Circuit arrangement for phase shifting a color signal prior to comb filtering and then cancelling the phase shift after comb filtering

摘要

A circuit arrangement processes a color signal reproduced from a record carrier, preferably a video tape, and modulated on a carrier wave of a first carrier frequency (color-under signal). The color signal modulated on the first carrier frequency is applied to a first mixer which converts the color signal to a second carrier frequency by mixing it with a first mixing signal. An output signal of the first mixer is applied to a comb filter circuit, whose output signal is applied to a second mixer. The second mixer converts the comb-filtered signal to a third carrier frequency by mixing it with a second mixing signal, in order to achieve an optimum comb filtering. The first mixing signal is subjected to a phase shift of a given magnitude by a first phase shifter after a given number of picture lines of the color signal. Each phase shift is in addition to the previously applied phase shifts and is selected so as to compensate for the possible non-integral ratio between the period of one picture line and the period of the color signal modulated on the first carrier frequency, in such a manner that a period of the color carrier wave on the input of the comb filter circuit begins at the beginning of the next picture line. A second phase shifter subjects the second mixing signal to a phase shift of the same magnitude but of opposite sign at the same instants.