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FLDigi: Decode HF Digital modes with no cables and maybe no radio

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I heard  a few people talking about a program called FLDigi.  So I thought I would see what it’s all about.

The good news is that FLDigi runs on lots of platforms – FreeBSD™; Linux™, OS X™ and Windows™.  The better news, if you just want to listen/decode the digital modes, is that you might not even need any messy connections between your PC and your radio.  And, if you are really clever, you might not even need a radio.  So, “in the best of all possible worlds”, Dr. Pangloss, you can decode real-time digital activity on the HF bands without even having a radio.  Imagine that!

Note:  This posting is primarily for Amateur Radio folks.  No technical background on digital HF modes is provided

So what is FLDigi?

FLdigi decoding CW from W1AW from a nearly HF Radio though a Laptop built-in microphone

This is from the software developers page

FLDIGI Digital Modes Software

Fldigi is a computer program intended for Amateur Radio Digital Modes operation using a PC (Personal Computer). Fldigi operates (as does most similar software) in conjunction with a conventional HF SSB radio transceiver, and uses the PC sound card as the main means of input from the radio, and output to the radio. These are audio-frequency signals. The software also controls the radio by means of another connection, typically a serial port.

Fldigi is multi-mode, which means that it is able to operate many popular digital modes without switching programs, so you only have one program to learn. Fldigi includes all the popular modes, such as DominoEX, MFSK16, PSK31, and RTTY.

Unusually, Fldigi is available for multiple computer operating systems; FreeBSD™; Linux™, OS X™ and Windows™.

You can get the FLdigi beginners guide and the software at the links below:

Beginners Guide –
Download Page –

Look Ma, no cables

I am going to keep this posting short.  I installed FLDigi on a Windows 7 machine and it started up without any problem.  When I strated thinking about cables to connect the output of my radio to the input of the sound card on my PC I thought I might try a shortcut.  The laptop that I used to install FLDigi has a built-in microphone.  What if I just tuned my HF radio to a digital station and just let the built-in microphone on the laptop listen to the audio from my nearby radio?  Well, it worked like a champ.

The image above shows FLDigi decoding CW from W1AW from the audio from the speaker of my TS-950 (without cables) into the build-in Mic of the Laptop.  The full setup is at the top of this posting.

Look Ma, no radio

So how lazy can one get?  How about if you don’t have an HF Radio?  Can you still use FLDigi to decode digital modes in real time from the HF bands if you don’t use or have a radio?

How about using a web-based Software Defined Radio and feed the audio of the SDR into FLDigi?  Actually, it’s possible.  The Windows operating system has the capability to route audio output of one application into the audio input of another application.  I gave this a try and had some limited success.

Note: See the image to the right.  This is Control Panels -> Sound.  On your PC you might have to “show disabled devices” and then enable the internal “Wave out mix” device.  The end effect is that audio from the Web-based Software defined radio will be fed into FLDigi.  So you are using a remote radio across the internet to feed audio signals into FLDigi on your PC on your desk.  Slick! 

If this does not work for you, then check another alternative at the end of this posting.

The Take

Digital modes on HF have been around for a very long time.  CW on HF has been around since the dawn of (radio) time – 100+ years.  Before computers had good digital sound cards and fast CPU’s you needed an external device to decode digital modes and decode CW.  A popular device was the PK-232.  With DSP-based sound cards and fast CPU’s you can do it all on your PC without any external hardware.

FLDigi is nothing new.  It does work and it’s fun to play with – especially with my accidental discovery that a simple built-in mic on a laptop as input to FLDigi is good enough to decode a strong audio signal from a nearby radio.  Of course, you would want to use a physical cable connection if you are really serious about using FLDigi for the long haul.  Another plus is that FLDigi runs on a variety of platforms including Mac OS X and Linux.  So you non-Windows folks – there you go.

Give it a try – it’s free.  Have fun.


Get a link to web-based Software Defined Radios here –

If you can’t get the Windows WaveOutMix to work then you might try this alternative – simple.  Just enable your built-in mic and let your mic listen to your PC speakers.  The picture below shows two applications running.  In one window is Web SDR that is tuned to a 40m RTTY station and playing through the laptop speakers (the window on the bottom).  In the other windows (top)  is FLDigi.  FLDigi  is listening on the built-in microphone and decoding the RTTY signal from Web SDR.  This actually works and works quite well.

Written by frrl

August 20, 2011 at 3:07 pm

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