Re: Centrol Radio system question

[mauried@commslab.gov.au (Maurie Daly)]

In article <BNIO2.590$w96.598@nswpull.telstra.net> "Notagunzel" <notagunzel@bigfoot.com> writes:
>From: "Notagunzel" <notagunzel@bigfoot.com>
>Subject: Centrol Radio system question
>Date: Wed, 7 Apr 1999 23:09:18 +1000

>Another question for any learned correspondents.

>The Victorian Centrol Train to Base radio system operates as a Land Mobile
>system, where the Base transmission is on a separate frequency to the Mobile
>transmit/Base receive.  Each Corridor is allocated a pair of frequencies,
>and when the Base transmits, all the sites on a particular frequency
>transmit simulcast.


There are a couple of possibilities.
The Motorola Radio system that VLF uses is a half duplex split frequency system
which uses a 2400 bps data burst at the start and end of the transmission 
,(analog FM ) to amongst other things unsquelch the loco radios so that a 
driver only hears what hes supposed to hear.
The unsquelching is based on the train number which is supposed to be keyed 
into the radio , and a corresponding train number transmitted.
Its possible that the Base Stations can operate full duplex , ie transmit & 
receive at the same time. (depends on whether the cavity filters are installed 
or not.) (the loco radios are half duplex only.)
If there is insufficient echo suppression at the controllers end and the Bases 
are operating full duplex then its possible that what comes in , will be 
retransmitted out again at a low audio level , this making it audible on the 
Base Stations output channel.
Also if a loco radio is unsquelching when it shouldnt be then a driver could 
hear what hes not supposed to hear.

Why this only affects the Bendigo line however beats me.



PS the system doesnt have to operate in the way that VLF use it , ie all Base 
Stations transmitting simultaenously.
The system is supposed to work out from received signal strength which Base 
Station to use at any given time to transmit to a particular train.
This is done by voting receivers which all provide signal strength info back 
to the Base station controller which is supposed then to only use the optimum 
Base.
The advantage of this method is that the mush zone is eliminated.
The mush zone is the area approx half way between two Base stations where
the signal strength from both of them is roughly the same.
FM systems cant handle multiple transmissions from differant Bases of equal 
signal strength , and this usually results in very distorted audio.
The system did work like this back in the 1980s when it was installed,but the 
voting seems to have now gone and all Bases Transmit together.

MD