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Technical Manual For The Radio Glen Transmit Selector
Change History:
0.0 18th January 1999 Draft created
0.1 sometime ago recovered after crash
1.0 11-FEB-2002 tarted up for web release
1.
Introduction And Copyright Notice
This document describes the Radio Glen Transmit
Selector system. All the information in this document should be
considered copyright and should not be re-used directly without
citation.
Web Release Note: This device has been
in use for a few years now. I'm not aware of any problems with
it though I think it is sometimes used to direct feeds from OBs
direct to the transmitter without going through a desk which I
wouldn't recommend. Very high RF currents caused by being right
next to the 50 Watt transmitter once caused some random
switching of the relays due to some grounding and mismatch
problem in the technical cupboard. Latching relays really do
sound like an unnecessary bit of retro don't they? But they are
so well behaved and bomb-proof, which is what you want for a
device in the transmission chain. I was surprised how well this
turned out, though I did my back in doing the metalwork for all
the sockets on the rear panel. The hysteretic behaviour of the
latching relays is especially handy - that is if one of the
coils is energised and the other coil is subsequently energised
then the relay stays in the original position. This effectively
deals with all situations of switch debouncing and all the
buttons being pushed at once - and I tried that often enough.
The picture above shows the main unit with the remote control
sitting on top of it. The remote unit is now computer controlled
by the automated system at SURGE. There are even LEDs above the
control switches which flash when you are about to be put on-air
by the system.
2. History
Around October 1998 the station and studio were
in a fair state and looking forward to spring of the following
year when the big change to 1W erp free-radiating might be made.
However, the desk was still wired such that the main output was
always connected to the transmission and distribution system. It
was not possible to take the studio off-air to do production or
shut down for maintenance. A transmit source selector was what
was needed, the same function which used to be provided by the
selector buttons and relay box underneath the big output meter
in the F-block studio. It would have been possible to make a
cheap selector with a rotary switch and an old box, but I
thought that it would be more interesting to make a full-scale
remote controlled switch unit. I considered 22V10 style
programmable logic for the state machine but was quite taken
with the idea of using latching relays. A bit of design work
showed that this was possible with 9 latching relays. While the
latching relays are quite costly, it made the whole design
terribly simple, avoided programming PLDs with their associated
long term maintainance problem, and was inherently non-volatile.
3. Description
The main box of the Transmit Selector system is designed to be
located in the technical rack of a small radio broadcast
station. There are four stereo balanced line inputs, either of
which can be routed to a stereo balanced output. The signal
routing is performed with relays and so the switching may not be
completely silent. The source is selected by one of four buttons
on the main box or by an identical pattern of buttons on one of
three remote control panels. The particular panel which can
control the switcher at any time is the panel which has the
'ACCEPT' lamp lit. This works on the standard offer-accept
system as widely used on mixing desks and switchers. The main
switch unit also has an 'OVERRIDE' button which can be used to
force the system into offer mode regardless of whether the main
panel has control. The main panel can therefore steal control
when needed, and this also serves to reset the system on
delivery or when the latching relays have been set to an unknown
state by vibration in transit. Also, if a remote panel has
control and is subsequently disconnected, the system can still
be switched from the main panel using override.
Remote panels connect to the main unit via a
sixteen way cable terminated with 25-way D-connectors. All power
for remote lamps is provided along this cable. Both main units
and remote panels are 2U size, though this is only due to handy
small boxes for the remotes only being available in 2U size.
Future units could be 1U if suitable hardware can be found. The
main unit will have to remain 2U due to the number of connectors
on the rear panel. All switch lamps are LED types.
As noted the latching relays are non-volatile;
They remain in their last position when power is disconnected.
The signal switching relays are ordinary non-latching types
however. The system is designed such that the switcher will
revert to the sustain programme input on channel one when power
fails. When power is restored, the originally selected studio
will be put back to the transmitter.
4.
Schematic Description Signal
Switching Relays switch01-01.sch
This sheet shows the signal switching relays. They are driven
from the latching selector relays. Note the sustain service
relay has the signal wired via the normally closed contact such
that sustain service is selected during a power failure. The
reverse biased diodes and resistors across the coils are a step
towards spark and emc suppression.
5. Schematic Description Latching State Relays switch01-02.sch
Sheet two has the four state relays which record which panel has
control, ie the accept lamp lit. The common ground of each
remote panel's select switches are routed through the
appropriate relay, so only one remote has control. The fifth
relay on the sheet indicates the offer state and allows any
remote panel to alter the state of the accept relays. Note that
when one panel selects its own accept relay to go on, it
switches all the others off via the diode ORing arrangement,
ensuring that only one panel can ever accept control. SOGNDx is
a potentially confusion label; It stands for SELECT/OFFER GND
and is low for the panel which has the accept lamp. In this
state that panel can both do selecting and offering.
6. Schematic Description Latching Source Relays Rollover
switch01-03.sch
The state of the four latching relays here determine which
source is selected to the output. Only one of the panels has the
bottom end of its switches grounded by the appropriate SOGNDx,
so only one panel can change the state of the select relays.
Note that when one relay is selected to go on, all the others
are switched off via the diode ORing arrangement. In this way
only one source can ever be selected. If the user happens to try
pushing two buttons at a time, the button pressed first will
remain lit, as that switch has already activated the off coils
for all the other relays, and the relays will not change state
with both coils activated. This provides an extremely neat
rollover action on the panel switches and avoids double source
selection.
7. Schematic Description Triac Annunciator Outputs and PSU
switch01-04.sch
This is the PSU and the outputs to the triac system connector.
The PSU is self explanatory, other than the voltage dropping
diodes. In an ideal world you would select the correct voltage
of transformer to start off with, though the choice of a toroid
is always a sensible option with audio equipment. Each of the
state relays' second normally open contact pair are taken off to
a 25-way D connector on the rear panel. This can be used to
switch a triac unit to light large mains annunciator lamps to
indicate who is on air, who has switch control, and when switch
control is on offer.
Rear panels of the main and remote unit.
Inside the main unit under construction.
You can see the remote panel in use
in the studio 1 rack in one of the pictures of the TBU
installed.
