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Intelevision, aghost is a replica of the transmitted image, offset in position, that is superimposed on top of the main image. It is often caused when a TV signal travels by twodifferent paths to a receiving antenna, with a slight difference in timing.[1]
Common causes of ghosts (in the more specific sense) are:
Note that ghosts are a problem specific to thevideo portion of television, largely because it usesAM for transmission. Theaudio portion usesFM, which has the desirable property that a strongersignal tends to overpower interference from weaker signals due to thecapture effect. Even when ghosts are particularly bad in the picture, there may be little audio interference.
SECAM TV usesFM for thechrominance signal, hence ghosting only affects theluma portion of its signal. TV is broadcast onVHF andUHF, which haveline-of-sight propagation, and easily reflect off of buildings, mountains, and other objects.
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 (orcable TV) can cause this condition.
Ghosting is not specific toanalog transmission. It may appear indigital television wheninterlaced video is incorrectlydeinterlaced for display on progressive-scan output devices. The mechanisms that cause ghosting in analog television may corrupt the signal beyond use fordigital television. 8VSB,COFDM, and other modulation schemes seek to correct this.
