decoding radio digital modes without a radio, using WebSDR and fldigi

I’m interested in decoding radio digital modes without actually having a radio. The idea is that by using a web-based software defined radio (to tune in the signals) and the fldigi program (to decode the signals) that I should be able to “listen” to digital modes and start to explore the digital parts of the amateur and broadcast radio bands.


I think I have all of the components, but I have yet to be able to get everything to work as expected. Here’s what I have:

WebSDR from . This is a software-defined radio tuning to the radio airwaves at the University of Twente in the Netherlands; it runs in your browser. There are 87 of these systems running around the world, listed at

fldigi from . This software for Windows, Linux and OS X, runs on your local system and takes an audio signal and decodes it in one of many digital modes.

Soundflower from . This is a Mac system extension that allows you to redirect the audio output of one program (e.g. your browser) into the audio input of another program (e.g. fldigi). Not strictly necessary because the fldigi can also just decode whatever it hears through the computer’s built in microphone. I picked up the recommendation from an article on HamRadioAndVision.

A number of people have done Youtube videos describing what they are doing to make this work. For example, OZ9AEC has a 6 minute Youtube from 2009 shows reception in single channel and multichannel mode. He doesn’t use Soundflower, just the built in mic of his Mac.

Not described here – and probably what I need to figure out next – are two key bits of information. Where do you tune to pick up digital signals? And, more importantly, which decoder do you use? It’s not hard to pick out which modulation is being used if you have some practice, but as of this writing I couldn’t readily tell you what WEFAX sounds like compared to RTTY. Still sorting through those details before I can claim to have mastered this.

Note also that this should work with a local software-defined radio as well, or even with a “hardware-defined radio” placed near your mic or patched in with a mic cable.


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