(TX) and travels through one path to the receiver (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. The technical term for this phenomenon is
ringing. •
Multipath distortion, because
radio frequency waves may take paths of different length (by reflecting from buildings, transmission lines, aircraft, 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 (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. 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.
SECAM TV uses
FM for the
chrominance signal, hence ghosting only affects the
luma portion of its signal. TV is broadcast on
VHF and
UHF, which have
line-of-sight propagation, and easily reflect off of buildings, mountains, and other objects.
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.
Hue shifting More severe cases of ghosting can show the ghost with a different
hue in than what was being intended to be
broadcast. Sometimes, ghosting can change the phase of the
color burst, thus confusing the television set and displaying the wrong color. ==Digital ghosting==