PUBLIC BETA · v1.0.262 · COMPLETE USER MANUAL

DECODIUM 4.0

“Shannon” — Complete User Manual

▸ Chapters 1-10 · 10 chapters · 20+ features · 3 platforms

10

CHAPTERS

The whole software

−23 dB

RAPTOR SNR

5 decode passes

ASYMX

ASYNC TX

No Even/Odd · No NTP

3 OS

NATIVE BUILDS

Win / macOS / Linux

Chapter 1 — Getting Started

1.1 What DECODIUM 4.0 is

DECODIUM 4.0 “Shannon” is an open-source platform for amateur radio digital modes, rewritten from scratch in C++ over six months of intensive development. It is heir and successor to DECODIUM 3.0 (“Raptor”) and introduces three structural innovations:

1. C++ multi-decode engine — the old Fortran core has been replaced by an OpenMP-parallel decoder that uses every available CPU core. Fortran code remains isolated only for LDPC, where the original mathematics by K1JT is unmatched.
2. Native FT2 mode — a new digital protocol with a 3.8-second T/R cycle, now certified ADIF 3.1.7 as SUBMODE=FT2 (unanimous 22:0 vote).
3. ASYMX — asynchronous transmission — the first HF digital mode in history that requires no clock sync (no NTP, no GPS).

The stated goal of v4.0 is twofold: deliver to the amateur radio world a decoder beyond the state of the art (−23 dB SNR sensitivity on FT2 Raptor, +1.5 dB on fading channels versus WSJT-X baseline), and provide a modern operating experience (integrated Live Map, automatic Call Roster, robust CAT for Yaesu/Kenwood/Icom, native cross-platform support).

1.2 Differences vs DECODIUM 3.0 and WSJT-X

For those coming from DECODIUM 3.0 or WSJT-X / JTDX / MSHV, here are the substantive differences:

Important: DECODIUM 4.0 is backward-compatible with DECODIUM 3.0 logs and settings. On first launch, the application detects any previous installation and offers to import the preferences.

1.3 System requirements

Minimum requirements

Supported operating systems

• Windows 10/11 x64 — Inno Setup installer, runtime included
• macOS 12.x+ Apple Silicon — native M1/M2/M3 DMG (build by 9H1SR)
• macOS 12.x+ Intel — support via Rosetta, reduced performance
• Linux x86_64 — portable AppImage (Qt 6.11), tested on Debian 12, Ubuntu 22.04+, Arch
• Raspberry Pi (ARM Linux) — LU7DID community port, ideal for portable stations and low-power use

Note on Windows 10: requires at least version 21H2. Earlier versions may have issues with Qt 6.11 and QML drag handles.

1.4 Project philosophy

DECODIUM 4.0 follows three guiding principles:

1. Technical transparency — every innovation is documented and credited. The LDPC decoder inherits K1JT’s mathematics (WSJT-X), the Qt UI framework is by The Qt Company, and this heritage is explicit in the code and credits.
2. Operational robustness — we prefer a slow and reliable decoder over a fast and unstable one. The HRD CAT bridge mode has been rewritten three times to guarantee data-mode persistence.
3. Community first, product second — the Telegram tester community is the primary development organ. Bug reports, suggestions, and feedback have absolute priority in the release cycle.

Chapter 2 — Installation

This chapter guides installation on every supported platform. Jump directly to the section that interests you.

2.1 Windows x64

Standard procedure

1. Open your browser at https://github.com/iu8lmc/Decodium-4.0-Core-Shannon/releases/latest
2. In the Assets section, download Decodium_1.0.262_Setup_x64.exe (~90 MB)
3. Double-click the downloaded file
4. If Windows shows the SmartScreen warning, click More info → Run anyway (the binary isn’t Microsoft EV-signed; it’s community-certified)
5. Follow the wizard. The installer will ask for:
   • Install folder — default: C:\Users\<your_user>\AppData\Local\Decodium
   • Create Desktop shortcut — recommended
   • Launch on finish — optional

User mode vs administrator

The DECODIUM installer requires PrivilegesRequired=lowest. This means:

• ✅ No admin password prompt
• ✅ Installs in user profile (not Program Files)
• ✅ Automatic cleanup of *.qmlc caches from previous installations
• ✅ Upgrade over 1.0.257/258/259/260 without issues

Upgrading from v1.0.257 or earlier

The installer detects a previous version and: 1. Automatically backs up decodium.ini 2. Uninstalls compiled QML files (.qmlc) from the old version 3. Overwrites the executable and resources 4. Preserves ADIF logs, preferences, and callsign dictionary

Important: If you’ve manually customized decodium.ini, make a separate backup before upgrading. The installer respects the file but in case of a crash you might lose it.

Uninstallation

Control Panel → Apps & Features → Decodium → Uninstall. Uninstallation removes the executable but preserves logs and user preferences. For a complete removal, manually delete:

• C:\Users\<your_user>\AppData\Local\Decodium\
• C:\Users\<your_user>\AppData\Roaming\Decodium\

2.2 macOS Apple Silicon

Native M1/M2/M3 build

The macOS build is personally maintained by Salvatore 9H1SR and is native ARM64 — no Rosetta emulation, full performance.

