Ghosting (television)

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In television, a ghost is an image on the screen which doesn't belong there, 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.

Here you can see the ideal situation for TV signals moving through space, the signal leaves the transmitter (TX) and travels through one path to the reciver (the TV set which is labeled RX)

Common causes of ghosts (in the more specific sense) are:

Mismatched impedance along the communication channel, which causes unwanted reflections.
Multipath distortion, due to the fact that radio frequency waves may take paths of different length (by reflecting from building, 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.

Here you can see how an object (in this case an aircraft) pollutes the system by adding a second path. The signal arrives at RX by means of two different paths which have different lengths. The main path is the dirrect path, while the second is due to a reflection from the plane

Note that ghosts are a problem specific to the video portion of television, which incidentally uses AM for transmission. The audio portion uses FM, which has the desirable property that a stronger signal tends to overpower interference from weaker signals. 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.

Co-channel interference is sometimes called ghosting but it is very different in nature so it will not be considered in length here, for more details see Television interference (Co-channel reception).

[edit] Fun experiment/method of investigating ghosting

If you find what is on TV a total bore, then it is possible to use the TV as a passive radar system. If you were to put a small yagi antenna on a broomstick it would be possible to find the direction in which the ghost is strongest. If you then measure the width of the screen and then the distance between the main image and the ghost, then it will be possible to work out the difference in the distance of the two paths between the transmitter and your antenna.

Note that the speed of light is 300×10⁶ 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 synch, 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 your TV aerial. , using in PAL TV, if the TV is 20 cm wide and the ghost is 5 cm to the right of the main image then

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

So the path difference is 4800 meters.

[edit] Pre echo

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 a RF leakage has allowed the TV signal to enter the tuner by a second route.

For instance if you encounter this, then check to see if someone has plugged in an additional aerial to a TV which has already got the communal TV aerial connection.