For
Analog TV the Answer is:
The TV picture image is being received multiple times,
a fraction of a second from the main picture signal.
For Digital LCD TV the Answer
is:
The speed at which the LCD screen up dates...
Why?
Any station can have ghosting, the Radio Frequency TV signal is bounced
off objects like
buildings, overpasses, hills etc. Your analogue TV receiver picks up
the "main TV signal" as well as any "TV signal echoes" and displays
them as ghosts. TV Stations use "high power" to get the signal into
your home, so people
with rabbit ears, "in roof" antenna and those that use wet pieces of
string can receive some watchable form of picture & sound; this
in turn creates powerful signal echoes.
It is a trade off to maximise people watching (for advertising revenue)
over quality watchable signals.
DiGiTaL TV
receivers eliminate ghosts but the ghosts have not gone away, the
DiGiTaL TV smarts filter the signals and track on the strongest one.
If a ghost and the main signal are of equal strength, the TV smarts
won't
find a picture and report "no or bad signal" yet it can look ok on a
analogue TV receiver.
LCD TV
can display a
false ghost effect when fast moving objects are shown on screen, a type
of trailing effect which is caused by the little Red, Green &
Blue LCD cells not switching off/on fast enough. It is not a
fault with the TV it is a limitation of the technology. As the LCD TV
gets older ths effect may be more noticable or less depending upon the
light conditions in the TV viewing room and back illumination of the
LCD screen (the fluro tubes with-in the LCD TV)
RF
Solutions:
Twist the TV antenna around a few degrees to acquire a stronger signal
(ghost or main, digital doesn't care which one)
Alternate Solutions:
Because of these poor picture or more correctly signal problems, TV
stations also re-transmit their signals in the UHF TV band Ch 21 to 69.
Usually from different locations in and around the city / area at lower
power levels and at a different signal polarity to the main TV signal
that services the area. This enables TV stations to reach areas blocked
by high rise buildings or in a low lying areas. You may wish to receive
both services to obtain "best" picture & sound per TV channel.
A polarised antenna is either "Vertically" (the long sticks go up and
down) and "Horizontally" (the long sticks go side to side).
North, South refers to magnet direction usually quoted as a compass
heading for pointing your TV antenna towards the TV
transmitter. Positive and Negative refer to
electrical power connection and usually only quoted when a
"mast head" (at antenna) or an inline TV amplifier is used to boost the
TV signal, which usually needs DC power injected into the coax
(antenna lead) to work.
FM radio signals also suffer ghosting, you hear them as distorted
audio, especially in a moving car on the road in the wet.
Ghosting is nothing more than the signal being received more than once,
usually a micro to a milli second later, it is the basic principle of
Radar, but that is another story...
73's for now.
Gary, VK3KHB