1. Download Decodium-1.0.262.dmg (or the zip) from the release
2. Double-click the .dmg
3. Drag the DECODIUM icon to the Applications folder
4. Eject the DMG from Finder

First launch and Gatekeeper

On first launch, macOS will block the app because it’s not Apple Developer-signed (the signature costs $99/year, unsustainable for community-driven GPL projects):

1. Go to Applications, find DECODIUM
2. Right-click (or Ctrl+click) → Open
3. The security dialog appears with a warning — click Open (this only happens once)
4. macOS will remember the authorization for subsequent launches

If the app still refuses to open, from terminal:

xattr -cr /Applications/DECODIUM.app

Removes quarantine attributes and allows execution.

Required permissions

On first launch macOS will ask for:

• Microphone — required to read audio from the radio
• Local network — for UDP broadcast (Log4OM, GridTracker, etc.)
• Documents / Downloads folders — for ADIF file access

Grant all requested permissions, otherwise decoding will silently fail.

2.3 Linux x86_64

AppImage (recommended)

The AppImage is a single executable file containing all dependencies (Qt 6.11, libsndfile, libfftw3, hamlib 4.7):

# Download
wget https://github.com/iu8lmc/Decodium-4.0-Core-Shannon/releases/download/v1.0.262-call/Decodium-1.0.262-x86_64.AppImage

# Make executable
chmod +x Decodium-1.0.262-x86_64.AppImage

# Launch
./Decodium-1.0.262-x86_64.AppImage

The AppImage requires no installation. You can move it anywhere (/opt/, ~/Applications/, desktop) and launch it from there.

Desktop integration

To have it in the application menu with an icon:

mkdir -p ~/.local/share/applications/
cat > ~/.local/share/applications/decodium.desktop << EOF
[Desktop Entry]
Name=DECODIUM
Comment=FT8/FT2 Digital Mode Decoder
Exec=/full/path/to/Decodium-1.0.262-x86_64.AppImage
Icon=decodium
Terminal=false
Type=Application
Categories=HamRadio;AudioVideo;
EOF
update-desktop-database ~/.local/share/applications/

Runtime dependencies

The AppImage includes almost everything, but the host system needs:

• FUSE (to mount the AppImage) — usually already present, otherwise apt install fuse libfuse2
• PulseAudio or PipeWire for audio
• ALSA for direct sound card access (optional)

On minimal distros (e.g. Debian server with graphical environment installed afterwards) you might need:

sudo apt install libxcb-cursor0 libxcb-icccm4 libxcb-image0 \
                 libxcb-keysyms1 libxcb-randr0 libxcb-render-util0 \
                 libxcb-shape0 libxcb-sync1 libxcb-xfixes0 \
                 libxcb-xinerama0 libxcb-xkb1 libxkbcommon-x11-0

Serial permissions (CAT)

To access the CAT serial port without sudo, add your user to the dialout group:

sudo usermod -aG dialout $USER
# Log out and back in

Verify with:

groups   # should show "dialout" among the groups
ls -l /dev/ttyUSB0   # should show "dialout" group

2.4 Raspberry Pi (ARM Linux)

The Raspberry port is maintained by LU7DID and is available as a separate repository. It’s particularly suited for:

• Portable FT2 stations with low power consumption
• Embedded installations (e.g. remote-rig on the rooftop)
• Educational and laboratory setups

Visit https://github.com/lu7did for the specific instructions.

Compatibility: - Raspberry Pi 4 (8 GB RAM) — full functionality, multi-pass decode - Raspberry Pi 5 — recommended, performance comparable to x86 - Raspberry Pi 3 — limited, not recommended for FT2 multi-decode

2.5 Installation verification

After installation, launch DECODIUM. The top bar must show:

Decodium 4.0 — FT8 — IU8LMC

where IU8LMC will be replaced by your callsign (empty on first launch).

At the bottom left there should appear:

• S: signal level indicator
• MON | TX | DEC | FT8 toggles
• CAT: Disconnected (yellow) — normal on first launch
• PWR: -- — will populate after CAT configuration
• Decodium 4.0 bottom right

If you see all of this, the installation succeeded. Proceed to Chapter 3 for configuration.

Chapter 3 — Initial Setup

This chapter guides step-by-step station configuration. It’s meant to be followed in order, because some choices (e.g. CAT) influence others (e.g. PTT).

3.1 Callsign and locator configuration

Open Setup from the hamburger menu (☰) at top-left, My Station section:

Tip: For the 6-character locator, use https://aprs.fi or http://www.levinecentral.com/ham/grid_square.php to calculate from your station’s GPS coordinates.

Press Save. The callsign appears in the title bar (Decodium 4.0 — FT8 — IU8LMC).

