A radio in your smartphone: how a DMR app works

Category: BasicsDifficulty: ★☆☆~7 minutes

You don't have to hold a radio and a hotspot to get on the digital air. A radio app turns an ordinary smartphone into a full member of a DMR network. Let's see how it works under the hood and where such a solution has its limits. For an overview of all the "no radio" approaches, see DMR without a radio; here we cover the app in detail.

What it actually is

It's a software DMR client: the phone connects to the network's server over the internet and behaves like a subscriber. You log in by callsign, pick a talkgroup, press the on-screen PTT button — and talk to the very same group as the "hardware" radios. In principle it's similar to DroidStar and like clients, but it's tied to a specific network and its services.

How the audio flows

The main focus is on the server. The phone's microphone produces ordinary audio (PCM); the app streams it to the server over a secure channel, and the server encodes the stream into AMBE+2 with its vocoder and injects it into the air just like a regular radio. On receive it's the reverse: the air is decoded on the server and arrives at your speaker. That's why the phone needs neither an AMBE chip nor a hotspot — the server does all the heavy lifting.

Why background receive matters A radio listens to the air all the time. So the app doesn't go "deaf" in your pocket, receive is kept alive in the background (a persistent service + network connection): minimize or lock the phone and calls still come through.

What it can do besides PTT

Groups and air. Subscribe to a talkgroup, see the "who's talking" list (Last Heard), switch groups on the fly.
Private calls. Tap a subscriber in your contacts and an individual call by DMR ID goes out, including app-to-app.
DMR-SMS. Text to a group or to one person, just like messages over the radio, only from a convenient keyboard.
Echo test. Say something — and a second later you hear yourself back: a quick way to check that the microphone, network and audio are all fine.

App or a radio with a hotspot

Each approach has its own niche:

The app — an instant start with no cost and no soldering, handy on the road and for getting to know the network. It depends on the internet and doesn't physically go out on the radio air.
A radio + hotspot — real radio air on UHF, the "grown-up" way to operate, but it needs hardware and setup.

In practice people combine them: at home and on the air — a radio through a hotspot; on the move — the app in your pocket. Both are equal subscribers of the same network.

Where to start Install the app, log in by callsign, subscribe to a common group and press PTT — that's a minute's work. After that you can build and connect your own hotspot to get on the real radio air. For how it all ties together, see the overview of how the network is built.