Video inversion technique to counteract slowly moving interference patterns in a video communications system

Claim 1

A video communications system comprising a plurality of similar video transmitters each of which serves to generate a composite video signal having video portions each representative of a line of an image and interspersed vertical and horizontal synchronization pulses the vertical synchronization pulses of each composite video signal preceding successive fields of the same and alternating in duration between two predetermined values for successive fields; a common transmission facility having a plurality of closely juxtaposed transmission channels for respectively carrying the composite video signals of said transmitters to remote video receivers, the juxtaposition between transmission channels being such that horizontal synchronization pulses appearing in one channel produce interference spikes in adjacent channels; each transmitter having coupling means for inverting the video portions of the composite video signal in every other field prior to coupling the same to said common transmission facility, said coupling means comprising a video inverting coupling path and a non-inverting coupling path, a flip-flop for alternately and successively enabling and disabling said inverting and non-inverting video paths, and control means operative in response to successive vertical synchronization pulses of one given duration to set said flip-flop to a first state and operative in response to the successive alternatelyoccurring vertical synchronization pulses of the other duration to set said flip-flop to a second state, said flip-flop being connected to said inverting and non-inverting video paths and serving in said first state to disable said inverting path and to enable said non-inverting path and in said second state to enable said inverting path and to disable said non-inverting path, whereby the video portions of each composite video signal are inverted in every other field; a plurality of similar remote video receivers coupled to said common transmission facility for respectively displaying the video signals from said plurality of video transmitters; each receiver including a coupling means, a flip-flop and a control means connected in the same circuit configuration as the aforementioned transmitter coupling means, flip-flop and control means, said receiver coupling means, flipflop and control means being similarly operative in response to the successive alternating vertical synchronization pulses of the received composite video signal to invert the video portions thereof in every other field prior to the display of the same whereby the video portions of the composite video signal inverted prior to transmission are reinverted prior to the receiver display of the same.