3.2 CAT configuration

CAT (Computer Aided Transceiver) is the communication protocol between software and radio. DECODIUM 4.0 supports three backends:

1. Hamlib — universal, recommended for most modern radios
2. OmniRig (Windows only) — alternative when other programs share the radio
3. HRD CAT bridge — for those using Ham Radio Deluxe as a central hub

3.2.1 CAT setup with Hamlib

Open Setup → Radio:

1. Rig — autocomplete field. Start typing the model:
   • FT-991A → Yaesu FT-991A
   • TS-590S → Kenwood TS-590S / 590SG
   • IC-7300 → Icom IC-7300
   • K3 → Elecraft K3 / K3S
2. Port — auto-selection of available ports:
   • Windows: COM3, COM4, etc.
   • Linux: /dev/ttyUSB0, /dev/ttyACM0
   • macOS: /dev/cu.usbserial-*, /dev/cu.SLAB_USBtoUART
3. Baud rate — fundamental, must match the radio menu:
   • Kenwood TS-590S: 57600 (recommended, max throughput) or 9600
   • Yaesu FT-991A: 38400 or 9600
   • Icom IC-7300: 19200 or 9600
4. Stop bits — usually 1, but for Yaesu FT-991A you might need 2
5. Handshake — None (default), Hardware (rare)
6. CI-V Address (Icom only) — 0x94 for IC-7300, 0xA4 for IC-7610

3.2.2 RTS / DTR

The RTS and DTR serial control lines are often used for hardware PTT. The DECODIUM 4.0 semantics are:

Important: If the radio is set for RTS-based PTT, leave RTS = ON and manage PTT via Setup → PTT (see section 3.4). Don’t confuse the two.

3.2.3 CAT Test

After configuring all parameters, press Test CAT. A successful connection returns:

✓ Rig: Kenwood TS-590S
✓ Frequency: 7.074000 MHz
✓ Mode: USB
✓ Power level: read OK

At the bottom, the CAT: Disconnected (yellow) indicator turns to CAT: Connected (green).

3.2.4 CAT troubleshooting

3.2.5 HRD bridge (advanced)

To use DECODIUM with HRD as central hub, enable Setup → Radio → HRD bridge:

1. Start HRD normally, connect to the radio
2. In HRD, enable the TCP server (Tools → TCP Server)
3. In DECODIUM, enter 127.0.0.1:7809 (or the address configured in HRD)
4. Test HRD — when you see ✓ HRD v6.x bridged, you’re set

HRD/Icom data-mode stability (v1.0.261): The new v1.0.261 has definitively fixed the issue of USB fallback during PTT settling. DATA mode is preserved across all TX/RX transitions and band changes. If you’re coming from v1.0.260 or earlier, the upgrade is strongly recommended.

3.3 Audio configuration

Audio I/O is the backbone of any digital mode. Wrong configuration = no decodes.

3.3.1 Audio hardware

DECODIUM supports three topologies:

1. USB Audio CODEC integrated in the radio (recommended)
   • Kenwood TS-590S/SG, Yaesu FT-991A, Icom IC-7300/7610 — single USB interface for audio + CAT
2. External interface (e.g. SignaLink USB, Tigertronics, RigBlaster)
   • Audio separate from CAT, more cables but greater flexibility
3. PC sound card + direct cables (legacy, not recommended)
   • Audio line from radio to PC sound card, manual level management

3.3.2 Device selection

Open Setup → Audio:

On Windows, disable driver audio effects:

• Control Panel → Sound → USB Audio CODEC → Properties → Advanced
• Disable “Audio Enhancements”, “Dolby Effects”, etc.

3.3.3 Level calibration

Audio levels are critical. Once CAT is connected and the radio is receiving on an active band:

1. Open the VU meter at bottom-left (S: indicator)
2. The bar should swing in the green zone (-30 dB to -10 dB)
3. Yellow zone (-10 to -5 dB) = too high, possible clipping
4. Red zone (>-5 dB) = overload, decoder will fail

To adjust the IN level:

• On the radio: USB IN level menu (Kenwood Menu 64, Yaesu Menu 113, Icom USB AF Output Level)
• On the PC: audio control panel → USB CODEC microphone level

To adjust the OUT level (for transmission):

• On the radio: USB OUT level menu (Kenwood Menu 65, Yaesu Menu 114, Icom USB MOD Level)
• On the PC: audio preferences → USB CODEC speaker volume

Tip Kenwood TS-590S: The optimal setup requires three menus in coordination: - Menu 63 = DATA IN — set to USB - Menu 64 = USB IN level — start at 5, increase until green zone - Menu 65 = USB OUT level — start at 5, adjust to obtain desired PWR

3.4 PTT (Push To Talk)

PTT is the signal telling the radio “transmit now”. DECODIUM 4.0 supports five PTT modes:

3.4.1 Available PTT modes

3.4.2 PTT CAT setup (recommended)

In Setup → PTT: 1. PTT method — CAT 2. PTT port — same value as CAT (e.g. COM3) 3. Save

PTT travels on the same CAT channel. No additional cable or configuration needed.

