The full name of the device is “ ma++ ingalls for Cycling '74: Soundflower (2ch)“ In the screenshot, the audio device I have selected is Soundflower. Select the FM modem in the upper left panel, and then, under FM Settings, choose the audio router device that you installed in Step 1.I made a bookmark in the lower left panel so that I can instantly jump to the right frequency. Tune the SDR to the national APRS frequency, 144.39 MHz.When CubicSDR starts, choose the RTL-SDR and click Start.GNU Radio – it should be possible to assemble a flowgraph in GRC that reads data from the SDR, processes the signal with a FM demodulator block, and sends the output to an audio device.įor the purposes of this post, I’ll be using CubicSDR, because I found it to be the easiest to use of all the options.You would need to run rtl_fm and pipe the output to an audio player, like SoX‘s play command We need to do two things to this stream of data before QTH.app can process it: it needs to be converted from I/Q format and processed by an FM demodulator.įortunately, there are a number of applications out there that do this, a subset, of course, that run on macOS: If you’ve ever listened to an APRS packet, you’ll know that it sounds like an old-school modem, because, well, it basically is.Ī Software Defined Radio (SDR) like the RTL-SDR captures a portion of the radio spectrum and sends this data via USB as a sequence of I/Q samples. Here you can see what my Input tab looks like, with both BlackHole and Soundflower installed: Step 2: Converting FM to AudioĪPRS signals are transmitted as AFSK (Audio Frequency-Shift Keying), which means that audio tones are transmitted using FM modulation. When you are done, you will see a new audio device in your System Preferences → Sound, in both the Output and Input tabs.
If you would like a commercially supported option, check out Loopback from Rogue Amoeba.Ĭhoose an option and install it using their instructions. It was originally supported by Rogue Amoeba, but is now in the hands of the original author.
In my experience, Soundflower implements more of the audio driver API so you get less error messages when it comes to setting volume and such, but it has more trouble keeping up with Mac releases in its support history. An audio router is a special macOS audio driver that allows you to pipe the audio output from one application to the audio input of another app.īoth of these function pretty much the same. This is the “secret sauce” to the whole setup. However, you can get a RTL-SDR dongle for around $36, plug it into an available USB port and start receiving radio signals.
To do that properly, you probably will need something like a Mobilinkd TNC3 or a SignaLink USB, both of which cost over $100. Most hams these days have one or more Baofeng radios on hand, and, while you can get your hands on them starting at $35, you would still need a way to interface it to your computer. To transmit, you will likely want to invest a bit more money in a quality radio. I’m going to be focusing on the receive aspect of APRS.
In this post, I’ll show you how to set up what I think might be the least expensive setup for receiving APRS packets over radio, beyond the cost of the Mac itself, of course. One minor goal I had when creating QTH.app was to make it inexpensive and easy to get started with APRS on a Mac.