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In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image.

An example of severe ghosting in an analog TV broadcast.

[edit] Common causes

A diagram of the ideal situation for TV signals moving through space: The signal leaves the transmitter (TX) and travels through one path to the receiver (the TV set, which is labeled RX)

In this illustration, 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 direct path, while the second is due to a reflection from the plane.

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.

Ghosting is specific to analog transmission. Digital television avoids the problems of ghosts altogether regardless of the transmission format. Co-channel interference is sometimes called ghosting, but it is very different in nature.

[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 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.

[edit] See also

• Ghost-canceling reference

[edit] References

• http://www1.electusdistribution.com.au/images_uploaded/tvrecepe.pdf
• http://www.ofcom.org.uk/static/archive/ra/publication/ra_info/ra323/ra323.htm#15