Fldigi on Linux — Complete Setup and Operating Guide

Live interface

Fldigi is running on this server as a systemd service. The live web interface shows the real-time waterfall from the server's connected radio and allows browser-based interaction with the modem.

What is Fldigi and why use it

Fldigi (Fast Light Digital modem Application) has been the standard Linux digital modes program since 2007. Written by Dave Freese W1HKJ and a team of contributors, it handles the audio signal processing for over 20 digital modes — receiving and decoding signals from your radio's audio output and generating audio tones for transmission through your radio's microphone or data input.

The core of Fldigi is a live waterfall display showing the received audio spectrum. Signals appear as colored vertical lines or patterns on the waterfall — you click on a signal to tune to it and Fldigi begins decoding it automatically. The decoded text appears in the receive window in real time, making it immediately obvious what mode a signal is using and what it contains.

Fldigi is the right choice for operators who want to work PSK31, RTTY, CW, Olivia, MFSK, and dozens of other traditional digital modes that predate FT8. It also has a built-in web server making it suitable for remote operation and the live interface running on this server.

Installation on Ubuntu and Debian

Ubuntu 22.04, 24.04 and Debian 11, 12

Fldigi is in the standard repositories for all current Ubuntu and Debian releases. Install it with:

This installs Fldigi plus two companion tools: Flrig (rig control front-end) and Flarq (ARQ file transfer). All three are useful for a complete digital mode station.

Install the latest version from source

The repository version may lag the latest release by several months. If you need recent bug fixes or features, build from source:

Replace the version number with the latest from the Fldigi website at w1hkj.com. The build takes 3–5 minutes on a modern machine.

Verify the installation

This prints the installed version. If you get "command not found" the installation did not complete successfully — check for error messages in the build output.

Fedora and other RPM-based distributions

Raspberry Pi OS

Fldigi installs and runs well on Raspberry Pi 3 and 4. Use the standard apt install method above — the arm64 packages are in the Raspberry Pi OS repositories. Performance is good for all modes at 48000 Hz sample rate.

Sound card setup and audio wiring

Getting the audio interface right is the most important part of setting up Fldigi. Poor audio levels, ground loops, and wrong device selection cause the majority of problems new users encounter.

What audio interface do you need

You need a way to get audio from your radio to your computer's input and from your computer's output to your radio's microphone or data input. Three common approaches:

• USB audio interface (recommended) — a dedicated USB sound card like the Signalink USB, Masters Communications DRA series, or a generic CM108-based dongle. Avoids ground loops, keeps station audio separate from computer audio, plug-and-play on Linux.
• Built-in sound card — works if your radio and computer share a common ground (same AC outlet). Prone to ground loop hum. Use the line-in jack, not the microphone jack.
• Radio's built-in USB audio — modern radios like the IC-7300, IC-7610, and Yaesu FTDX-10 have built-in USB audio. Linux sees them as a standard USB sound card. No additional hardware needed.

Wiring a basic radio-to-computer interface

The basic wiring connects four signals between radio and computer:

• Radio audio output → Computer line-in — the received signal for Fldigi to decode
• Computer audio output → Radio microphone or data input — the transmit audio from Fldigi
• PTT line → Radio PTT input — keys the transmitter when sending
• Ground — common ground between radio and computer

Configuring audio in Fldigi

Go to Configure → Sound Card → Audio → Devices and configure these settings:

• Capture (input) — select your USB audio interface or radio's USB audio device
• Playback (output) — select the same device
• Sample rate — set to 48000 Hz for best performance
• Channels — Mono for most setups, Stereo only if using dual-channel operation

Finding your audio device name

The device in Fldigi's dropdown will appear as "USB Audio Device" or similar. If you have multiple USB audio interfaces connected they will all appear — select the one connected to your radio.

