Ghosting (television)

Ghosting (television)

In television, a ghost is an unwanted image on the screen, appearing superimposed on the desired image. In a more specific sense, a ghost is a replica of the desired image appearing fainter and offset in position with respect to the primary image.

Common causes of ghosts (in the more specific sense) are:
* Mismatched impedance along the communication channel, which causes unwanted reflections. The scientific or engineering term for this phenomenon is ringing.
* Multipath distortion, because radio frequency waves may take paths of different length (by reflecting from buildings, clouds, etc.) to reach the receiver. In addition, RF leaks may allow a signal to enter the set by a different path; this is most common in a large building such as a tower block or hotel where one TV antenna feeds many different rooms, each fitted with a TV aerial socket (this is known as pre-echo). By getting a better antenna or cable system it can be eliminated or mitigated.

Note that ghosts are a problem specific to the video portion of television, largely because it uses AM for transmission. TV is also transmitted on VHF and UHF, which have line-of-sight propagation, and easily reflect off of buildings, mountains, and other objects.

The audio portion uses FM, which has the desirable property that a stronger signal tends to overpower interference from weaker signals due to the capture effect. Even when ghosts are particularly bad in the picture, there may be little audio interference.

Note also that ghosts are specific to analog transmission. Digital television avoids the problems of ghosts altogether, although it has its own artifacts such as block errors. DVB and ISDB use COFDM modulation, which performs well under conditions that produce analog ghosting, while ATSC uses 8VSB, which performs poorly under such conditions.Fact|date=July 2007

Co-channel interference is sometimes called ghosting, but it is very different in nature.

Ghosting can also occur in KVM switches, or in VGA video cables, due to poor electromagnetic shielding.

Fun experiment/method of investigating ghosting

It is possible to use the TV as a passive radar system. By putting a small yagi antenna on a broomstick, it would be possible to find the direction in which the ghost is strongest. By measuring the width of the screen and then the distance between the main image and the ghost, it is possible to work out the difference in the distance of the two paths between the transmitter and the antenna.

Note that the speed of light is 3×108 m/s.

In the PAL and SECAM systems, the line is 64 microseconds from start to finish. As the first 12 microseconds of the line are taken up by things like the front porch, back porch, horizontal sync, and the colour burst, only 52 microseconds of time exists for each horizontal line.

In NTSC the active line duration is 52.2 microseconds, and the total line length is 63.55 microseconds.

From these facts, it is possible to measure the difference in the path length between the two paths the TV signal takes between the transmitter and the receiver.In PAL, if the TV is 20 cm (8") wide, and the ghost is 5 cm (2") to the right of the main image, then:

t = 64×10−6 × 5/20 seconds = 16×10−6 seconds

The resulting path difference is 4800 meters (4.8km, or 3 miles).


If the ghost is seen on the left of the main picture, then it is likely that the problem is pre-echo, which is seen in buildings with very long TV downleads where an RF leakage has allowed the TV signal to enter the tuner by a second route. For instance, plugging in an additional aerial to a TV which already has a communal TV aerial connection (or cable TV) can cause this condition.

ee also

*Ghost-canceling reference



Wikimedia Foundation. 2010.

Нужно решить контрольную?

Look at other dictionaries:

  • Ghosting (televisión) — El Ghosting o Imagen fantasma es un término utilizado en radiodifusión de señales de televisión analógica. Este efecto introduce una réplica de la imagen transmitida, desplazada en posición, que se superpone (suma) a la imagen que se está… …   Wikipedia Español

  • Ghosting — can have different meanings:* ghosting (television), a double image when receiving a distorted or multipath input signal in analog television broadcasting.* ghosting (identity theft), a form of identity theft, whereby a person takes on the… …   Wikipedia

  • Television interference — (TVI) is a term for anything that can adversely affect the signal reaching the TV or the TV s ability to function correctly. It is usually an indicator of electromagnetic compatibility problems.In this page only conventional UHF (or VHF) AM TV… …   Wikipedia

  • Ghosting — La palabra Ghosting puede referirse a: ghosting (televisión), aparición de una doble imagen en la recepción de un señal de entrada distorsionado. Término de radiodifusión de señales de televisión analógica (broadcasting). desenfoque de movimiento …   Wikipedia Español

  • ghosting — ► NOUN ▪ the appearance of a secondary image on a television or other display screen …   English terms dictionary

  • Television antenna — A Winegard HD 7084P 68 element VHF/UHF aerial antenna A television antenna, or TV aerial, is an antenna specifically designed for the reception of over the air broadcast television signals, which are transmitted at frequencies from about 41 to… …   Wikipedia

  • ghosting — /goh sting/, n. Television. the appearance of multiple images, or ghosts, on a television screen. Cf. ghost (def. 7) [GHOST + ING1] * * * …   Universalium

  • ghosting — noun Date: circa 1957 a faint double image on a television screen; also the formation of such images …   New Collegiate Dictionary

  • ghosting — noun a) The practice of hiding prisoners from inspection from (possibly hostile) outside inspectors. b) The effect that interference caused by a multi path reception problem has on a television screen …   Wiktionary

  • ghosting — ghost|ing [ goustıŋ ] noun uncount the appearance of a double image on a television screen, caused by problems with your RECEPTION …   Usage of the words and phrases in modern English

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”