Fldigi and amateur radio on Linux


Author: Joe Barr

Dave Freese has just released version 1.2 of Fldigi, a popular new program for Linux and FreeBSD which enables amateur radio operators to join their radios and their computers at the hip and create a new kind of ham shack: a digital ham shack. Here’s the story behind both the rising popularity of “sound card” digital modes in amateur radio and how Fldigi lets you enjoy enjoy them on Linux.Hams use their own jargon, which is just as obscure to non-hams as IRQ, USB, or DDR are to non-computer geeks. I’ve included a brief glossary of terms used in this story, but for the inquisitve, here is a link to a more complete reference work explaining ham jargon.

According to Wikipedia, there are about 3 million amateur radio operators — or hams — around the world. There have been amateur radio operators for as long as there has been radio. The ARRL, which is the national association for amateur radio in the United States, was founded in 1914 and counts 150,000 active members today.

Hams communicate by radio, but only on government-allocated frequency bands where they are allowed to operate. They chat, they invent, they have contests to see how many stations they can contact within a given time frame, and when natural or man-made disasters occur, they are usually the first to establish any kind of communications between the stricken area and the rest of the world.

Early hams used Morse code sent by CW almost exclusively. Over time, the popularity of morse code — also called simply code or CW — has waned and interest in voice communications — an analog mode as opposed to the on/off digital of CW or RTTY (radio teletype) — has grown. Learning Morse code and passing a test on it has long been a barrier to entry for new hams. You could get a beginner’s license without passing the code test, but you couldn’t progress to the next higher level without it. The FCC has recently decided to do away with the code test, and beginning in February of 2007, it will no longer be required.

In 1999, a new kind of digital mode appeared. It’s called PSK, for Phase Shift Keying, and it’s known as a “sound card” digital mode because it uses a computer’s sound card to encode and to decode the transmitted signal. Older digital modes like RTTY have been revitalized as “sound card” modes, and completely new modes like PSK and MFSK have appeared. The new modes are proving as popular with long time hams as they are with new hams, like me.

There are several reasons why “sound card” digital modes are growing in popularity. First of all, other than the original equipment setup, using these modes is as easy as typing at your keyboard and reading from your computer screen. They are the amateur radio HF equivalents of instant messaging.

PSK in particular offers further advantages. You don’t have to compete with expensive rigs generating megawatts of power to do global QSOs with PSK, for example. Even with my modest setup, running between 25 and 50 watts of power into a ground-mounted vertical antenna, I’ve enjoyed PSK QSOs with Cuba, South America, and Burkina Faso in Central Africa.

Meet Dave Freese, W1HKJ

Dave Freese retired from the Coast Guard in 1980. He is a graduate of both the Coast Guard Academy (BS-1960) and the Navy Postgraduate school (MSEE-1966). He has been a licensed ham since 1957. He started his programming career in 1966 and has worked on everything from “I have programmed everything from big IBM iron to embedded microprocessors. The applications range from machine control to business management and nearly everything in

He’s been a Linux user almost from its inception, using SCO Unix and Minix before starting to use Linux. He started using sound card digital modes in 2001, and has written more than 20 GPL licensed programs over the years. I asked why he chooses to release his software under the GPL and he said:

I have no need for additional income and I like the concept of shared knowledge. Some of my work is original, but much depends on the generosity of others such as the author of gmfsk. The greatest flattery one can receive in the GPL world is for someone to take your
code and build a better application or perhaps apply your ideas in a new
and unique way. GPL is not for everyone though as you do expose yourself to a very wide audience of very skilled programmers. You need to realize that there is no criticism only critical analysis when you get feedback that is not flattering. But that is why the GPL code
continues to get better all the time…

At present there are 11 programs offered on my web site, all GPL. Prior to Linux development I wrote applications for DOS and Windows. I made a half hearted attempt at the shareware world, but turned everything into give away software after a few months. fldigi has been the most intense and took place over the last 9 months. It has been intense for the dedicated crew of testers from all over the globe. They allowed me to test their operating systems, their computers and sometimes their radio equipment. Along the way we have become very good friends.

