How to use this manual

The manual is divided into three parts:

Command names are given as they appear in the English-language interface (Monitor, Setup > Radio, Halt), because that is the reference language of the code. If you run Decodium in Italian, Latvian, Spanish or any of the other available languages, the commands are in exactly the same places, and the translated wording is given wherever it could cause confusion.

A note on feature availability. Decodium is built for Windows, macOS and Linux with different decode backends and libraries. Some features described here may not appear, or may appear disabled, depending on the platform, the radio you have connected and the build options of the package you downloaded. The manual points this out where it matters.

Part I — Install and configure

1. Introduction

1.1 What Decodium 4 is

Decodium 4.0 Core Shannon is a complete digital station for weak-signal modes. It grew out of the WSJT-X / Decodium line and moves it onto a different architecture: a Qt 6 / QML interface, decoders and modulators progressively rewritten in C++, integrated CAT handling with several backends, GPU-accelerated waterfall and panadapter, live map, DX Cluster, PSK Reporter, QSO logging and a persistent decode history in a database.

With Decodium 4 you can:

• receive and transmit in FT8, FT4, FT2, Q65, MSK144, JT65, JT9, JT4, FST4, FST4W and WSPR;
• control the radio over CAT (Hamlib, native backends, Ham Radio Deluxe, OmniRig, TCI);
• run manual QSOs, Auto Sequence, Auto CQ, Multi Answer Mode and DX-Pedition modes;
• send spots and logs to PSK Reporter, QRZ Logbook, Cloudlog, LoTW and external software over UDP, ADIF TCP or N1MM;
• control the station from a tablet or phone through the remote web dashboard.

The name Core Shannon refers to Claude Shannon and to the idea behind the project: treating weak digital communication as a problem of information, synchronisation, robustness and operational reliability — not as an exercise in modulation.

1.2 What has changed since Decodium 3

If you are coming from Decodium 3 "Raptor", three things are substantially different:

1. A new interface. The Qt Widgets interface has been replaced by a QML interface with panels you can dock, undock and rearrange, and with a layout that persists between sessions.
2. A C++ engine. The FTx-family decoders and modulators have been migrated to C++, with comparison tests against the previous routines. Legacy components remain for some specialist VHF/UHF modes.
3. Integrated services. Decode history in SQLite, remote web dashboard, DX Cluster, PWR/SWR/ALC telemetry and station diagnostics are part of the program and need no external tools.

The CHANGELOG.md file in the sources gives the detail release by release.

1.3 FT2 in two lines

FT2 (Fast Track 2) is the digital mode created within this project and certified in ADIF as MODE=MFSK with SUBMODE=FT2.

Chapter 6 is devoted entirely to FT2 and its options.

1.4 Licence and sources

Decodium 4 is distributed under GPL-3.0. The full licence text is in the COPYING file included with the sources and the packages. The source code is public on GitHub, with full commit history and downloadable releases.

2. Requirements and installation

2.1 Requirements

Supported operating systems

• Windows 10 / 11, 64-bit
• macOS on Apple Silicon (and on Intel where the package is available)
• Linux x86_64 and aarch64, as an AppImage

Recommended minimum hardware

• A recent quad-core CPU for operating FT2 with asynchronous decoding
• 4 GB of RAM (8 GB recommended with Live Map and panadapter running)
• A GPU with up-to-date drivers: Decodium uses Qt Quick/RHI and takes advantage of hardware acceleration for the waterfall, panadapter and map
• A sound card, or the radio's USB interface
• An internet connection for NTP, DX Cluster, PSK Reporter, data and program updates

On more modest machines there is Slow-PC mode (section 4.9), which reduces graphics load and interface refresh rate.

2.2 Downloading the package

1. Open the Releases page of the official repository.
2. Download the package for your system: - Windows: x64 installer; - macOS Apple Silicon / Intel: .dmg file; - Linux: AppImage for x86_64 or aarch64.
3. Install the package or make the file executable, then start Decodium.

