0.0 18th January 1999 Draft created
0.1 sometime ago recovered after crash
1.0 11-FEB-2002 tarted up for web release
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.
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.
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.
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