WARNING!!! Deep dark Geek stuff
Bear with me, please.
Back in the day, radios were introduced that were muted until a audio tone "switch" was thrown. When the switch was activated, the radio would "open" so voices could be head. Those tones were very low, 60 to 250 Hz, and filtered out of the speaker's audio stream - voices are normally 300-3,000 Hz and not affected. Transmissions and interference that didn't have the correct tone would remain muted.
Originally, radios had a mechanical reed that would vibrate at a particular audio frequency, always responding to the "raw" audio even when the speaker was muted. When the reed seriously vibrated because the radio heard the right harmonic audio tone, the reed's motion caused a magnet on the end to generate a second signal that activated a switch opening up the radio. This was the original CTCSS or Private-Line (PL) system. ("Private" is misleading- an "open" radio doesn't care about private tones and can still hear everything.)
That worked so well a second sequential tone was added for paging, and possibly do other things like blow a horn or turn on the lights. Both tones would have to be received in sequence for anything to happen. This became the famous "Two-Tone" one-second-three-second tone paging system. There are many similar systems.
The next paging evolution was to open the radio whenever either reed vibrated for a long time, like five seconds. This became a "group" or long tone page.
[Sidebar: DCS/DPL codes are a time-varied single low audio tone - around 100 Hz - that send-pause-send-pause-etc. in a particular sequence sending a binary number. Same thing as CTCSS, but different.]
Station alerting was introduced as a house-wide radio that would be quiet at night (sleepy-time) but raise Holy Hell when the station's unique tones were received, in theory louder than snoring firefighters. Effectively, a fixed-install pager that made noise and turned on the lights. That was a good plan, and has evolved even further with the introduction of digital and IP systems. Unfortunately everyone's idea is better than everyone else's, so there is no single standard. Folks who have been around radio long enough can ignore the voices, but their ear can detect their familiar 2-tone sound and bolt them into action.
Nowadays radios are computers with an antenna. Mechanical vibrating reeds are long gone, so the audio tones can be more complex. CTCSS, MDC, 2-tone, 5-tone and DTMF (telephone dial tone) audio tone sequences are decoded by a radio and cause it to do something.
The commonly deployed 2-tone system has several hundred possible codes, enough for each station in a large district. 5-tone and DTMF can have hundreds of thousands unique codes, with the advantage of using wildcards in the radio's decode mask. A Chief or particular apparatus can be alerted with "12345". Using wildcards a "12
???" decode mask will activate on both "12
345" and "12
999". The actual coding is up to the system designer, and once again there is no standard.
For your particular location, it is going to take some investigation, and decoding and opening on a unique station code may be difficult or impossible. Some higher-end scanners will allow you to program in a decode sequence for a station, most will not. Of those that do most will only decode a 2-tone sequence, but that is going to cost you. Many newer scanners will require a CTCSS or DCS code to open, but then you hear all traffic, not just a call to a particular station.
Download "Audacity" to find audio tones. A standard tone chart can be downloaded from
https://www.midians.com/pdf/tone-signaling-charts.pdf. Audio tones have decimal precision, but you need to allow 2 or 3% error. Reality: they are not that precise.
Digital and MDC-1200 transmissions are a whole other complex can of worms to figure out.
Hopefully, knowing the how and why will help you (and others) figure out your local system. Reverse-engineering can be a rewarding challenge.
