Analog-digital converter with variable threshold levels

#This invention relates to an analog-digital converter for converting analog signals into digital signals, and more particularly, to an analog-digital converter in which the threshold level for converting the analog signals, such as image signals, into binary signals is changed in response to changes in the level of the analog signals.

#In the field of pattern recognition, image signals from an image pick-up device, such as an industrial TV camera, are converted into binary signals based on a constant threshold level.

#In this case, no binary signals with long-term stability have been obtained since the gains and the DC levels of the image signals vary at relatively slow speeds due to variations in temperature and brightness under various conditions.

#FIG. 1 is a diagram explaining the fundamental principle of this invention;

#FIG. 2 is a schematic block diagram of an exemplary embodiment of this invention;

#FIG. 3 is a waveform diagram in the form of a time chart explaining the system shown in FIG. 2;

#The principle of this invention will be explained with reference to FIG. 1. The image sample is illustrated as a simple model or object 1 divided into a plurality of picture elements in a 16 by 24 coordinate arrangement of picture elements located with reference to axes in the vertical and horizontal directions, respectively. Further, the brightness of each picture element is to be represented by binary levels, i.e., the level "1" for a bright element and the level "0" for a dark picture element, even though the actual brightness is generally multilevel. Therefore, in the sample model of FIG. 1, the hatched portion corresponds to the level "0" and the unhatched portions correspond to the level "1". X and Y represent the axes of the discrete image coordinate system.

#A signal S.sub.1 designates a video signal derived from an image pick-up device, such as an industrial TV camera, by which object 1 is picked up. The lower and higher levels of the signals S.sub.1 designate the black and white portions of the object, respectively. It is assumed that the signal S.sub.1 corresponding to one horizontal scanning period H of the image is obtained repeatedly every period as that part of the object along the dotted line 6 is scanned. Since the object is static in a pattern recognition system, the same data will be scanned along dotted line 6 of the object 1 during each period.

#When the signal S.sub.1 is converted into a binary signal with a threshold level L.sub.1, the binary signal S.sub.3 is obtained. However, if the threshold level of the signal S.sub.1 changes to L.sub.2, such as illustrated by the signal S.sub.2, for a significant period of time, an incorrect binary signal S.sub.4 is obtained if the signal S.sub.2 is converted into a binary signal using the threshold level L.sub.1. But, when the signal S.sub.2 is converted into a binary signal based on threshold level L.sub.2, which is compensated in response to the variation of signal S.sub.1, the correct binary signal S.sub.3 is obtained. In order to compensate the threshold level in accordance with this invention, the following principle is utilized.