#radio #blog *2024-09-13* I've been struggling with my [[KiwiSDR Project]] for a bit, but we're mostly in the refinement process of that project. I'll be ordering things and tweaking it for a while but it's time to move on to the next thing - my pile of RTL-SDR's. ### What the heck is an RTL-SDR I'm going to cut a lot of important history about this. An RTL-SDR is a cheap (~$30) USB dongle that was intended to be a receiver for European TV signals. You'd plug it into your laptop and be able to watch topless women sell orange juice on Italian TV, as they do on Italian TV (after a certain hour). However, some enterprising individuals realized you could use the tuner to basically tune in about anything and this is basically how modern [Software Defined Radio](https://en.wikipedia.org/wiki/Software-defined_radio) ended up being popular. Go read that link, I can't explain everything to you. When I talk about an RTL-SDR, I'm talking about the USB radio that can tune in a bunch of things. It can tune about 2.2MHz of spectrum in at a time which is more than enough to do some cool things. If you'd like a good history on them and a more basic introduction about things you can do with them, go look at the [RTL-SDR blog's About page](https://www.rtl-sdr.com/about-rtl-sdr/). ### So Why Are They Interesting? They're cheap, but that's the start. This used to be the domain of _really expensive stuff_ but at $30 a pop, this turned into a game changer. They provide a constant 2.2MHz stream of radio data into your computer per RTL-SDR. Ok, I'll drop the nerd talk. That may not make a whole lot of sense to non-radio people, and it may not make a whole lot of sense to people into radio. When you say, tune the radio in your car to a radio station, it's only tuned into what you're listening to. The RTL-SDR is pulling in the whole radio dial (not really, but close) at a time. ### Ok, So Why Is That Interesting? There is a project called [trunk-recorder](https://github.com/robotastic/trunk-recorder) that takes advantage of the fact that you can pull in such a wide part of the spectrum. It can do many things, but one of the primary ways you use it is as a police scanner. Yes, police, ambulances, fire trucks, whatever, we're just going to call it a police scanner for sake of conversation. Your city's safety people use radios. Back in the day you could go to the store and buy a scanner to pick up the signals. But your city got bigger and needed more radio channels to communicate. But for sake of argument, your city has a police, fire, public parks, and ambulance service but they only have 3 frequencies total. This is where trunking comes in. You'll have one radio channel that acts as the "trunk". It's a data stream attached to a computer that tells the radios what frequencies to transmit and receive on. The remaining frequencies are split up virtually with the computer and trunk data stream to make more radio space than you had before. In the olde worlde, if you were lucky you could get a radio that would be able to understand the trunk and the other tuner of your radio around to follow one of those service frequencies. When you can capture the entirety of the spectrum with multiple RTL-SDR's, your computer can tune in the trunk and record _every single conversation_ happening on the radio system because it's able to tune in all the frequencies. Granted, you can't listen to them all at once, but at you record them and listen to them at your leisure. In a bigger city like Columbus where I live, this means you can use something like 5-6 RTL-SDR's and record every single radio conversation happening within the entire county. It's not only dispatchers talking to cars, but radio to radio conversations because your radios are able to work on every frequency at once (so to speak). It's like being able to listen to every phone conversation in a county at once. Legal? Yes, these are your airwaves. ### So the KD8ORQ RTL-SDR's. So I have a few of these things. I currently have 3 of them configured with a laptop to have Hilliard and Upper Alrington's services, but the problem is being able to consume and listen to the conversations in a way that matters. I could easily record all the conversations in the county from 100's of public services, but I have no way to make them listenable. This is the current project and hopefully I'll have more to report on this soon. ### My trunk-recorder setup I took some notes on this here: [[Trunk Recorder Server]]. You can also listen to my recorded trunk on OpenMHz: [Central Ohio Interoperable Radio System](https://openmhz.com/system/coirsdwhu)