Setting audio levels correctly

Correct audio levels are critical for clean transmission. Use alsamixer or your system's volume control to set levels:

• Receive level — the waterfall should show colored signals without clipping. Watch the Fldigi signal level meter — peaks should reach 50–75% of full scale during normal signals
• Transmit level — the ALC meter on your radio should show minimal deflection. Any ALC action means you are overdriving the audio and causing splatter on adjacent frequencies

PulseAudio and PipeWire

On modern Ubuntu systems audio goes through PulseAudio or PipeWire rather than directly to ALSA. In Fldigi select the PulseAudio device in the sound card settings rather than the ALSA hardware device directly. This avoids conflicts with other applications using the same audio interface.

Rig control — Hamlib and Flrig

Rig control lets Fldigi read your radio's current frequency and mode, and change them automatically when you switch modes. It also controls PTT (push to talk) to key the transmitter when sending.

Method 1 — Via Flrig (recommended)

Flrig is a separate rig control application that acts as a broker between your radio and any software that needs CAT control. Run Flrig as a daemon, then connect Fldigi, CQRLog, and any other applications to Flrig. This is cleaner than having each application fight over the serial port directly.

In Flrig select your radio model and serial port, click Connect, and verify it reads the radio's frequency. Then in Fldigi go to Configure → Rig Control → Flrig and enable "Use Flrig for CAT". Fldigi connects to Flrig on port 12345 automatically.

Method 2 — Hamlib direct connection

Configure Hamlib directly in Fldigi at Configure → Rig Control → Hamlib:

• Rig — select your radio model from the dropdown
• Device — your serial port, usually /dev/ttyUSB0
• Baud rate — must match your radio's CAT baud rate setting
• PTT via — RTS, DTR, or Hamlib depending on your setup

Serial port permissions

Linux requires your user to be in the dialout group to access serial ports:

Common rig settings for popular radios

PTT control options

Understanding the Fldigi interface

The Fldigi interface has several distinct panels that work together. Understanding what each one does makes operating much more intuitive.

The waterfall display

The waterfall is the heart of Fldigi. It shows a scrolling spectrogram of the received audio — frequency on the horizontal axis, time moving downward, and signal strength shown as color (dark blue = weak, bright yellow/white = strong). Signals appear as characteristic patterns:

• PSK31 — thin vertical lines, very narrow bandwidth
• RTTY — two parallel vertical lines separated by 170 Hz
• Olivia — wide smeared band of multiple tones
• CW — single vertical line with on/off keying pattern
• MFSK — staircase pattern of tones sweeping across bandwidth

Click anywhere on the waterfall to move the tuning marker (the red triangle at the top) to that frequency. Fldigi immediately begins trying to decode whatever is at that frequency.

Receive and transmit text windows

The upper text area shows received text — everything decoded from the selected signal appears here in real time as it comes in. The lower text area is where you type your transmitted text. Text you type appears in the transmit window and is sent when you click TX or press the assigned function key.

Mode and frequency indicator

The bottom status bar shows the current mode, the audio frequency offset within the passband, your radio's frequency (if rig control is active), and signal quality indicators. The mode button in the bottom-left opens a quick mode selection menu.

Signal and power meters

The S-meter shows received signal strength. The power meter shows transmitted audio level — watch this to avoid overdriving your radio's input. Both update in real time during receive and transmit.

Digital modes — PSK31, RTTY, CW and more

Fldigi supports over 20 digital modes. Here are the most commonly used ones with their characteristics and typical frequencies.

Operating guide — making contacts

Making your first PSK31 contact — step by step

1. Tune your radio to 14.070 MHz USB — the main 20m PSK31 frequency
2. In Fldigi select Op Mode → PSK → BPSK-31
3. Click RX (or press F6) to start receiving
4. The waterfall fills with narrow vertical lines — each one is a PSK31 station
5. Click on any active signal — the red marker moves there and decoded text appears in the receive window
6. Read the text until you see a station calling CQ ("CQ CQ CQ de W1XYZ W1XYZ pse k")
7. Note the station's callsign — click it in the receive window to auto-fill the log fields
8. Click TX to transmit your reply — use your pre-configured reply macro
9. Exchange signal reports, names, and QTH, then send your 73 macro to close
10. Log the contact and return to receive

Calling CQ on PSK31

To call CQ yourself rather than answering one, find a clear spot on the waterfall — a gap between existing signals — and position your tuning marker there. Click TX and send your CQ macro. Listen for replies for 30–60 seconds after each CQ. If no reply try a different part of the waterfall or a different time of day.