2.3 Windows

Run the installer and follow the wizard. On first start Windows may ask for network access: you need to allow it if you want NTP, DX Cluster, PSK Reporter, the web dashboard or automatic updates.

The Windows packages are digitally signed; the notes on code signing are in the CODE_SIGNING_POLICY.md file in the sources.

2.4 macOS

If macOS blocks the application because of quarantine, remove the attribute from the installed app:

sudo xattr -r -d com.apple.quarantine /Applications/Decodium.app

The exact application name may differ depending on the package that was built.

macOS asks for microphone permission the first time Decodium opens the audio input: without that permission no signal reaches the decoder.

2.5 Linux (AppImage)

Make the AppImage executable and run it:

chmod +x Decodium*.AppImage
./Decodium*.AppImage

Some distributions need libfuse2, or you can extract the contents manually:

./Decodium*.AppImage --appimage-extract
cd squashfs-root
./AppRun

To access serial ports your user must belong to the right group (dialout on Debian/Ubuntu, uucp on Arch). You need to log in again after adding yourself.

2.6 Updates

Decodium includes an update check (Setup > Advanced > DATA UPDATES). When a newer version is available a window appears with the release notes and a link to the package. Updating does not erase settings, logs or decode history.

3. First-time configuration

This chapter covers the minimum you need in order to make your first QSO. The complete reference for every setting is in chapter 14.

Open the settings with the ⚙ Setup button in the top bar.

3.1 Station — Setup > Station

Fill in the STATION DETAILS section:

The STATION INFO section (Station Name, QTH, Rig Info, Antenna, Power (W), WSPR Power) has no effect on transmission: it feeds the ADIF log and the external services. Filling it in now saves you from incomplete logs later.

3.2 Radio and CAT — Setup > Radio

In the BACKEND CAT section choose how Decodium talks to the radio:

In addition to these, Decodium can work with Ham Radio Deluxe when the radio is already under HRD control.

In the CAT CONTROL section set the serial port, speed, stop bits, handshake and PTT method (CAT, RTS, DTR, VOX). On Linux it is better to use the stable /dev/serial/by-id/... paths rather than /dev/ttyUSB*, which can change at every reboot.

In the SPLIT OPERATION section choose the split mode:

• None — no split, the TX tone stays inside the audio passband;
• Rig — split handled by the radio;
• Fake It — Decodium shifts the VFO frequency to keep the TX tone in a clean part of the filter.

Fake It is the best choice with most modern radios, because it keeps the transmit tone away from the filter edges and reduces transmit distortion.

Verify the link using the DIAGNOSTICS section: you must see the radio's real frequency, band changes must work in both directions, and PTT must put the radio into transmit when you press Tune.

Data modes. Decodium tries to preserve the DATA/PKT modes of Icom, Yaesu and Kenwood radios, so that the radio is not forced onto USB/LSB when the correct setting is DATA-U. If you notice the radio dropping out of data mode when Decodium changes frequency, check the mode configured in this section.

CAT profiles. You can save several CAT configurations as profiles and recall them from the Profile menu. It is the quickest way to switch between two radios, or between your home setup and your portable one.

3.3 Audio — Setup > Audio

In the AUDIO DEVICES section select:

• Input: the device receiving audio from the radio (USB codec or sound card);
• Output: the device sending audio to the radio;
• Channel: Mono, Left, Right or Both, according to your wiring.

In the LEVELS section set the RX and TX levels. Decodium has automatic RX levelling, enabled by default, which protects against clipping without making you chase the slider: if you turn it off, aim for a receive level that keeps the noise floor around 30–50 dB on the indicator.

For transmission the usual digital-mode rule applies: no compression, no processing, ALC at rest. Raise the TX level until you reach the power you want, and no further. Section 16.2 explains how to read the ALC telemetry to check this.

On Windows the audio path is restarted deliberately aggressively after each transmission, to avoid undrained buffers and stuck carriers. If you notice a brief silence after every TX, that mechanism is working correctly.

3.4 Clock synchronisation

For FT8, FT4 and — even more critically — FT2, time synchronisation is the difference between decoding and decoding nothing at all.