3.4.3 PTT RTS or DTR setup

For legacy radios or external setups: 1. PTT method — RTS or DTR depending on cable 2. PTT port — can be same as CAT or a different serial port (if you have a dedicated PTT cable) 3. Test PTT — press the button; the radio must switch to TX (red LED lights)

3.4.4 PTT test with TUNE button

At any time, the TUNE button in the toolbar generates a 1500 Hz test tone and activates PTT. Use it to:

• Verify PTT actually fires
• Tune the antenna (SWR)
• Adjust output PWR level

Important: TUNE generates a continuous signal for 5-10 seconds. Don’t leave it on for long to avoid stressing the radio’s final stage. Use SWR ≤ 1.5 for prolonged TUNE.

3.5 Time synchronization

FT8, FT4, synchronous FT2 and other modes require the PC to be UTC-synchronized within ±1 second.

3.5.1 TIME SYNC indicator

At top, next to the frequency display, there’s the TIME SYNC line with current UTC time:

• White → synchronized, all OK
• Yellow → drift within 500 ms, tolerated
• Red → drift > 1 second, decoding compromised

3.5.2 Configuring NTP

Windows 10/11:

Control Panel → Date and Time → Internet Time → Change settings
✓ Synchronize with Internet time server
Server: time.windows.com (default) or pool.ntp.org

macOS:

System Preferences → Date & Time → Set automatically
Server: time.apple.com

Linux:

sudo systemctl enable --now systemd-timesyncd
timedatectl status   # verify

3.5.3 ASYMX — no NTP needed

If you operate only in FT2 ASYMX, time sync is not required. ASYMX is designed precisely for scenarios where NTP/GPS are not available (emergency, maritime, portable). See Chapter 5.

Chapter 4 — Operating in FT8

FT8 is the most widespread digital mode in the world. DECODIUM 4.0 offers full WSJT-X-compatible support, with the Raptor decoding engine improving sensitivity.

4.1 Typical workflow

An FT8 QSO in DECODIUM follows the classic flow:

1. Select the band (bottom toolbar: 160, 80, 60, 40, 30, 20, 17, 15, 12, 10, 6, 4, 2, 70cm)
2. DECODIUM auto-tunes the radio to the standard FT8 frequency (e.g. 7.074 MHz for 40m)
3. Passive decode — you see signals in the Full Spectrum
4. Click a station calling CQ — DECODIUM prepares TX messages
5. Start the QSO by pressing TX (or let auto-sequencing run)
6. Auto-log at end of QSO

4.2 Decode windows

4.2.1 Full Spectrum (left)

Shows all decodes across the entire audio band (50 Hz - 3000 Hz). Columns:

4.2.2 Signal RX (right)

Shows only decodes on the selected frequency (shown at top: 1395 Hz). Useful for following an ongoing QSO without distractions.

4.2.3 Live Map

World map with yellow dots for each recent decode. Green lines indicate active paths:

• IN→ME = stations you receive
• ME→DX = if you’re spotted on PSK Reporter
• BAND = yellow dots without lines = raw decodes

Click a dot = filter Full Spectrum on that station.

4.3 Transmitting

4.3.1 Replying to a CQ

1. In Full Spectrum, double-click a line starting with CQ
2. DECODIUM populates the messages automatically:
   • TX1: <DX> IU8LMC JN71
   • TX2: <DX> IU8LMC -10
   • TX3: <DX> IU8LMC R-10
   • TX4: <DX> IU8LMC RR73
   • TX5: 73 to <DX>
3. Press TX at bottom (or let auto-sequencing run)
4. Radio switches to TX, transmits for 13 seconds, returns to RX
5. If DX replies, autoSeq flow proceeds automatically through to log

4.3.2 Calling CQ

1. Press TX6 at bottom (preset to CQ IU8LMC JN71)
2. DECODIUM alternates RX and TX automatically
3. When another station replies, you’ll see their message in Signal RX
4. The software proceeds with autoSeq flow: receive report → send report → receive RR73 → send 73 → log

4.3.3 Auto-sequencing (autoSeq)

The SEQ button in the toolbar enables full QSO automation. When SEQ is green:

• DECODIUM auto-advances from TX1 → TX2 → TX3 → TX4 when it detects coherent replies
• Logs at end of QSO without intervention
• Stops transmission if no reply in 3 consecutive slots

4.3.4 Hold mode

The HOLD button (orange when active) blocks automatic changes to TX messages. Useful when:

• You’re working a difficult station and don’t want the software to change callsigns based on spurious decodes
• You want to maintain QSO with the specific DX even if another station calls

4.4 Logging

4.4.1 ADIF log

Every completed QSO is automatically logged to ~/.decodium/log.adi (or %APPDATA%\Decodium\log.adi on Windows). The format is ADIF 3.1.7 standard, compatible with any logbook software.

