In television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

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

Common causes

Common causes of ghosts (in the more specific sense) are:

• Mismatched impedance
  Impedance matching
  In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
  
   along the communication channel
  Channel (communications)
  In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
  
  , which causes unwanted reflection
  Reflection (physics)
  Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
  
  s. The technical term for this phenomenon is ringing
  Ringing (signal)
  In electronics, signal processing, and video, ringing is unwanted oscillation of a signal, particularly in the step response...
  
  .
• Multipath distortion, because radio frequency
  Radio frequency
  Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
  
   wave
  Wave
  In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
  
  s may take paths of different length (by reflecting from buildings, clouds, etc.) to reach the receiver
  Receiver (radio)
  A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
  
  . 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
  Hotel
  A hotel is an establishment that provides paid lodging on a short-term basis. The provision of basic accommodation, in times past, consisting only of a room with a bed, a cupboard, a small table and a washstand has largely been replaced by rooms with modern facilities, including en-suite bathrooms...
  
   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

Video

Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 portion of television, largely because it uses AM

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

 for transmission. TV is also transmitted on VHF and UHF, which have line-of-sight propagation

Line-of-sight propagation

Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...

, and easily reflect off of buildings, mountains, and other objects.

The audio portion uses FM

Frequency modulation

In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

, which has the desirable property that a stronger signal tends to overpower interference from weaker signals due to the capture effect

Capture effect

In telecommunication, 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 will be demodulated....

. Even when ghosts are particularly bad in the picture, there may be little audio interference.

Ghosting is specific to analog

Analog television

Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...

 transmission. Digital television

Digital television

Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...

 avoids the problems of ghosts altogether regardless of the transmission format. Co-channel interference

Co-channel interference

Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....

 is sometimes called ghosting, but it is very different in nature.

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.