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In television, a ghost 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 two different paths to a receiving antenna, with a slight difference in timing.Jorma Hyypia, ''Beating TV Interference'', '' Popular Mechanics'' , June 1980 page 126
Common causes of ghosts (in the more specific sense) are:
*
Analog ghosting
Common causes of ghosts (in the more specific sense) are:
* Mismatched impedance
In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...
along the communication channel, which causes unwanted reflections. The technical term for this phenomenon is ringing
Ringing may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
.
* 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
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
*Digital audio, representation of sound ...
portion uses FM, which has the desirable property that a stronger signal tends to overpower interference from weaker signals due to the capture effect
In a radio receiver, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency or channel will be demodulated.
FM phenomenon
The capture e ...
. 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
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
and UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
, which have line-of-sight propagation
Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions travelin ...
, 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 (orcable TV
Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with broadc ...
) can cause this condition.
Digital ghosting
Ghosting is not specific toanalog
Analog or analogue may refer to:
Computing and electronics
* Analog signal, in which information is encoded in a continuous variable
** Analog device, an apparatus that operates on analog signals
*** Analog electronics, circuits which use analo ...
transmission. It may appear in digital television when interlaced video is incorrectly deinterlaced for display on progressive-scan output devices. The mechanisms that cause ghosting in analog television may corrupt the signal beyond use for digital television. 8VSB, COFDM, and other modulation schemes seek to correct this.
See also
*Ghost-canceling reference Ghost-canceling reference (GCR) is a special sub-signal on a television television channel, channel that receivers can use to compensate for the ghosting (television), ghosting effect of a television signal distorted by multipath propagation between ...
* Onion skinning
References
* * {{Authority control Television terminology Radio electronics Visual artifacts