Originally Posted by kc9sbd
Wow, this is pretty cool info. Your wife working in lake geneva and having seen this is pretty cool as well. I heard it at 630ish pm tonite so i dont know if it checks after fire calls or whatnot, but i do appreciate the quick responses. Thanks again.
He is Michigan, not Wisconsin...I think a few people are getting their wires crossed (no pun intended) here. His wife probably recognized the sound because her city or county uses a similar system so it sounds the same.
A modern outdoor warning system is comprised of an encoder of some sort at the PSAP (and backups off-site usually) and decoders at each of the noise makers. When the dispatcher pressed the big red button, the encoder sends out various data bursts via radio or wireline to each of the sirens to start them up or shut them down. In reality, it's a two-way conversation so there is usually a lot of squawking...especially in a big city that has a lot of sirens. The systems I am familiar with have a typical 25w or 50w mobile radio mounted at the PSAP and in each siren control cabinet so they can communicate back and forth.
Back when the outdoor warning system was built out in the 50's during the Cold War there were initially 3 choices: Alert, Attack and Cancel/All-Clear but then the Cancel/All-Clear was deleted leaving just Alert and Attack. Alert is a steady tone (mechanical siren rotor winds up and stays at max RPM) and Attack is a wailing rising/falling tone (mechanical siren rotor RPM varies as motor powers up/shuts off in sequence). Currently there are also test modes like the silent test that make the siren head rotate but not power up the rotor, or the full test that everything powers up for a couple seconds and then shuts down.
In order for the newer sirens to know which mode to operate in, the encoder sends out different sets of data depending on what the operator wants them to do. They also send out the polling data on regular timed intervals and then report back if any of the units failed or didn't respond. If you listen to the process on a regular basis and live by one of the sirens in question, you will know what the sirens are doing just by the type of data bursts after a while. Some of the systems have an external control box like shown in the link above, or some are interfaced into the dispatch console. The data types vary also and range from DTMF, FSK or two-tone. I got a old Motorola Intrac two-tone encoder interfaced into a Centracom console as part of a surplus acquisition a while ago, and even though it's 30 plus years old it could still be used. The better part was the Centracom interface that says "TORNADO", "ATTACK" and "CANCEL". Older systems like the Intrac don't do remote polling, just the newer FSK system as far as I know.
I think Federal Signal, American Signal and Whelen are probably the 3 big outdoor warning equipment sources these days, plus just about everything is electronic now but there are still plenty of mechanical sirens around (which sound better IMHO).
Google "outdoor warning sirens" or similar if you really want to get your nerd on...I'd go on for pages but I'll suppress my inner demons