Decodium includes:

• an internal NTP client (Setup > Advanced > NTP TIME SYNC);
• the DecoSyncTime panel, which shows the offset, the synchronisation state, the mean DT of the decodes and the decode latency.

Enable Enable NTP and check that the panel reports a synchronised state. A mean DT that is consistently non-zero across many stations points to a local clock problem, not to propagation.

3.5 Operating mode

Choose the mode from the selector in the top bar. For your first QSO:

1. Start with FT8 on a standard frequency, so you can verify audio, CAT and timing under known conditions.
2. Check that decodes appear in Full Spectrum within a period or two.
3. Only then move to FT2, which is far less forgiving of timing and audio configuration errors.

4. The interface

4.1 Window structure

The main Decodium 4 window consists of:

4.2 The top bar

The buttons available in the top bar are configurable (Setup > UI Buttons), both for visibility and for order:

To reorder the buttons: long-press and drag. To go back to the original arrangement use Restore default button order in Setup > UI Buttons.

4.3 Waterfall and panadapter

The waterfall shows how the spectrum evolves over time; the panadapter shows the instantaneous spectrum, with the audio frequency scale along the bottom.

Main operations:

• left click on the waterfall: moves the receive audio frequency;
• click with a modifier (as configured): moves the transmit audio frequency;
• mouse wheel: zoom or pan, depending on the area;
• palettes: selectable and customisable from Setup > Colors > SPECTRUM.

The waterfall can be undocked into its own window, docked again, or minimised. In a compact layout it uses a lower history floor, reducing memory and texture use without losing the visible part.

4.4 Full Spectrum

Full Spectrum lists every decode in the current period. The visible columns are configurable in Setup > Display > DECODES and include, among others, distance (Dist), azimuth (Az) and frequency.

• double click on a row: selects that station as your QSO partner and prepares the messages;
• Clear: empties the list;
• the tail button (Go to the latest decode) returns the view to the bottom when you have scrolled away.

Rows are coloured according to the rules in Setup > Colors > DECODE COLORS: new DXCC, new continent, new CQ or ITU zone, new grid, new callsign, CQ in the message, 73/RR73, already-worked stations (B4), LoTW users and transmitted messages.

4.5 Signal RX

Signal RX is the surgical view: it shows only what concerns the QSO in progress and the stations calling you. During a pile-up this is the panel to watch, because Full Spectrum becomes unreadable.

4.6 TX panel

The TX panel contains:

• messages TX1…TX7, which can be generated and edited;
• the state of the QSO and of the partner;
• the operating controls: Auto (Auto Sequence), Call, Hold, Tune, Halt;
• the TX power and the quick transmit controls.

The order of the TX panel buttons is customisable too, and can be restored with Restore default TX panel order.

Halt (or Esc) stops transmission immediately: this is the command to commit to memory.

4.7 Layout: dock, undock, restore

Every main panel can be:

• undocked into an independent window (Detach), useful with several monitors;
• docked back into the layout (Dock);
• hidden and then restored (Restore …);
• moved between columns by dragging;
• returned to its initial state with Reset Layout.

The layout is saved when you close the program and reloaded at the next start, including undocked windows and their positions.

4.8 Compact mode

Compact mode reduces the height of headers and rows, so more decodes fit on the screen. It is the right choice on laptops and small displays.

4.9 Slow-PC mode

Slow-PC mode reduces interface and graphics load: lower waterfall refresh rate, fewer animations, less work in the graphics thread. Enable it if you notice stutter in the interface or delays in decoding on older machines: in FT2 a struggling interface can make you miss the transmit slot.

4.10 Themes and appearance

In Setup > Display > ASPETTO / TEMA you can choose the theme (including the dark theme), the interface colours and the fonts. Setup > Display > FONT sets the decode font: use a monospaced font, otherwise the message columns will not line up.

Part II — Operating

5. The QSO: from receive to log

5.1 Starting reception

1. Press Monitor: the button changes to STOP and the waterfall starts to scroll.
2. Check that the RX level is not clipping.
3. Wait one or two periods: decodes appear in Full Spectrum at the end of each slot.