4.4.2 Automatic uploads

In Setup → Logbook you can enable automatic upload to:

• LoTW (Logbook of The World) — requires TQSL configured
• eQSL — username/password
• Club Log — username/password
• QRZ.com — API key
• Cloudlog / Wavelog — endpoint URL + API key

FT2 submode and LoTW: ARRL confirmed that LoTW support for SUBMODE=FT2 requires updating the TQSL configuration file. In the meantime, upload uses SUBMODE=DATA as a workaround. The definitive fix is in implementation on the ARRL side.

4.4.3 Export and backup

From Logbook → Export:

• Complete ADIF — entire logbook
• ADIF by band — split per band
• ADIF by mode — split per mode
• CSV — for spreadsheet import

4.5 Macros and MAM (Memory Auto Macro)

4.5.1 Classic macros

The TX1-TX6 buttons can be customized. Right-click on a TX → Edit macro:

%CALL% — current DX callsign
%MYCALL% — your callsign
%MYGRID% — your locator
%RPT% — received SNR report
%SENT% — report you sent

Custom macro example: %CALL% %MYCALL% TU 73 DE %MYGRID%

4.5.2 MAM — Memory Auto Macro

MAM is an advanced feature that stores recurring QSO sequences. Typical application: contest or activations where you repeat the same flow dozens of times.

Open MAM Window from the hamburger menu. Create a compound macro: 1. Trigger — activation condition (e.g. “after QSO log”) 2. Sequence — series of actions (e.g. “change band, call CQ”) 3. Stop — stop condition (e.g. “after 10 QSOs” or “manual”)

The feature is detailed in Chapter 7 (Advanced Features).

Chapter 5 — Operating in FT2

FT2 (Fast Transmission 2) is the original digital protocol developed by the DECODIUM team and certified ADIF 3.1.7 with a unanimous 22:0 vote on March 22, 2026.

5.1 What FT2 is

FT2 is an HF digital protocol designed to be 3-4 times faster than FT8 while maintaining comparable sensitivity. Technical specs:

5.1.1 Standard FT2 frequencies

Dedicated sub-band within each band’s digital segment:

DECODIUM auto-tunes these frequencies when you select the band in FT2 mode.

5.2 Enabling FT2

5.2.1 Switching to FT2 from FT8

1. On the mode toolbar (bottom-left), click the FT8 button which opens the menu
2. Select FT2
3. DECODIUM automatically changes:
   • Radio frequency to FT2 sub-band
   • Decoder to dedicated FT2 engine
   • T/R cycle to 3.8 seconds
   • Interface with FT2-specific elements (AP, QQ buttons visible)

5.2.2 FT2 mode buttons

In the toolbar new controls will appear, specific to FT2:

5.3 Raptor Engine (DEEP)

The DEEP button activates the multi-pass decoding engine. When DEEP is green:

1. The decoder runs 5 independent passes on the same RX slot
2. Each pass tries an optimal sync with different methods (classical correlation, MMSE, EMA)
3. Results are combined with a best-of strategy
4. The final decode is the one with valid CRC and best SNR

The gain is significant:

• Standard decoder: −16 dB SNR floor
• DEEP (Raptor 5-pass): −23 dB SNR floor (+7 dB of sensitivity)

Computational trade-off: DEEP uses more CPU. On modern PCs (quad-core 2.5+ GHz) it’s negligible. On Raspberry Pi 4 it might slow the interface — disable DEEP if you see lag.

5.4 ASYMX — Asynchronous mode

ASYMX is FT2’s most radical innovation. It eliminates Even/Odd sync of classic FT8. Result: no NTP, no GPS, no “slot loss” if you change band at the wrong moment.

5.4.1 How to enable ASYMX

1. Band 40m, FT2 mode active
2. Press AP (Async Period) in toolbar — the button turns yellow
3. ASYMX is active. Transmission starts at first available TX without waiting for “even/odd slot”

5.4.2 Complete ASYMX sequence

T=0      You transmit CQ (2.47s waveform)
T=2.5    Waveform ends, radio returns to RX
T=2.8    You decode DX's reply (~300ms guard time)
T=3.1    You decode the reply (decoder <400ms)
T=3.5    Auto-fire TX2 (report)
T=6.0    You decode DX's TU
T=6.0    QSO complete, auto-log

Total: ~6 seconds for complete QSO.

5.4.3 When to use ASYMX

ASYMX is ideal for:

• Maritime operations — boats and sailing vessels without always-available GPS
• Emergency/EmComm — scenarios where time infrastructure is compromised
• Portable/Field Day — setup simplification, no PC sync required
• Propagation experiments — fast test sequences
• High-rate contexts — DXpedition with tight pile-ups

5.4.4 Compatibility

Important: The ASYMX signal is identical to the standard FT2 signal from the RF point of view. Anyone using WSJT-X or JTDX will receive your ASYMX signals perfectly. The difference is only in TX timing: you transmit freely, others receive normally.

