Importance

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Abstract

A method and apparatus of compatibly and simultaneously transmitting and receiving an encoded signal with the normal visual and aural carrier of a television signal is disclosed. The encoded signal is used as a modulating signal to frequency modulate the visual carrier frequency signal prior to amplitude modulating the visual carrier frequency signal with a source of composite video signals. Normally, the introduction of additional sideband power even in the form of constant amplitude components will cause visual distortion to a received picture due to the normal Nyquist shaped bandpass characteristics of the intermediate frequency amplifier in the television receiver. This distortion is reduced or eliminated in several ways. The maximum allowable frequency modulation deviation can be limited so that the visual distortion is substantially reduced. Where greater frequency modulation deviation is required, the interfering beats which appear as horizontal lines are made less visable by adjusting the frequency modulation deviation so that these lines roll vertically at a maximum rate. A more desirable approach is to amplitude predistort the frequency modulated visual carrier frequency signal in a complimentary mode to the slope of the video intermediate frequency amplifier circuits of the television receiver. In this manner, an incidental amplitude modulation would be added to the television visual carrier at the transmitter, and this incidental amplitude modulation is in the opposite polarity of the slope of the modulation taking place in the receiver. In other words, by predistorting the frequency modulated visual carrier frequency signal, the signal is in effect pre-equalized to the Nyquist bandpass characteristics of the television receiver intermediate frequency amplifier. The predistortion of the frequency modulated visual carrier signal may be accomplished passively with a bandpass filter having its lower frequency slope at the television picture carrier frequency and having bandpass response characteristics which are complimentary to the Nyquist bandpass characteristics of the television receiver intermediate frequency amplifier. Instead of a passive bandpass filter, similar results are obtained by active, pre-equalizing circuits in the transmitter. Specifically, a phase opposite correction signal derived from the encoded signal is added to the composite video signal prior to amplitude modulating the frequency modulated visual carrier frequency signal. In any case, normal aural carrier generation methods avoid any interference by the addition of a frequency deviation of the visual carrier. The received signal is detected to separately reproduce the encoded signal.

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First Claim