In article , Edwin Spector
writes


The Natural Philosopher wrote:

Edwin Spector wrote:
...
It's probably worth spending a couple of minutes making the joint as 'nice'
as
possible. A discontinuity in a TV feed could produce a reflection which
could
give a ghost on the telly image.


Unlikly given

(a the small refelction that has been demonstrated = 20dB
(b teh fact that teh distances involved are small. you need a fair few
meters to give a discernile ghost.

a line is 60Us long, roughly, so for 600 dots per line rough image
quality, 100ns or less is indiscernible.

That equates to 3meters of cable. Quite a bit

Good points. However, just to clarify, the 20dB return loss was for the
reasonably good joint. The bad joint had a return loss of about 11.6 dB. That's
about 7% of the power being reflected. (20dB reflection corresponds to 1%). I'm
not sure how noticable that ghost would be.

Also, don't forget that signals travel more slowly down cable than in air, by a
factor of (say) 0.5 to 0.7 (.) So, it would only take 2m of cable to give a
discernable ghost (by your test).

I added the note about reflections after a colleague's account of a bad ghost
which was traced to a cable joint made using a 30A mains junction box. Who ya'
gonna call?

Edwin.

In fact there a very well written article on a subject called pre echo
where there is a possibility of direct pick up off poor cables directly
from the transmitter and the delayed signal via a distribution system.
Very good site this is if you're into TV etc...

http://www.wrightsaerials.tv/Resources/Pre-Echo.pdf
--
Tony Sayer