An analog VLSI, scale invariant method for edge detection
A simple technique for detecting adjustable contrast in a visual scene is presented. The circuit elements can be used to detect contrast in any array of sensors or processing elements where spatial relationships among neighboring elements define contrast or the presence of an edge. This technique eliminates the need for a differential pair, thereby allowing more than two inputs to be compared for contrast in a single processing step. The circuit elements first smooth erroneous edges in the array through the use of a resistive network, then, the mean (scaled by an adjustable amount) of a pixel and its neighbors is compared to the harmonic mean of the same pixels to detect the presence of contrast within the pixel neighborhood. Comparison between the mean and harmonic mean allows the detection of contrast to be scale-invariant as long as the transistors remain in subthreshold operation. This circuit offers the massively parallel processing inherent to focal plane processing within an 18% fill factor in a 2 mu m process, 6.8 mu W typical power dissipation per element, and 0.67 ms response time at low power subthreshold operation. Results for a proof of concept, 8 x 8 array of pixels with light inputs, as well as a purely electronic input, 4 x 4 array are presented.
analog vlsi focal plane processing photo detectors