I’m hoping to build up a series of static pages with various helpful information here. This is the first one, a guide to scanner listening. It’s not really presenting any new information, but hopefully it’ll help someone: scanner listening is one of those things where tutorials present the most basic, obvious information, and then some guides exist for really complicated stuff, but there’s nothing to cover the middle ground.

Frequencies

The first thing you’ll need is a list of the radio frequencies used in the area. The Internet makes this pretty easy, though. Here are some excellent resources:

• Radio Reference‘s database
• Cityfreq, although I find it’s got a decent amount of errors
• FCC Advanced License Search, for more advanced use

Understanding the System

It’s helpful to have some understanding of how things work. Here are a few technologies probably going on in the background when you’re listening.

CTCSS (“PL Tone”)

A PL tone (CTCSS code) is nothing more than a continuous tone superimposed on a radio signal at frequencies (roughly) outside the range of human hearing, between 67 Hz and about 260 Hz. What happens is that you set up all the radios in your fleet to transmit a certain PL tone, and to only unmute when they hear a signal with that tone. This helps out a lot in cutting out unwanted noise.

Repeaters

In their simplest form, a repeater is little more than two radios hooked up to each other, situated somewhere like on top of a mountain, hooked up to a really good antenna. Especially across larger areas, you might not be able to hear everyone on your frequency, but the repeater is in such a good location that it can. (This is especially relevant when people try using portable radios instead of their high-powered vehicle radios.) Nearly all public safety communications these days take place through a repeater, where the units transmit on one frequency (the repeater’s input frequency), but receive on another (the repeater’s output frequency). Because you’re listening to the repeater output, if you can’t hear a given unit, it stands to reason that no one else can.

Trunking

Trunking’s not that complicated a subject, but I find it hard to explain. What you need to understand is that, especially in crowded areas, we’re starting to run out of frequencies to use. If you think about it, a town probably needs about a dozen channels at least. It’s a really good idea to give at least two each to the fire and police departments, one for dispatch and one for unit-to-unit communications, etc. That’s four. If the ambulances are separate from the fire department, they need one or two more. Add another for the public works, and another for the water department. And the building inspectors. And the health inspectors. And the school system. And the trash collectors. Let’s call it a dozen channels. You now need to get a license from the FCC for two dozen frequencies, since you’re going to want repeaters on each of them, each of which needs an input and output frequency. And then you need to buy and set up a dozen repeaters.

When you’re done all of that, you’ll find that, at any given time, only one or two of the repeaters are in use at any given time. So rather than licensing a dozen ‘pairs’ of frequencies, you might license five. Since it’d get confusing, we stop saying that a radio is on a given channel, and instead use the term talkgroup. We use one of our pairs of frequencies as a control channel, where radios go to get a frequency assignment, and we make it computer-controlled, so that it happens in a fraction of a second.

So when someone wants to talk on the Public Works talkgroup, (s)he will key their microphone to talk. To them, it’ll beep (a ‘talk permit tone’), and then everyone will hear them talk. In the background, though, their radio has just had a quick ‘conversation’ (not a technical term) with the trunking controller, the computer that makes the whole system work. Their radio identifies itself, indicates what talkgroup it belongs to, and requests a frequency assignment. The controller will verify that the radio is part of the system, and that it’s authorized to transmit to a given talkgroup, and then respond by instructing all radios on that talkgroup to switch to a given frequency. All the radios switch over, hear whatever the user has to say, and then the radios switch back to listening to the control channel, awaiting further instructions.

The reason this matters to scanner listeners is that the trunking controller cares nothing about trying to make sure that a given talkgroup is always assigned the same frequency. It’s almost guaranteed, in fact, that if you listen to any given frequency in a trunking system, it’ll be a jumble of all sorts of different conversations from different talkgroups, one after another.

Newer scanners are designed to listen to the control channel and follow a given talkgroup, letting you listen just like the ‘real’ users can. It’s possible to listen to a trunked system with a non-trunking (conventional) scanner, but it’s a painful process.

Digital Voice

The newest trend is to use digital voice. The reigning standard is APCO’s Project 25 digital voice, known commonly as P25 voice, IMBE, or CAI (Common Air Interface). Motorola is, by far, the biggest maker of IMBE gear right now, under the name ASTRO. Without a radio capable of doing digital, all you’ll hear is noise kind of sounding like a saw, only much, much worse. Digital scanners are rare, and most cost $400 or more. (Which is about a tenth of what the departments pay for new digital radios.)

