Chrominance detection circuit comparing the polarity of high frequency signals separated by a field or frame.

摘要

A circuit to detect the presence of the chrominance signal in a composite signal is disclosed. Advantage is taken of the fact that the polarity of luminance signals in two fields or frames is the same whereas the polarity of chrominance signals is opposite. The circuit uses a band pass filter to select the chrominance and high frequency luminance signals, a polarity detector, a field or frame delay, and an XOR having the delayed and nondelayed outputs of the polarity detector as inputs. The output can be used to modify the decoding circuits when a purely black and white image is present.