Yea, they have been simulcasting paging, including tone and voice
here in this country for MANY years. UHS Oscillators and offsets of
1 to about 8Hz causes any nulling that will happen in overlap areas
to move around in the overlap area. It is very important that ALL
audio be as perfectly in phase as possible we used to use Allen
Aviation (I think) delay lines. the problem is if you have multiple
receivers. Then you have to thoes togeather with delay lines going
to a single site for re distribution. Good luck!
AC0Y
--- In Repeater-Builder@yahoogroups.com, Al Wolfe [EMAIL PROTECTED]
wrote:
Back in the 1970's several of the European broadcasters
experimented
with simulcasting with multiple transmitters on the same
frequency. It seems
to me that they settled on 50 htz for an offset (carrier frequency
difference) between adjacent transmitters. This is low enough to
not be a
problem with PL tones and high enough to mask the beat note issue
in the
overlapping mush zones. Not sure how they maintained their
frequency
stabiliy back then.
Al, K9SI
Date: Wed, 04 May 2005 08:53:07 -0400
From: Kevin Custer [EMAIL PROTECTED]
Subject: Re: voting receivers with simulcast transmitters
Joe,
Did you mean offset when you said stability? I'd agree that
1/2, to a
few Hertz would be annoying. In testing here, and as shown in
practice,
simple systems sound better if run at about 10 - 20 Hz offset.
This
makes the beating more tolerable without being able to be
reproduced
(very well) by the listening speaker. This is also why it is
nice to
have high pass filtering in the listening receivers. Radios
with PL
filters do nicely, something like the Com-Spec TS-64's PL filter
works
well. Unfortunately, many made for ham rigs don't have adequate
(if
any) high-pass filtering even if the radio has PL decode.
Simulcast
Systems are one area that benefit from Total HPF of a PL filter,
where
Notch Filtering would do no good for the Simulcast beats in the
very low
frequency range; 60 Hz.
Of course, at 10 Hz offset, a few Hz. of instability at each
transmitter
could result in something very annoying; as the two drifting
transmitters could come within a few Hz. of one another or worse
yet,
zero beat.
I remember one particular instance many years ago where we did
testing
of two transmitters that were close together and run at 67 Hz
offset.
You could decode this PL tone when you heard both transmitter
sites, but
they didn't have HSO's and drifted enough that PL decoding was
not
reliable.
Kevin Custer
mch wrote:
To work well, you will need more than 'a few Hz' stability. Even
1/2 Hz
is very noticable and annoying.
Joe M.
Thomas Oliver wrote:
You will need the three transmitters to have uhso (high stab
oscilators)
to
keep them within a few hz of each other, you will have to delay
the audio
so all three transmitters transmit the audio at the same time.
I do not
know what effect the multipath from buildings will have on the
recieved
signal. I think it is worth a shot.
tom n8ies
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