If you decode nothing but the waterfall shows signals, the problem is almost always the clock (section 3.4) or the wrong mode for that frequency.

5.2 Calling a station

1. Double click on the station's row in Full Spectrum or Signal RX. Decodium sets the partner, generates the messages and picks the correct slot.
2. Check the TX audio frequency: in the FTx modes good practice is to transmit in a clear part of the spectrum, not on top of the station you are calling.
3. Check the messages generated in the TX panel.
4. Enable transmission.
5. With Auto Sequence enabled, Decodium completes the QSO on its own: report, confirmation, RR73/73.

5.3 Calling CQ

1. Select TX6 (the CQ message).
2. Enable transmission, or Auto CQ to call repeatedly.
3. Watch Signal RX for direct replies.
4. Let Auto Sequence do the work, or step in manually.

With Auto CQ enabled Decodium goes back to calling CQ at the end of each QSO, subject to the protections you have set (watchdog, SWR check, retry limits).

5.4 Auto Sequence and its options

Setup > TX > AUTO SEQUENCE gathers the rules that govern the automation:

Quick QSO is designed for contests and high-rate activity: it reduces the QSO to four exchanges while still producing a valid log entry.

5.5 Retries and closing the QSO

Setup > TX > FT2 UTILITY holds the QSO closing parameters, with separate values for FT2, FT4 and FT8:

These options exist for one purpose: to close weak QSOs without occupying the frequency indefinitely. If you chase marginal DX, raise the boost for very weak partners; if you operate contests, keep the retries low.

5.6 Logging the QSO

When the QSO closes Decodium opens the log confirmation window (if Prompt to log QSO is enabled in Setup > Reporting > LOGGING). The QSO is written to the local ADIF log and, where configured, forwarded to QRZ Logbook, Cloudlog, LoTW and the external software you have connected.

Alt+L logs the current QSO without touching the mouse.

5.7 Worked-before (B4)

Decodium recognises stations you have already worked by callsign, band and mode together. A QSO on 20 m FT2 does not mark the same callsign as worked on 20 m FT8 or on 40 m FT2. The B4 colour in Full Spectrum therefore indicates a real duplicate, and the "new DXCC / grid / zone" highlights stay trustworthy even when you change mode during a session.

6. Operating FT2

FT2 is the mode this project was born around, and it remains its main focus. This chapter gathers everything about FT2 in one place.

6.1 What to expect from FT2

The FT2 slot is 3.75 seconds and each transmission lasts about 2.52 seconds. That means two things:

1. The QSO rate is far higher than FT8 and appreciably higher than FT4: that is its advantage in contests, pile-ups and DXpeditions, where the bottleneck is time per QSO rather than sensitivity.
2. The time available to decode, decide and prepare a transmission is short. Audio configuration, clock accuracy and CPU load matter much more than they do in FT8.

FT2 is not a "better FT8": it is a fast mode, optimised for conditions where the signal is present and time is the scarce resource.

6.2 Asynchronous decoding

Decodium decodes FT2 asynchronously: instead of waiting for the end of the slot and processing everything at once, it works on overlapping batches while the signal is still arriving. This shortens the delay between the end of the partner's transmission and the message appearing on screen — time that in FT2 you need in full in order to answer in the right slot.

The state of asynchronous decoding is shown by the Async FT2 (A) button in the top bar.

6.3 The transmit scheduler

The FT2 TX scheduler has one non-negotiable rule: never start a transmission that cannot finish inside the slot. A truncated transmission is decoded by nobody and merely pollutes the frequency.

Hence the following protections:

If you see your transmissions starting and then being cut off, the first thing to check is time synchronisation; the second is the conservative TX window.

6.4 The FT2 robustness options

These options are for signals that are weak, unstable or fading. Do not enable them all "just in case": each one costs either CPU time or latency.

A sensible starting point: leave the defaults, enable FT2 partner-memory if you work DX in QSB, and enable Conservative FT2 only when you are chasing signals at the limit.

