Ghost (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.
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.
Ghosts of other types, besides the more specific sense above, are caused by interference from other television transmissions. Such interference between two broadcast television transmissions is rare because stations operating at the same frequency are spaced far enough apart to avoid this effect except under exceptional circumstances. However, it is not rare for an improperly shielded cable to act as an antenna and pick up broadcasts. As cable television allocates frequencies differently than broadcast television, the interference is nearly always between unrelated broadcasts.
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.