Another plus for PSK is its narrow bandwidth, which means that more stations can share the same band without interfering with each other’s ability to send and receive distant signals. Unless, of course, you are over-driving your signal. That’s the number one sin of new PSK users, myself included: driving the output signal way past what’s required.

I saw an IRC discussion about lower-case PSK being faster than upper-case, and I immediately thought someone was trying to pull my leg. I’ve often joked about lower case taking up less space on disk drives, or being faster over Ethernet because of lower wind resistance. But in the case of PSK, it’s not a joke. It’s true. That’s because PSK, like Morse code, uses variable length character encoding. The lower case characters require fewer baud — think bits — and therefore don’t take as long to send.

One of the hams I interviewed for this story, Dick Swan, K4FAZ, has been a ham for over 70 years. That’s not a typo. He was first licensed in 1936, and he is still going strong. Dick told me he got interested in amateur radio at the age of 12, when he was playing hide and seek, and an amateur radio operator found him peeking in a basement window while he was playing the game.

My interview with Dick began during a PSK QSO on the 20 meter band, but had to be finished by email because of QRM.

Dick told me, “The ham who grabbed me by the collar turned out to be my mentor and I got a Class C license when I was 14 years old. My first rig was a single 6L6 oscillator and of course on CW.” He kept building rigs over the years, added phone (voice) to his capabilities, and upgraded to a Class A license. As for the digital modes, he said “I started on RTTY in the 70s and then in the 80’s was on PACTOR and AMTOR and packet in the 90’s. Have been on PSK since about 2001. I love the digital modes although I have just been active using the computer and sound card for about 5 years.”

Equipment needed

If all you want to do is to listen in — to read what others are saying — all you need is audio cable running from your HF radio to the sound card on your PC. But if you want to both send and receive, you’ll need the proper license and:

  • An HF transceiver which allows computer control
  • A computer with serial port and sound card
  • Software like Fldigi to encode and decode the signals
  • An audio cable from your transceiver audio output to your PC’s sound card line-in
  • An audio cable from your PC’s sound card speaker output to your transceiver’s microphone input
  • A cable from your PC’s serial port to your transceiver’s computer control port

The specifics for those last two items are rig dependent, varying not just by manufacturer, but possibly model by model as well. If you’re handy with cables and solder, they are easy to make once you have the correct connectors and know the pin-outs required. If you’re like me, and choose to remain solder-free, you can get those made for you at practically any Ham store.

Also note that on my Kenwood TS-450SAT, and many other radios, an additional piece of equipment is needed: an interface to convert the serial signal into meaningful instructions to the transceiver, and vice versa. There are a number of firms out there making interface devices for virtually every brand and model of transceiver.

Behind the waterfall

Click to enlarge

The Fldigi UI might seem complex at first blush, but when you look more closely it’s quite easy to grok. There are three main sections to the UI. The top section — not counting the command bar — is all about logging contacts.

Keeping logs is very important to Hams, and Fldigi makes it easy to automate the process. Type in the call sign of the station you’re working, for example, then click on QRZ. If you’re connected to the Internet, Fldigi will query the QRZ.com website and fill in the details for the call sign based on what it finds there. You can also configure Fldigi to work from a CD version of the QRZ database if you prefer.

The center section, made up of two scrollable panes with a white background, shows the traffic sent and received during the session. Immediately below the text panes is a row of programmable macro-driven buttons which send pre-entered text when you click them. That way you won’t suffer repetitive stress injury while sending CQ or bragging about your station particulars.

The bottom section is the waterfall, where all the action is, at least in terms of moving lights and active displays. The “waterfall” extends nearly all the way across the window. Next to it, immediately to the right, is the digiscope. Beneath them both are controls for tweaking their displays.

I used both Fldigi version 1.11 and several pre-release versions of 1.2 (with Hamlib version 1.2.6) while working on this story. Fldigi 1.2 has now been released. Like many other Linux programs, Fldigi has traditionally depended upon the Hamlib project in order to converse with and control the radio. Beginning with the 1.2 release, Fldigi offers you the choice of using the Hamlib interface or its own native interface.

Setting it up

Before using Fldigi for the first time, you’ll need to configure it for the make and model radio you’re using, the serial port you’re using to communicate with it, and information, the device name for your sound card, and personal information like your call sign and location.