5.5 Quick QSO (QQ)

QQ further optimizes the QSO flow, reducing from 5 to 4 messages.

5.5.1 QQ vs standard flow

Standard FT2:

TX1: CQ MYCALL MYGRID
TX2: DXCALL MYCALL +05
TX3: DXCALL MYCALL R+03
TX4: DXCALL MYCALL RR73
TX5: 73 TO DXCALL

QQ FT2:

TX1: CQ MYCALL MYGRID
TX2: DXCALL MYCALL R+05    ← report sent already confirmed
TX3: DXCALL MYCALL RR73
TX4: TU MYCALL              ← closes the QSO

Savings: 1 full cycle (~3.8 seconds).

5.5.2 QQ activation

1. Press QQ in toolbar — button turns green
2. Next QSO will use the optimized flow
3. If the partner station doesn’t support QQ, automatic fallback to standard cycle

5.5.3 TU Detection

DECODIUM automatically recognizes the TU MYCALL message as valid QSO end. No need to press anything — log is automatic on TU reception.

5.6 FT2 best practices

5.6.1 Tone spacing and SSB

FT2 has 167 Hz RF bandwidth, against FT8’s 50 Hz. This means:

• ✅ More robust to fading and Doppler (wider band = fewer catastrophic fluctuations)
• ✅ Easier multi-decode (tones more spaced, less overlap)
• ⚠️ Faster band saturation — with 10 FT2 stations in 3 kHz SSB you already have a crowded market

5.6.2 Power and SWR

FT2 is medium-duty cycle (~50% TX). Tips:

• Typical power: 20-50 W sufficient for most QSOs
• SWR < 1.5:1 recommended
• For QRP activations, 5 W enough to work DX on 20m with decent propagation

5.6.3 Synchronous vs ASYMX

Chapter 6 — Decoder and Sensitivity

This chapter is for those who want to understand how DECODIUM 4.0 achieves superior decoding performance. It’s not required to use the software, but it’s essential for those who want to optimize setup or contribute to development.

6.1 Decoder architecture

The DECODIUM 4.0 decoder is structured as a 4-stage pipeline:

1. Audio pre-processing (FFT, AGC, RMS norm)
       ↓
2. Sync acquisition (Costas correlation, MMSE channel estimation)
       ↓
3. LDPC decoding (Min-Sum normalized, 5-pass best-of)
       ↓
4. Post-processing (CRC validation, EMA averaging, message extraction)

Each stage is isolated in separate C++ modules. The principle: if a new algorithm proves superior in one stage, it can be replaced without touching the others.

6.1.1 Audio pre-processing

Incoming audio is transformed into three parallel representations:

RMS normalization operates on three simulated audio channels to make the decoder robust to level variations: if the RX signal drops suddenly, the decoder doesn’t lose sync.

Important: RMS norm is not radio-side AGC. It’s an internal decoder normalization. On the radio you can (and should) leave AGC as you prefer for listening.

6.1.2 Sync acquisition

DECODIUM searches for the 16-tone Costas pattern at the start of every slot. Three parallel strategies:

1. Classical correlation — WSJT-X standard method. Fast but fading-sensitive.
2. MMSE adaptive estimation — uses all 16 tones as pilots to estimate the channel and correct the signal. +1.5 dB on fading channels.
3. EMA-weighted multi-period — for stations repeating the call, accumulates information across consecutive slots.

The Best-of-N selector picks the sync with the best correlation score. This avoids locking onto interfering signals on a false-positive sync.

6.1.3 LDPC decoding

LDPC (Low-Density Parity-Check) is the error correction code used in FT8, FT4, FT2. DECODIUM uses original K1JT (Joe Taylor) mathematics but with two optimizations:

1. Normalized Min-Sum — replaces classical Sum-Product. Reduced complexity per iteration (~30% faster) with equivalent performance on AWGN channels.
2. Multi-pass best-of — 5 independent attempts with different syncs. The final CRC-14 confirms validity. Only decodes passing CRC reach the log.

Where the LDPC code lives: isolated in lib/ft2/decode174_91_ft2.f90 — the only Fortran file remaining in DECODIUM 4.0. The choice is deliberate: K1JT’s math is perfect, rewriting it in C++ would add risk with no measurable benefit.

6.1.4 Post-processing

After decoding, every message passes through:

1. CRC-14 validation — if CRC fails, decode discarded (no log, no display)
2. EMA averaging — for stations seen in previous slots, applies weighted average to stabilize SNR/DT
3. Callsign hash extraction — the 77-bit payload is decompressed into callsign + grid + text message
4. Best-of-N consolidation — if multiple passes found the same message, the decode with the best SNR is kept

6.2 Sensitivity table

DECODIUM Lab internal measurements, AWGN channel, 12 kHz sample rate:

Fair comparison vs WSJT-X: WSJT-X reports −21 dB floor on FT8 and ~−17 dB on FT4. DECODIUM 4.0 without DEEP is equivalent. With DEEP enabled the gain is real but costs CPU (~25-30% more per slot). On modern PCs it’s negligible.

