DIYbanter - View Single Post

On 7 Dec 2003 14:32:09 -0800, (Martin
Pentreath) wrote:

At the moment my aerial gets a reasonable analogue picture, but with
some ghosting. Would I be right in thinking that if I got a digital
set-top box this setup would be more forgiving of ghosting? I think
the signal itself is fairly strong, but there's just a bit of
reflection off nearby trees and things.

Ghosting is the result of signals being reflected from some other
object between the transmitter and your receiving antenna. Because
the signal takes a longer time to reach you (think of it as corners of
a triangle, you get a ghosted image offset to the right on an analogue
picture.

You can roughly work out the extra distance by measuring the offset as
a percentage of the total screen width. On a standard 625line/50Hz
TV (which is what you have, one line takes just under 64 microseconds
in total, but thre is about 53 microseconds worth visible because of
synchronisation time. So your extra delay is roughly the proportion
of the screen for the ghosted image multiplied by 53 uS.

The TV signals travel at approx. the speed of light (299,792,458
metres/sec); so from this you can work out the extra distance.
Generally the transmitter is a lot further from you than the
reflecting object, so to a good approximation, the distance you
calculate is the distance the object is from you.
Sometimes, if the objects are very close, e.g. surrounding walls and
other objects, the ghosted image will be so close that it just appears
to be a fuzziness of the verticals of the picture.

With digital TV, you would not see ghosting directly, since the
digital information is placed on an analogue signal and the decoder
uses that. The decoder is taking the signal and making decisions
millions of times per second (the data bit rate) about whether the
signal is 1 or 0 and uses that. However it is doing that by
sampling the level of the signal at that rate. This works well in
principle, but there are problems with it. Interference to the
signal or poor signal will mislead the decoder about whether given
bits should be 0 or 1. Currently digital TV transmissions use a
very low power in comparison with the analogue TV, which is one of the
reasons why people are having to use better antennas for digital TV.

If you factor in an additional delayed signal (which is what ghosting
on analogue is), the decoder will see it as interference to the wanted
signal. Depending on how strong the delayed signal is relative to
the wanted one, the decoder may make mistakes as it will with
interference.

Up to a point, the decoder can take care of this because the
transmitted digital signal has extra data added that is used to help
the decoder detect and correct such errors.
Beyond a certain rate of errors, these mechanisms fail and the result
is the the picture and sound will freeze or the picture breaks into
small blocks (called macroblocks). Obviously you don't want this.

Therefore, when an antenna is set up for digital reception, it is not
a case of pointing and hoping or looking on the screen for best
picture or minimum ghosting. A proper installer will have a piece
of test gear which measures the rate of errors coming in the signal.
This takes account of all the interference and multiple reception
paths, and generally the objective is to minimise the error rate.

You could try with your existing antenna, but don't be surprised if
you have to get a new, more directional one which will give a stronger
signal and because it is more directional, reject unwanted reflected
signals.

..andy

To email, substitute .nospam with .gl