Terry Casey wrote:
In article , says...
The manufacturers of many of the modulators sold for communal TV system
use claim that the output is so clean that filter combiners are
unnecessary. But I use filters anyway. The fact is that I seem to get
better results.
I wonder if they have any active stages AFTER the filter, possibly
because they might be used in installations with less than perfect
matching?
Mebbe so, I hadn't thought of it. The out-of-channel noise from them is
impressively low, but with twenty of them it adds up.
We used to use some tunable modulators, although most of them were
fixed. The idea of the tuneable one was that you had a near instant fix
if one of the modulators developed a fault. The downside was that the
broadband noise from the unfiltered output degraded the headend by 1dB
per modulator ...
That sounds about right! The most amazing thing I ever saw like that was
at the old Visnews place at Park Royal. They had a system done by
'experts' who I dare not name even now. The modulators were terrible. No
kidding they drifted 1/4 of a channel if you put your hand on the case.
Their outputs were combined with a 20-way resistive star network. The
result was the most horrendous garbage you've ever seen. The modulator
outputs after the combiner were -10dBmV and the noise was about 6dB
below that! Of course there was a lot of amplification following, and a
lot of cross mod because in attempts to get rid of the snow and
patterning people had turned everything up. Hilariously, before I went
to it they had the local aerial firm round and they'd metered the C Pal
signal at one outlet and diagnosed 'weak signal' so they'd fitted a
massive high gain aerial! This was looking at C Pal which was about five
miles away. Happy days!
Obviously you will add noise to the amplified source but not to the rest
of the system.
Well yes, but better to add a little noise there than let the channel go
through the system 10dB down.
Bill