The radio system
Starcom21
Fire and Police departments here used to run on analog VHF, one agency, one frequency, permanently. They have since moved to Starcom21, a digital radio network covering all of Illinois. It’s built on Project 25 (P25), the digital standard most public safety agencies in the country are moving to.
The P25 network is available across the entire state. Crews sent to another county can still reach their own dispatch and talk directly to the agencies around them.
How a shared network works
On the old system, each agency owned a frequency outright. On Starcom21, many agencies share a small pool of voice frequencies, and the network hands one out for the length of each transmission.
That network has a control channel: a separate frequency that carries no voice, only instructions. When someone keys up, the control channel assigns a voice frequency and tells every other radio in that conversation where to tune to. This happens instantly and to the actual personnel on the system, it works more or less just like a traditional radio.
A couple of terms worth knowing:
- Talk group: the digital equivalent of a channel. It’s the group of radios that hear a given conversation. Most agencies have a handful of their own.
- Site: a tower location, each with its own set of voice and control frequencies. All of McHenry County is site 146 on the statewide system.
Fireground channels
Fireground channels are the short-range radio traffic at an incident. Crews talking to each other and to command on scene, rather than to dispatch.
Illinois departments share a statewide set of these. They’re simplex, meaning radio-to-radio with no repeater in between. That has two consequences for listeners: they’re hard to pick up from any distance, and because the whole state shares them, you’ll occasionally hear a distant agency bleed through.
A recent update to our scanning software lets us fold these channels into our streams, but only if we’re close enough to the incident in progress.
MABAS Division 5, which is all the Fire Departments inside of McHenry County, has started using Starcom21 Talk Groups for incidents for many events these days. The dispatch centers assign them an available channel for their incident, rather than the older frequencies.
Equipment
Traditional Scanners Don’t Work
Analog scanners and most other types of digital scanners cannot follow this system. They have no way to read the control channel, so they can’t tell where a call has moved to. To monitor Starcom21 directly you need a scanner that handles P25 Phase II and copes well with simulcast (sites where several towers transmit the same signal at once). Those scanners run several hundred dollars and up.
How Our Feeds Work
There’s a cheaper route, and it’s what powers this site.
Our feeds come from several inexpensive Software Defined Radio (SDR) tuners small USB devices that pass raw radio signal to a computer instead of decoding it themselves. A free program called SDRTrunk does the rest: it reads the control channel to learn where each call is going, then decodes every active voice channel at the same time. A single scanner can only follow one call at a time, which is the real advantage here.
SDRTrunk then pushes each individual call out to Broadcastify Calls and to the matching Broadcastify stream.
Setting this up is genuinely technical, but the SDRTrunk wiki is well written and a reasonable place to start if you want to try it yourself.