Surprisingly, given how expensive the whole thing is, decoded digital voice sounds horrible. (Possibly because it’s 9,600 bps… We whine and complain that a 128kbps (128,000 bps) MP3 sounds horrible.) It also doesn’t ‘degrade’ well: with an analog signal, you might start hearing some static on the voice signal as it gets weak, but with the digital signal, you start hearing random buzzing noises, or just nothing at all, in the middle of their talking. As much as I’ve read up on digital standards, and owning several ASTRO radios, I’m still at a loss to explain exactly why anyone would want to use digital voice. The current implementation, at least, is really not that great.

Codes & Terminology

Police 10-codes

10-codes are pretty common in public safety. APCO has developed a standard list for police use. That’s where the common “10-4” comes from, along with the root of “What’s your (10-)20?” The APCO 10-codes are easy to find online. New Hampshire, though, has its own set, which uses “10-5” instead of “10-4” to mean “Message received,” etc. Departments use them in varying capacities; some don’t use more than a couple, others (like the NH State Police) rely on them very heavily. It’s handy to find the list the organizations you listen to use, print them out, and keep them by your scanner. Over time you’ll come to recognize most of them.

Miscellaneous Terminology

This one probably varies too much to be useful, but here are a few I’ve come across:

• “DK” is used by the Boston PD to mean “Drunk.”
• An “FIO” (Boston PD) is basically “Field Intelligence” information on someone who isn’t doing anything meriting criminal charges; they’ll collect information anyway. (Sort of like a “Stop and identify” thing.)
• “94C” references the criminal code for drug use in Massachusetts; it seems to be used to refer to marijuana, but technically refers to any illegal drug.
• A directed patrol (possibly “DP”) is a check (patrol) of an area that has had problems in the past; thus they were ‘directed’ to patrol the area specifically. You’ll often here these called in so it’s recorded that they checked.

Fire Terminology

• Fully-involved basically means that the building is consumed by flames.
• DWH, PWH refer to Doubtful Will Hold and Probably Will Hold. “Will hold” refers to their ability to control the fire; a DWH, then, means that they doubt that they’ll be able to contain the fire.
• Knockdown refers to most of the flames being, well, knocked down. After a fire is knocked down, firefighters can go in and get the rest.
• Extension refers to the spread of fire, especially into walls, etc. You’ll often hear something along the lines of, “The bulk of the fire has been knocked down; crews are now checking for extension.”
• FAST is an NYFD term; it refers to a company that will be on the scene of a fire in the event that it is necessary to rescue any firefighters.
• Command is used to refer to one of the companies on scene who is in ‘charge’ of a given incident; the first company arriving on scene will have command, but will sometimes relinquish command.
• Still alarm: It seems to be a fire that is called in by someone, as opposed to a fire alarm activation. (I’ve never been 100% confident of this.)
  • A box alarm is a fire alarm that has been activated, either a smoke detector or pull station
• Fireground refers to a frequency, often simplex, that is used for communications on the scene of a fire.
• Entrapment refers to someone being trapped inside of a car, usually as the result of a serious accident.
• Mutual Aid is a set of agreements whereby neighboring towns will assist with each other’s fires. The term can mean a few different things:
  • A frequency is often designated as a Mutual Aid frequency, so multiple departments can communicate. (154.280 / 154.190?)
  • Various fire companies will just be dispatched “mutual aid” to an address in another town.
    • Sometimes it’s just mutual aid station coverage, defined below
  • There’s also automatic mutual aid, where they’ll be dispatched automatically, as opposed to by request This isn’t a big distinction, although the odds are probably greater that they won’t be needed.
• Tone out refers to issuing a series of tones to alert fire pagers (e.g., Minitors), which basically listen to the fire dispatch frequency but don’t open squelch until they hear the right set of tones.
• Station coverage: A department might ‘tone out for station coverage’ if multiple calls are happening simultaneously, so that, in the event of another alarm coming in, they have adequate manpower to respond. (Mutual aid companies will sometimes be dispatched if a given town is spread thin fighting a bona fide fire.)