Ghost-canceling reference

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 within the duration of one video line. Local television receivers generate their own version of this signal, and use the comparison between the local and remote signals to tune out any ghost images on the screen.