6.3 Optional filters

In Setup → Filters are available post-decode filters that reduce false positives at the cost of sensitivity.

6.3.1 FDR (Frequency Domain Resilience)

Filter rejecting decodes with inconsistent SNR across passes. If pass 1 says −18 dB and pass 5 says −8 dB, it’s probably a QRM artifact.

When to disable FDR: if you’re convinced some valid decodes are being lost. Typical case: QRP stations at marginal propagation where every decode matters.

6.3.2 Spectral Mask

Filter rejecting decodes outside the expected spectral mask. Useful in crowded bands where adjacent signals can induce false syncs.

6.3.3 Sliding Window AGC

Internal AGC applied to the audio buffer before FFT. Not to be confused with radio AGC. Improves dynamic range on signals with highly variable levels within the slot.

6.4 Bandwidth and selectivity

DECODIUM works by default on 3 kHz of audio bandwidth (50 Hz - 3000 Hz). You can modify in Setup → Audio → Bandwidth:

Caution: reducing bandwidth also reduces the number of stations simultaneously decodable. Use only if you have specific out-of-band QRM to exclude.

Chapter 7 — Advanced Features

7.1 CALL — Direct call (v1.0.262)

The 📞 CALL button is the v1.0.262 novelty. It’s located in the TX toolbar next to ACQ, and allows calling a specific callsign without waiting for their CQ.

7.1.1 When CALL is useful

CALL solves three concrete scenarios:

1. Skeds with friends — agree on a time, they don’t CQ, you call them directly
2. Fast replies to missed CQs — DECODIUM decoded a CQ but you clicked 100ms too late
3. Targeted DXpedition activations — call repeatedly to a rare station without waiting for their CQ

7.1.2 CALL workflow step-by-step

1. Click 📞 CALL — the CallDialog opens
2. Enter the target callsign — e.g. F4CQS, JA1XYZ, VK7AB
3. Configure parameters:

4. Press ▶ Start
5. Decodium starts calling at the next available slot: F4CQS IU8LMC JN71
6. The CALL button in the toolbar turns green with tooltip retry N/M
7. Edge-detector on transmittingChanged counts actual retries (not attempts failed by CAT errors)
8. Automatic stop if:
   • Max retries reached
   • Timeout expired
   • The target replies (automatic handoff)
9. If the target replies, natural handoff to existing autoSeq flow: R-XX → RR73 → log

7.1.3 Operating conventions and etiquette

CALL is a powerful tool. Misusing it can create QRM. Guidelines:

🚫 Don’t use CALL: - To call stations clearly in QSO. Wait for their 73. - To call DXpedition in tight pile-up without coordination. You just create QRM to the existing pile-up. - Continuously in series toward 10 different stations. Useless split is chaos.

✅ Use CALL: - When you have a confirmed sked and the time has come - For fast replies to CQs decoded but missed by a few ms - When you want to complete an interrupted QSO with a previous station (calls + grid already exchanged) - In EmComm where calling directly is standard practice

7.1.4 CALL ↔︎ AutoSeq ↔︎ Hold interaction

7.2 Call Roster

The Call Roster is a dedicated window that automatically tracks callsigns heard during the session.

7.2.1 Activate the Call Roster

Hamburger menu (☰) → Tools → Call Roster. Or shortcut F7.

The window shows three tabs:

• Heard — all stations decoded in the current session
• Calling — stations currently calling CQ
• Roster — your personal “watch” callsign list (see 7.2.3)

7.2.2 Roster columns

7.2.3 Personal roster

Right-click on a station → Add to roster. The personal roster is a list of “interesting stations” you want not to miss.

DECODIUM emits audio/visual alert when a station from your roster decodes. Configurable in Setup → Roster → Alerts.

Tip: Use the roster for Most Wanted DXCC, contest target list, recurring sked partners.

7.3 DX Cluster

7.3.1 Cluster connection

In Setup → DX Cluster:

7.3.2 Cluster filters

DECODIUM applies filters to avoid being overwhelmed:

• By band — only active band or multi-band
• By mode — FT8, FT2, FT4, CW, SSB, etc.
• By DXCC — only entities of interest
• By distance — exclude local clusters (e.g. > 1000 km)
• By age — only recent spots (e.g. last 5 min)

7.3.3 Spotting

You can spot stations you decode: right-click on a decode → Spot to cluster. DECODIUM sends a spot to the connected cluster.

7.4 PSK Reporter

PSK Reporter is the worldwide “who hears whom” database. DECODIUM automatically uploads all valid decodes.

7.4.1 Enabling

In Setup → PSK Reporter:

• Enable upload — ✓ recommended
• Upload interval — 5 minutes (default)
• Include FT2 decodes — ✓ (requires updated PSK Reporter plugin)

7.4.2 Upload verification

In Help → PSK Reporter status you can see:

• Last upload performed
• Number of decodes uploaded in the session
• Any HTTP errors

Privacy: PSK Reporter publicly exposes your callsign + grid + station heard. If you don’t want this, disable upload. It’s entirely optional.

