sub.a). This signal is then converted into a two-value (i.e. black and white) signal by an all-or-nothing converter. This is done by a comparator (1) comparing the analog signal (V.sub.a) with a threshold (V.sub.d); and to avoid losing wanted information, in spite of varying background densities, it is necessary for the relative values of the analog signal (V.sub.a) and the threshold (V.sub.d) to vary so that the threshold remains somewhere between local maximum and minimum modulation levels over a range of absolute values. In the present invention successive modulation peaks are detected by a peak detector (2) and their values stored e.g. on a capacitor. First means (a resistor 3) decay the stored value slowly so that while pale image representing peaks occur frequently the threshold as applied by circuit (6) at a value slightly below the peak value. Second means (transistor switch 4) are controlled to cause rapid decay of the stored peak value after the binary output signal has remained at black level for a run of more than 10 or so image points. Switch (5) serves to clear memory between successive scan lines.

A system for converting an analog image signal into a digital image signal is provided. The system comprises an analog-to-digital signal converter for producing a digital signal by subjecting an incoming analog signal to threshold processing with a threshold level. The system also comprises a peak/valley extracting circuit for extracting peak and valley points corresponding to thin lines and narrow gaps between lines, respectively, in the original image scanned. The extracted peak and valley points are utilized in the analog-to-digital conversion processing so as not to lose information as to thin lines and narrow gaps between lines. Especially, in extracting peak and valley points, the concentration or level of a particular picture element is compared with that of the adjacent picture elements at least in two directions.

A video detector circuit is disclosed which develops a threshold level from a white or background peak signal in regions of low contrast, and from a combination of both white and black peak levels in regions of high contrast. The circuit is adjustable to compensate for variable affection performance of the detector.