The white (VHF frequency) and blue (UHF frequency) are separate systems. The blue SCU unmutes when it digitally receives a signal from the cad and it is broadcast on blue 8. The white SCU unmutes when it receives a QCII signal on Tac 19 (used to be called white 3 hence the name).
From LACountyfire.com:
Here is a step-by-step run down:
1. Dispatcher pushes <ENTER> to dispatch call.
2. CAD looks at units assigned and their status, and sets off station SCU if units are in quarters.
3. If units are in quarters, the SCU (Station Control Unit) does three things:
A. Turns on station speakers for a preset amount of time (usually about a minute) to allow any transmission on Blue 8 to come through
B. Sends a signal to the electrical room to turn on the "night lights" (which usually entails at least a light on the apparatus floor and dorm)
C. Activates a circuit in the SCU to emit a tone into the speakers. This tone can vary from station to station, as the circuit board in each station's SCU can vary slightly, and does not come from Command and Control.
-The logic is built into the SCU to not set off the tone or turn on the speakers if only a BC is sent out on a call. However, some battalion headquarter's SCUs can turn on the light in the BC dorm and apparatus floor only. This is because the BCs have their own pager to wake them.
4. After about ten seconds of purposely waiting (for all the above stuff to happen in the station), the dispatcher pushes the <WARBLE> button on the console (to make the up-down-up-down sound), and then the <TRANSMIT> button to voice the call. If there are several calls lined up, there may be limited voice dispatch ("Rescue, 38s", Structure Fire, 132s", etc.) or just no dispatch.
Although most of the time the MDTs go off before the SCU, sometimes if there are a lot of units on the ticket, the SCU might go off first, leaving us to wonder what kind of call we are going on.