Working RTTY

RTTY is popular during contests and with DX expedition stations. Switch to Op Mode → RTTY and tune to 14.080 MHz. RTTY signals appear as two parallel lines on the waterfall — click between them to tune. The Mark/Space indicator at the bottom of Fldigi shows which line is which.

For RTTY contests the exchange is typically callsign, RST, and serial number (or state/province for domestic contests). The pace is faster than PSK31 — many contest stations run at 60 wpm or more. Use macros for the exchange to keep up.

Weak signal modes — Olivia and MFSK

Olivia is the best choice when the band is marginal and PSK31 is failing. The most common variant is Olivia 8/500 on 14.106 MHz. Olivia is significantly slower than PSK31 but can decode correctly at signal levels where PSK31 produces nothing but garbage. Switch to it when conditions are poor or when working stations at the edge of your propagation path.

CW with Fldigi

Fldigi includes a CW decoder that reads Morse code from the audio. It works reasonably well on clean machine-generated CW but struggles with hand-sent CW at irregular speeds. For sending, Fldigi generates CW audio tones at your configured WPM speed — type your message and Fldigi encodes it as CW and keys the transmitter. Use the WPM control to set your sending speed.

Understanding signal reports

The standard RST report used in digital modes is typically 599 regardless of actual signal quality — it is a conventional exchange rather than a meaningful assessment. In PSK31 ragchews you will often give a more honest report or describe conditions in plain text. In RTTY contests 599 is always exchanged as the standard.

Completing a QSO efficiently

A well-run PSK31 QSO follows a predictable structure that both operators recognize:

• CQ call → response → signal report exchange
• Name and QTH exchange
• Rig and antenna exchange (optional, common in ragchews)
• 73 and sign-off

Keep macros ready for each stage. Many operators have 8–12 macros covering every stage of a typical QSO so they rarely need to type during a contact.

Macros and automation

Macros are the key to efficient operating in Fldigi. They let you send pre-written text blocks with a single keypress, with variable substitution filling in callsigns, RST, and other values automatically.

Macro variables

Example macro set for PSK31

Accessing and editing macros

Macros are stored in sets of 12, assignable to function keys F1–F12. Access them via Configure → Macros. You can have multiple macro sets saved and switch between them — useful for having one set for PSK31 ragchews and another for RTTY contesting.

Logging contacts and LoTW upload

Fldigi has a built-in logbook that records all your contacts. Access it via View → Logbook.

Logging a contact

When you click a callsign in the receive window Fldigi pre-fills the log entry fields. After the QSO completes, verify the callsign, RST sent and received, and mode are correct, then click Save. The contact is added to your Fldigi log and optionally exported to ADIF.

ADIF export for LoTW and other loggers

Fldigi saves contacts to an ADIF file at ~/.fldigi/logbook.adif. Upload this to LoTW via TQSL:

Replace N0CALL with your callsign. TQSL signs the log and uploads it to LoTW automatically.

Sending contacts to CQRLog automatically

Enable real-time logging to CQRLog in Fldigi's settings — completed contacts appear in CQRLog as they are logged, with no manual export/import needed. See the Integration section below for setup details.

Running headless on a server

Fldigi can run without a display using Xvfb (virtual framebuffer). This is how the live interface on this site works — Fldigi runs as a background service and its built-in web server exposes the waterfall and controls via a browser.

Install Xvfb

Systemd service for headless Fldigi

Nginx proxy for web access

Fldigi's built-in web server runs on port 7362. Add this to your nginx config to proxy it at a clean URL:

Hardware compatibility

Radios with built-in USB audio (no external interface needed)

Popular external USB audio interfaces

Integration with CQRLog and WSJT-X

CQRLog integration

CQRLog can receive QSO data directly from Fldigi in real time. In CQRLog go to Settings → TRX Control → Fldigi and enable the connection on port 7362. When you save a contact in Fldigi it appears in CQRLog automatically — no manual export needed.