6.5 Log and message protections

The last option is useful when you need to report a sequencing problem: it produces the exact trace of the states that were passed through, which is what is needed to reproduce the bug.

6.6 Callsign handling and hashes

FT2, like FT8 and FT4, compresses callsigns. Long or compound callsigns travel as a hash and are resolved by the receiver using callsigns already seen. Decodium includes protections against ghost calls (callsigns that appear to decode but are not really present) and against unresolved payloads: if a message cannot be attributed with confidence, it is not used to advance the QSO sequence.

This behaviour is deliberate: an incomplete QSO is preferable to a QSO logged with the wrong callsign.

6.7 FT2 on limited CPUs

On slow machines:

1. enable Slow-PC mode;
2. enable FT2: adaptive decode (CPU saver);
3. consider FT2: skip redundant end-slot decode;
4. reduce the waterfall height and the visible history;
5. close Live Map if you do not need it.

The priority in FT2 is always the same: timing first, sensitivity second, interface looks last.

7. Multi Answer Mode and the caller queue

7.1 What it is

Multi Answer Mode (MAM) lets you handle several callers in parallel, keeping more than one QSO active at a time. The MAM panel shows the active streams and lets you add stations by double-clicking them.

The operator remains responsible: MAM organises the work, it does not authorise it. Before using it on a busy frequency, make sure your signal and your configuration are clean.

7.2 Multi-stream

Setup > TX > FT2 UTILITY contains:

• FT2/FT8 MAM multi-stream (MSHV, sperimentale) — enables handling of several simultaneous streams;
• MAM multi-stream: max stream simultanei — how many streams to allow.

The option is marked experimental: it increases the number of simultaneous transmissions and therefore also the opportunities to get things wrong. Start with two streams.

7.3 Caller queue

The Caller Queue panel collects the stations calling you and lets you order them by operational criteria. It is the natural tool when you are calling CQ from a sought-after station and replies arrive faster than you can work them.

8. DX-Pedition and contest modes

8.1 Fox / Hound / SuperFox

In Setup > Advanced > OPERATING MODE:

The OTP settings (Setup > Advanced > OTP: OTP Enabled, OTP Seed, OTP Interval, OTP URL) govern the generation and distribution of the codes used to verify an activity: they concern whoever organises an expedition, not the individual caller.

8.2 DX-Pedition Workspace

The DX-Pedition Workspace is a dedicated layout that places Full Spectrum · Decode, Signal RX · QSO Lock and Log · QSO Entry side by side in a single view, with the UTC time and the operator's callsign. It is designed for operator shifts on a sought-after station, where you need band activity, the running QSO and log entry all in view at once.

8.3 Contest

Setup > Advanced > CONTEST sets the current activity:

• Activity: None, Field Day, Fox, Hound;
• FD Exchange and RTTY Exchange: the exchanges required by the rules;
• Contest Name, Indiv Name, NCCC Sprint: parameters for the specific formats.

The choice of activity changes message generation: always check the TX messages before you start transmitting in a contest.

9. Advanced decoding

Setup > Advanced > ADV DECODING gathers three recovery technologies, which can be enabled individually or managed automatically:

With Auto Mode enabled Decodium switches the three technologies on only when they are needed, based on measured conditions; the Live state field shows which ones are active at any moment.

The cost of these options is CPU time. On slow machines, enabling them all by hand makes matters worse rather than better: Auto Mode is the sensible choice in most cases.

Other decode options are in Setup > Decode:

• DECODE PARAMETERS — decode depth and general parameters;
• JT65 VHF/UHF — parameters specific to VHF/UHF use;
• SIDELOBE CONTROL — sidelobe control;
• DECODE FILTERS — filters applied to decodes (see section 12.4).

10. Live Map, Astro and Time Sync

10.1 Live Map

The Live Map shows the stations you decode and the paths between your station and theirs, with markers coloured by direction and state. It clears automatically when you change band or mode, and when you empty the decode lists, so that stale activity does not linger on the map.

