Getting Started with FLDIGI

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FLDIGI is a digital modem program that permits the transmission of digital information through an amateur radio. Digital transmissions in amateur radio are restricted to the voice channels set aside by the ITU. To accomplish this, audio tones are used to represent bit patterns ("1" and "0"). Many audio tone encoding methods exist that are used in HF, VHF, and UHF transmissions and each of these encoding methods vary in complexity, noise tolerance, and error correction techniques. FLDIGI employs "software modems" to achieve the necessary audio encoding necessary for the various modes. FLDIGI has a very large selection of modes that it can encode/decode, thus, to encompass the setup of every mode available in FLDIGI is beyond the scope of this article. What this article will do is pick a single digital modem as an example, and go through the process of buidling a full working setup. This article will consist of four sections. They are:

  • Downloading and Installing FLDIGI
  • Initial station setup
  • Testing of Encoding/Decoding for a particular software modem
  • Interfacing of the computer to the amateur radio

The digital mode (software modem) that will be covered is MT63-2000 - a mode that finds use in ARES in the US and a recommended mode to be used when using forms from the NBEMS (Narrow Band Emergency Management System) section of FLDIGI.

Downloading and Installing FLDIGI

FLDIGI can be found here [1]. The software is available for Windows, Linux, and Mac OSX. Download the version that applies to you. Linux distros may also have versions in their repositories. Installation is straightforward.

Initial Station Setup

When you start FLDIGI for the first time, you will be prompted to include basic information about
 FLDIGI - Operator Info
FLDIGI - Operator Info
yourself and your station location. Go ahead and enter your callsign, name, QTH (location), and other details that may be asked of you.

Modem Setup

In this example, we will be setting up the software modem MT63-2000 - a noise tolerant encoding scheme recommended for emergency communications.

Under the Configure->Modems option, click on the MT63 tab and insure that the following settings are made:
 MT63 - Modem Settings
MT63 - Modem Settings
  • 64-bit (long) interleave (checked ON)
  • 8-bit extended characters (UTF-8) (checked ON)
  • long receive integration (checked OFF)
  • Transmit lower start tone (checked ON)
  • Transmit upper start tone (checked ON)
  • Tone Duration (secs) (Set 4)
  • Allow manual tuning (checked OFF)

You may refer to the thumbnail on the right.

Audio Setup

For the purposes of initial setup, we are going to allow the computer speakers and microphone to be the devices that we will use to transmit and receive signals.
 Audio Settings
Audio Settings
A recorded file of an MT63-2000 transmission will be used to test the software modem receive ability. Refer to the thumbnail image for the necessary port settings. The devices in Mac OSX and Linux will be shown differently.

Testing of Encoding/Decoding for the particular software modem

As stated before, audio tones are used to encode digital information, and an amateur radio is used to transmit these audio tones over RF. We can test a Digital HF station before transmission by using the speakers and microphone as our transmit and receive paths respectively.


To see what happens with encoding, you can use one of the transmission buttons. Before doing so, insure that your speakers are set low - as the MT63 encoding scheme is complex - thus the sound generated is harsh - similar to a "buzz saw". Click on the "CQ >|" button. If you do so, FLDIGI will start a transmission sequence using your call sign. If your call sign is VA3ZZZ, the transmission sequence is:



You will hear a high pitch come from your speakers announcing the beginning of transmission, then the "buzz saw" sound representing the text shown above.


To test the decoding of your setup, play the following audio file Media:Mt63-2000-va3por.ogg on your computer. What you should see is "MT63 2K" appear in your waterfall as the first tones are playing, then when the "buzz saw" sound kicks up, you should see the following sequence:



If you see the above text, that means that the MT63-2000 modem has been properly set up!


Interfacing of the computer to the amateur radio

Interfacing your computer to your radio can be done in one of two ways - loose coupling and direct coupling.

Loose Coupling

Loose coupling essentially is using FLDIGI without a cable at all. With this method, you simply make use of the radio microphone and speaker and the microphone and speaker of your computer.

Thus, if you wish to transmit, you simply turn the volume up on your computer speakers and send your message. The computer will generate the necessary tones/sounds representing the data and you place your radio microphone above the computer speakers with the push-to-talk (PTT) depressed. To receive, you simply place the radio speaker close to the computer microphone. This method has the advantage of a very simple, totally ad-hoc, on-demand setup. The great disadvantage is that if any background noise other than the data audio stream can corrupt transmission. To help mitigate background noise, noise tolerant modes like MT63 can be used in open field conditions.

Direct Coupling

Direct Coupling involves coupling the computer's soundcard device to audio inputs/outputs of the radio. How to do this totally depends on the make and model or radio. The manufacturer's owner's manual must be consulted to properly create the interface. Many radios have CAT ports that allow the interfacing of a computer sound card to the radio with PTT control. USB ports are also being introduced to allow for full sound and data control. One example is the Icom 7200 which simply requires a single, off-the-shelf USB cable to achieve full connectivity. THe advantage of this setup is that it removes the background audio noise as a factor, and full control of the radio can be perfomed on the computer - which can simplify radio operations for novice operators.