Ghost-canceling reference (GCR) is a special sub-signal on a
television
Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
channel that receivers can use to compensate for the
ghosting effect of a television signal distorted by
multipath propagation
In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and ...
between transmitter and receiver.
In the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, the GCR signal is a
chirp
A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser syste ...
in
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
of the
modulating signal from 0 Hz to 4.2 MHz, transmitted during the
vertical blanking interval
In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fra ...
over one video line (line 19 in the U.S.), 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 an
adaptive equalizer that removes ghost images on the screen.
GCR was introduced after its recommendation in 1993 by the Advanced Television Systems Committee.
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References
External links
Official GCR specification
Television technology
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