During transmission the map is deliberately updated more slowly: this is a priority decision, because the graphics thread must not steal time from transmission.

If you do not need the map, closing it frees GPU and memory — it is the first panel to sacrifice on modest hardware.

10.2 Astro

The Astro panel provides the astronomical data needed for EME and VHF/UHF work: moon position, azimuth and elevation, Doppler, degradation. The AzEl data directory is configurable in Setup > Audio > DIRECTORY.

10.3 Time Sync (DecoSyncTime)

The DecoSyncTime panel brings together everything about timing:

• NTP state and the offset being applied;
• mean DT computed over the decodes received;
• decode latency;
• the number of samples used for the estimate.

How to read it:

There is also an estimate of sound card drift in ppm, with a green/amber/red indicator: significant drift progressively shifts the slot timing and is typical of cheap USB codecs.

11. Logging, online services and integrations

11.1 Local QSO log

Decodium maintains the local ADIF log and the QSO Log window, where you can enter and edit QSOs. The behaviour options are in Setup > Reporting > LOGGING: confirmation before logging, required fields, report format.

11.2 PSK Reporter

Decodium sends observations to PSK Reporter and lets you search your own spots from the dedicated panel. Sending is queue-driven: when there are no observations waiting, no background traffic towards the service is kept alive.

11.3 QRZ Logbook, Cloudlog and LoTW

For Cloudlog, Decodium normalises the API endpoint and distinguishes HTTP, authentication and proxy failures in its error messages: if an upload fails, the error tells you which of the three it is.

The LoTW data also drives the highlighting of LoTW users in Full Spectrum (the LoTW marker colour).

11.4 DX Cluster

The DX Cluster panel connects to a telnet node and shows the spots, with filtering. Configuration is in Setup > Reporting > DX CLUSTER (host, port, login callsign, filters).

11.5 Integration with external software

If you use JTAlert or GridTracker, the UDP Server must be enabled and the port must match the one configured in the external program.

11.6 Audio recording

Setup > Reporting > RECORDING handles recording of the received audio, which is useful for later analysis and for attaching real samples to a bug report.

12. Decode history, filters and alerts

12.1 Decode History

Decodium stores decodes in an SQLite database, which you can browse from the History window. You can search by:

• callsign;
• band;
• mode;
• UTC date range;
• result limit.

Results can be exported to ADIF. The history survives closing the program and supports post-activity analysis: checking propagation, reviewing band activity, reconstructing a doubtful QSO.

The live in-memory views are capped so that RAM use does not grow during long sessions; the complete history remains in the database.

12.2 Audio alerts

Setup > Alerts > AUDIO ALERTS lets you attach a sound to specific events: new DXCC, new grid, a callsign you are looking for, a direct call. It is how you work in the background without staring at the screen.

12.3 Filters: blacklist, whitelist and territories

Setup > Filters contains:

ALWAYS PASS takes precedence: it is the right place for the callsign of a friend you never want to miss, even when you are filtering aggressively.

12.4 Decode filters

Setup > Decode > DECODE FILTERS acts further upstream, on the decoder's work and on what gets published to the lists. Used sensibly it reduces CPU load on a crowded band.

13. Remote web dashboard and FT2-Link

13.1 Remote web dashboard (iPad, phone, PWA)

Decodium includes an internal web server that lets you control the station from another device on the same local network.

Enabling it

1. Open Setup > Display > REMOTE WEB SERVER (iPad / mobile PWA) or Setup > Reporting > REMOTE WEB DASHBOARD (LAN).
2. Enable the server and set the port and access credentials.
3. Find the IP address of the station computer.
4. On the tablet or phone, open http://IP-ADDRESS:PORT in the browser.
5. Add the page to the home screen to use it as an app (PWA).

Security — read this before enabling it

• Use the server on the local network only. Do not expose it to the internet and do not open ports on your router.
• Set a non-trivial password: anyone who can reach the dashboard can command your station, and therefore your transmitter.
• On shared networks (hotels, contest clubs, exhibitions) keep the server off when you are not using it.

