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Television Interference (ghosting) This is caused by having more than one path from the transmitter to the TV set, this can be caused by Reflections off large objects, or RF leaks or Reflections in the coaxial cable system. This is most common in a large building such as a tower block where one TV antenna feeds many different rooms each fitted with a TV aerial socket. By getting a better antenna or cable system it can be eliminated or mitigated. Fun experiment/method of investigating ghosting If you find what is on TV a total bore, then it is possible to use the TV as a passive Radar system. If you were to put a small Yagi Antenna on a broomstick it would be possible to find the dirrection in which the ghost is strongest. If you then measure the width of the screen and then the distance between the main image and the ghost, then it will be possible to work out the difference in the distance of the two paths between the transmitter and your antenna. Note that the Speed Of Light is 300×106 m/s. PAL and SECAM the line is 64 Microsecond s from start to finish. As the first 12 microseconds of the line are taken up by things like the Front Porch , Back Porch , Horizontal Synch , and the Colour Burst only 52 microseconds of time exists for each horizontal line. In NTSC the active line duration is 52.2 microseconds, the total line length is 63.55 microseconds. So from these facts it is possible to measure the difference in the path length between the two paths the TV signal takes between the transmiter and your TV aerial. For example using a PAL TV if the TV is 20 cm wide and the ghost is 5 cm to the right of the main image then t = 64×10−6 × 5/20 seconds = 16×10−6 seconds So the path difference is 4800 Meter s 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 a RF leakage has allowed the TV signal to enter the tuner by a second route. For instance if you encounter this, then check to see if someone has plugged in an additonal aerial to a TV which has already got the communal TV aerial conection. |
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