Ghost-canceling reference

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Ghost-canceling reference, or GCR, is a special sub-signal on a television channel that receivers can use to attenuate the ghosting effect of a television signal split into multiple paths between transmitter and receiver.

In the United States, the GCR signal is a chirp in frequency of the modulating signal from 0 Hz to 4.2 MHz, transmitted during the vertical blanking interval over one video line, shifted in phase by 180° once per frame. Television receivers generate their own local versions of this signal, and use the comparison between the local and remote signals to tune out any ghost images on the screen.