If you’re using 1.11 or earlier, you’ll need to create a .fldigi subdirectory in your home directory first, so Fldigi will have a spot to save its log and configuration files. Version 1.2 will create it for you if you don’t have one.

Obviously, you’ll need to have all the requisite cables correctly connected to your rig. Power on your radio first, then fire up Fldigi. Now you can click on Configure and tend to those chores before proceeding.

Each time you start Fldigi, first time or not, you’ll need to select the Interface tab from the Configure option and initialize the interface so that the two devices — the rig and your PC — can have their conversation.

Don’t be a lid: tune your station!

Before you start sending on the air, however, you have one more thing to do. Find an open area of the band of your choice and tune your station: the sound-card and the rig, so that you are transmitting a clean signal at the lowest possible power required to communicate. The number one sin of new sound-card digital mode operators is over-driving the signal and polluting the band with interference.

Some experienced ops I have met tell me they have found 25 watts of power is their sweet spot. That may or may not be true for you, but sending at 50 watts or lower should be your guideline. More than that will generate QRM in the form of side-bands and harmonics, which cause needless interference and hurt others’ ability to communicate. It will also earn you a reputation as a “lid,” which is ham-speak for lamer.

Proper tuning involves setting three different output levels: the audio output from your sound card, the mic gain on your transceiver, and the power used to transmit the resulting signal. The Fldigi help file, included in your download of Fldigi, explains in great detail how to tune your sound-card output. Read it and heed it.

When you can transmit the tuning carrier at 50 watts or lower, without ALC deflection, you’re ready for the next step, which is to actually send a CQ or TEST signal. You may need to further tweak your mic gain and/or power when transmitting data, so keep an eye on your rig’s meters and adjust accordingly when you transmit.

Three popular modes

Click to enlarge

By default, Fldigi starts up in PSK31, which operates at 31 baud per second. That’s a comfortable speed for handling keyboard input, which is what it is asked to do. There are several other types of PSK, some of which are faster, some more robust. But PSK31 is far and away the most widely used.

The waterfall display normally shows a bandwidth of approximately 3,000 hertz. On the 40 meter band, for example, it might show signals from 7070 to 7073 kilohertz. A well-tuned PSK31 signal spans 60 Hz, so quite a few signals may appear in the waterfall at any given time. When you move the mouse in the waterfall, a pair of yellow lines moves with you. When those yellow lines are directly over a signal you’re interested in reading, a click of the mouse changes the yellow lines to red. The red lines indicate the portion of the spectrum that Fldigi is decoding and displaying the results for in the upper text area. If you move the mouse around, the yellow lines still follow, but Fldigi is only listening to the signal between the red lines.

The space between the lines widens and narrows as you change operating modes. Select RTTY and it spreads to 170Hz, MFSK16 is wider still.

The image to the right shows a PSK and an MFSK16 signal nearly side by side, so it’s easy to see the difference in bandwidth between the two modes.

The sound of each mode is unique, too, and your ears quickly become accustomed to distinguishing between them. That’s especially true of the MFSK16 mode, which sounds like a snake charmer flutist recorded at 33 RPM being played on an accordion at 45.

PSK, RTTY, and MFSK are probably the top three in terms of popularity of all the sound-card digital modes, but there are a number of others, and new ones will probably appear in the future. These three are good for live conversations at the keyboard. MFSK16 can transmit images as well as text. None of these three offer error detection or correction, so QSOs — like the interview with Dick Swan for this article — are subject to disruption by man-made or natural interference.


I’ve been using Fldigi for a couple of weeks now and have barely scratched the surface of its capabilities. I’ve tried RTTY and MFSK16 in addition to PSK, but there are so many others: DominoEX, Feld-Hell, Olivia, and Throb, to name a few of the more exotic-sounding. And did I mention that you can copy CW with Fldigi, as well?

This is an excellent piece of work by Dave and his team of testers, and they can always use more help to make sure it works on even more rigs. If you’re a licensed ham, and want a break from your phone or speed key, give Fldigi and the sound card digital modes a try. Maybe you can provide an XML file defining the characteristics for your rig so others with the same radio can benefit from your contribution. But be careful: PSK on Fldigi is as addictive as chocolate crack.


  • News