Ghost-canceling reference
Encyclopedia
Ghost-canceling reference, or GCR, is a special sub-signal on a television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 channel
Television channel
A television channel is a physical or virtual channel over which a television station or television network is distributed. For example, in North America, "channel 2" refers to the broadcast or cable band of 54 to 60 MHz, with carrier frequencies of 55.25 MHz for NTSC analog video and...

 that receivers can use to attenuate the ghosting
Ghosting (television)
In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image on an analogue broadcast.-Common causes:Common causes of ghosts are:...

 effect of a television signal split into multiple paths between transmitter and receiver.

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, the GCR signal is a chirp
Chirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...

 in frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of the modulating signal from 0 Hz to 4.2 MHz, transmitted during the vertical blanking interval
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...

over one video line, shifted in phase by 180° once per frame, with this pattern inverted every four lines. 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.

GCR was introduced after its recommendation in 1993 by the Advanced Television Systems Committee.
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