The step-by-step guide for newcomers is in the doc/WEBAPP_SETUP_GUIDE.en-GB.md file in the sources, including the troubleshooting section and the description of the API endpoints for advanced use.

13.2 FT2-Link (experimental)

FT2-Link is an open protocol for reliable chat and messaging, under development inside Decodium. It is publicly documented and independent of the modem: the same ARQ layer can run over different waveforms.

The intended flow is: beacon or CQ on NARROW, HELLO reply, negotiation of the best common profile, a switch to W500 or W2300 for the data, and a fall back to NARROW if the wide profile does not hold up.

The FT2-Link working period is 15 seconds, unlike the 3.75 s of FT2.

Caution. FT2-Link is experimental material, and the W500/W2300 profiles are not standard FT2 and must not be presented as such. Check the regulations that apply to the bandwidth you intend to use, in your band and in your IARU region, before transmitting.

Part III — Reference

14. Settings reference

The settings are organised into thirteen tabs. Every section is listed below, in the order in which it appears, with what it does.

14.1 Station

14.2 Radio

14.3 Audio

14.4 TX

14.4.1 Ready Profiles

The ready-made profiles apply a coherent set of parameters in one action:

The active profile is marked ● active. You can start from a profile and then change individual options: your changes stay yours and are not overwritten until you apply another profile.

14.4.2 Watchdog

Keep the watchdog enabled. It is the protection that stops a forgotten carrier sitting on the band while you are out of the room.

14.5 Display

Process priority. High (recommended) is the advised setting. Realtime is flagged with a warning: on some systems it can leave the computer unresponsive or unstable. Do not use it unless you are making specific measurements.

14.6 Decode

14.7 Reporting

14.8 Frequencies

Management of the working frequencies: the list per band and mode, adding, editing, loading and saving to .qrg / .qrg.json files, with the option to merge (Merge Working Frequencies) an external list into your own instead of replacing it.

14.9 Colors

14.10 Advanced

14.11 Alerts

AUDIO ALERTS — attaching sounds to events (new DXCC, new grid, a callsign you are looking for, a direct call).

14.12 Filters

BLACKLIST, WHITELIST, ALWAYS PASS, EXCLUDE TERRITORY, FILTER OPTIONS (section 12.3).

14.13 UI Buttons

TOP TOOLBAR (button visibility) and TOOLBAR BUTTON ORDER (order, with restore to defaults for both the toolbar and the TX panel).

15. Keyboard shortcuts

Transmission

Controls (Ctrl)

Actions (Alt)

Other

The complete list can be consulted inside the program under Keyboard Shortcuts.

16. Diagnostics and troubleshooting

16.1 I am not decoding anything

Work through this in order:

1. Is Monitor running? The button must read STOP.
2. Is the waterfall scrolling? If it is frozen, the problem is the audio input device, not the decoder.
3. Is the RX level sensible? A noise floor that is far too low, or clipping, both prevent decoding.
4. Is the clock synchronised? Check the Time Sync panel. In FT2 and FT4 a few tenths of a second are enough to lose everything.
5. Is the mode right for the frequency? FT8 on an FT4 frequency decodes nothing.
6. Is the audio channel correct? With a stereo codec, the wrong channel is the same as silence.

16.2 I transmit but nobody answers

• Check that the PTT works and that the radio really goes into transmit.
• Check the telemetry in Setup > Radio > DIAGNOSTICS: the power must match what you expect and the ALC must stay at rest. A reading of ALC -- means the value is unavailable, whereas 0 is a valid value: these are two different situations.
• Check the SWR: Check SWR does not block Tune, so you can measure and correct, but it does protect real transmission and Auto CQ.
• Make sure you are not transmitting outside the slot: in FT2 the conservative TX window (section 6.3) rejects late transmissions, and that is the correct behaviour.

16.3 Graphics and performance problems

The CPU pressure indicator in Setup > Display > UI — PERFORMANCE counts the overload events for the session: if the severe counter keeps rising, the computer is not keeping up and you should lighten the configuration.