With rig control via Flrig shared between both applications, CQRLog also reads your frequency and mode from the same Flrig instance that Fldigi uses. This gives you a fully integrated station where your logger, digital mode software, and rig all talk to each other.

WSJT-X alongside Fldigi

Fldigi and WSJT-X both need exclusive access to the sound card, so they cannot run simultaneously on the same audio device. Use separate USB audio interfaces — one for Fldigi, one for WSJT-X — if you want both available at once. Alternatively close one before opening the other.

Rig control can be shared via Flrig — both Fldigi and WSJT-X connect to the same Flrig instance. Only one can transmit at a time but both can read frequency and mode simultaneously.

Flarq ARQ file transfer

Flarq extends Fldigi with ARQ (Automatic Repeat reQuest) error-correcting file transfer over HF radio. Install it with sudo apt install flarq. Flarq connects to Fldigi on port 7322 and adds reliable binary file transfer capability on top of the existing audio interface. Useful for sending images, documents, and other files over HF when internet is unavailable.

Troubleshooting common problems

No audio input — waterfall is flat or very dark

• Check that the correct audio device is selected in Configure → Sound Card → Audio → Devices → Capture
• Run arecord -l in a terminal to list available capture devices and verify yours is present
• Open alsamixer and check that the capture volume is not muted or at zero
• Check that your radio's audio output is connected to the correct input on your interface
• Verify the radio's audio output level — some radios need the AF gain turned up for the data output

Waterfall shows activity but nothing decodes

• Make sure you are in the right mode — PSK31 signals on the RTTY decoder produce garbage
• Click directly on the center of the signal, not to the side of it
• Check that AFC (Automatic Frequency Control) is enabled — it helps lock onto drifting signals
• The signal may be too weak to decode reliably — try a stronger signal on the waterfall
• Verify your sample rate is set to 48000 Hz — mismatched sample rates cause distorted decodes

Fldigi won't key the radio (PTT not working)

• Check that your user is in the dialout group: groups should include dialout
• Verify the correct serial port is selected in the PTT settings
• Try the RTS line instead of DTR or vice versa — some interfaces use different pins
• If using VOX, check that your radio's VOX sensitivity is set high enough to trigger on the audio level Fldigi generates
• Use ls -la /dev/ttyUSB* to verify the port exists and check its permissions

Transmitted signal is distorted or causing splatter

• Reduce the transmit audio level — the ALC on your radio should show little or no deflection
• Check for ground loops — a hum in the audio causes intermodulation in the transmitted signal
• Make sure your radio is in the correct mode — digital modes should use USB or USB-D/DATA mode, not FM
• Disable any speech processing or compression on your radio — these are designed for voice and distort digital signals badly
• Check that your radio's microphone equalizer is bypassed when using the data port

Hum or noise on received audio

• Ground loop — use audio isolation transformers on the audio cables between radio and computer
• Switch to a USB audio interface with optical isolation like the Signalink
• Make sure the radio and computer are on the same AC circuit and share a common ground
• Keep audio cables short and away from power cables and antenna feedlines

Rig control not working — can't connect to radio

• Check that the serial port exists: ls /dev/ttyUSB*
• Verify you are in the dialout group: groups | grep dialout
• Check the baud rate matches your radio's CAT baud rate setting exactly
• Try a different Hamlib rig model number — some radios have multiple valid entries
• Close any other application that might be using the same serial port
• Try rigctl -m [model] -r /dev/ttyUSB0 -s [baud] f to test Hamlib independently

Fldigi crashes on startup

• Delete the Fldigi configuration directory and start fresh: rm -rf ~/.fldigi
• Check for missing audio library dependencies: ldd $(which fldigi) | grep "not found"
• Try running from a terminal to see the error message before the crash
• Check available disk space — Fldigi can crash if it cannot write to its log files

Frequently asked questions

Related tools