16.4 Audio stuck, or a carrier after transmission

Decodium includes a TX audio watchdog aimed precisely at this case. If it happens:

1. use Halt (or Esc);
2. check that the PTT has been released at the radio;
3. check the audio output device in Setup > Audio;
4. if it happens systematically, capture a diagnostic log and report the case.

16.5 CAT ports that change or appear twice

On Linux use the /dev/serial/by-id/... paths, which stay stable across reboots. If the same device appears twice (as /dev/ttyUSB0 and as a by-id path), choose the by-id path and not both: they are the same physical device.

16.6 Reporting a bug usefully

Decodium includes a bug report window. A useful report contains:

• the exact program version (About Decodium);
• the operating system and platform;
• the radio and CAT backend in use;
• the operating mode and frequency;
• a description of the steps that reproduce the problem;
• the diagnostic log and, if the problem concerns decoding, a WAV file of the signal;
• for FT2 sequencing problems: enable FT2 state transition census (log only) and attach the trace.

17. Files, folders and user data

Decodium uses the standard system folders provided by Qt for:

• configuration settings;
• the QML cache;
• the SQLite decode-history database (db.sqlite);
• CTY.DAT and the DXCC data;
• the ADIF log;
• diagnostic logs;
• audio recordings.

The exact locations depend on the platform. The most reliable way to find them is to read the startup log, where Decodium prints the main paths (AppDataLocation, CacheLocation, database path).

For a complete backup of your configuration you need: the settings folder, the ADIF log and the history database.

Decodium also includes a storage migration step: when the data layout changes between versions, existing files are converted on first start. Before a major update, a copy of the data folder is still the sensible precaution.

18. Building from source

18.1 Dependencies

Depending on the platform you need:

• CMake (3.20 or later) and a recent C/C++ compiler;
• Qt 6 with the Quick, Qml, QuickControls2, Widgets, Multimedia, Network, SerialPort, Sql and WebSockets modules;
• Hamlib;
• a Fortran compiler (gfortran) for the legacy components that are still present;
• Boost and the other dependencies listed in the build files.

18.2 Build scripts

The repository contains ready-made scripts for the main platforms:

The Windows-specific notes are in doc/building on MS Windows.txt and the adjacent files; the macOS port is documented in doc/MACOS_PORTING_v1.2.0.md.

18.3 Tests

The tests/ directory contains the unit tests and the comparisons between decoder implementations, including the tests dedicated to FT2 and FT2-Link. To run them:

ctest --output-on-failure

The stage-comparison tests are the right tool for verifying that a change to the decoder has not introduced a sensitivity regression.

19. Glossary

20. Credits, licence and contacts

The project

• Martino Merola — IU8LMC — creator of FT2, DECODIUM lead developer
• Salvatore Raccampo — 9H1SR — C++ core, macOS and Linux builds and packages
• The testers and contributors of the FT2 community, whose reports have driven much of the work on recent releases
• The community translators, who maintain the program's localisations

Where the code comes from

Decodium descends from WSJT-X, developed by Joe Taylor K1JT, Steve Franke K9AN, Bill Somerville G4WJS and the project's other authors, and distributed under the GPL. The AUTHORS, THANKS and COPYING files included with the sources give the full attributions and licence. This project's own contributions — the FT2 protocol, the rewritten decoders and modulators, the QML interface, the integrated services — are documented in the commit history and in CHANGELOG.md.

Licence

Decodium 4.0 Core Shannon is free software distributed under the GNU General Public License version 3. You may use, study, modify and redistribute it under the terms of that licence. The program comes with no warranty, to the extent permitted by applicable law.

Where to get help

• Project website: ft2.it
• Community and support: community.ft2.it
• The FT2 Digital Mode Telegram channel
• Technical reports: the Issues section of the GitHub repository

A final operating note

Decodium controls a transmitter. Responsibility for what goes on the air — band, bandwidth, power, times, authorisations — rests with the licensed operator. The program's experimental features, in particular the wide FT2-Link profiles and the multi-stream modes, should only be used after checking the regulations that apply in your own band and IARU region.

73 de IU8LMC