7.5 Astro Tools

Astro is the propagation tools suite integrated in DECODIUM.

7.5.1 Solar dashboard

In toolbar → Astro. The dashboard opens with:

• Solar Flux Index (SFI) — HF propagation indicator
• K-index and A-index — geomagnetic disturbance
• Sunspot Number — solar spots (SFI proxy)
• MUF/LUF map — Maximum/Lowest Usable Frequency in real time

7.5.2 Greyline tracker

DECODIUM displays the terminator line (greyline) on Live Map. Stations near the line are propagatively favored for DX on HF.

7.5.3 Auroral oval

When K-index > 4, DECODIUM highlights the auroral oval area on Live Map (high polar absorption). Useful for those seeking EME / VHF contacts.

7.6 In-depth Live Map

The Live Map isn’t just decorative — it has operational features:

7.6.1 Map filters

Buttons above the map:

• IN→ME — only stations receiving you (requires PSK Reporter)
• ME→DX — only stations you decode
• BAND — only active band
• All — everything

7.6.2 Click on a dot

Single click on a station → filters Full Spectrum on that callsign. Double click → opens QRZ.com (configurable in Setup → Lookup) for details.

7.6.3 Counters

Bottom-right of the map:

• Active paths — number of active paths this slot
• In band — stations decoded on current band
• UTC clock — UTC time synced with system

Chapter 8 — UI Customization

DECODIUM 4.0 is highly customizable. This flexibility is designed to accommodate different operating styles: from the contester wanting everything at a glance, to the home operator preferring a relaxing interface.

8.1 Themes

In Setup → Interface → Theme:

The theme changes immediately without restart.

8.2 Waterfall palette

In top toolbar → Palette. Four options:

Tip: To search for weak signals, use Shannon Dark. To quickly identify strong stations in a pile-up, use Heat.

8.3 Font scaling

In top toolbar → Aa 100%. Three sliders:

• A− reduces global font size
• 100% restores default
• A+ increases global font size

Range: 80% → 150%. Useful for operators with visual issues or for public demonstrations.

8.4 Compact mode

For multi-monitor setups or small screens, Setup → Interface → Compact mode enables:

• Reduced internal padding
• Toolbar collapse (icons only)
• Hides redundant labels
• Maintains accessibility via tooltip

8.5 Pop-out windows

All main windows can be detached from the main window:

• Live Map → Pop button top-right
• Full Spectrum → Pop top-right
• Signal RX → Pop top-right
• Call Roster → Tools menu → Pop-out

Pop-out windows maintain live decoder connection. Useful for:

• Multi-monitor setup where you want the map on monitor 2
• Streaming/video where the map is a separate scene
• “Only watch Signal RX and hide everything else” operations

8.6 Multi-instance (MultiRig CLI)

DECODIUM 4.0 supports parallel multiple instances for stations with multiple radios. Launch via CLI:

# Instance 1 - main radio
decodium --instance=1 --rig-port=COM3 --config=primary.ini

# Instance 2 - secondary radio
decodium --instance=2 --rig-port=COM4 --config=secondary.ini

Each instance has:

• Separate configuration (distinct .ini)
• Separate ADIF log
• Independent windows
• Separate audio device

Important: The two instances must not share the same sound card. Use two separate USB CODEC interfaces.

8.7 Interface languages

In Setup → Interface → Language:

• 🇬🇧 English (default)
• 🇮🇹 Italiano
• 🇪🇸 Español
• 🇩🇪 Deutsch
• 🇹🇷 Türkçe

Language change is immediate without restart. Translated: all menus, dialogs, tooltips. Not translated (because technical): parameter names (SNR, DT, Freq), protocol terms (CQ, RR73, 73), and standard ham mode abbreviations.

8.7.1 Contributing translations

Translations are Qt Linguist .ts files. To contribute: 1. Fork the GitHub repository 2. Modify translations/decodium_xx.ts with Qt Linguist 3. Pull request

Translators are credited in the program’s About page.

Chapter 9 — Troubleshooting

This chapter collects real issues reported by the Telegram community during Public Beta. For each: symptom, probable cause, tested solution.

9.1 CAT problems

9.1.1 “Cannot open port”

Symptom: at startup or pressing Test CAT, error Cannot open port COM3 (or Linux/macOS equivalent).

Possible causes: 1. Another program is using the same port (WSJT-X, JS8Call, FlDigi) 2. Missing permissions (Linux: not in dialout group) 3. USB-serial driver not installed or wrong 4. Defective or unplugged cable

Solutions:

Step 1: Close all other ham software and retry
Step 2: Verify the port exists
  - Windows: Device Manager → Ports (COM and LPT)
  - Linux: ls -l /dev/tty* | grep USB
  - macOS: ls /dev/cu.*
Step 3: On Linux, add user to dialout group:
  sudo usermod -aG dialout $USER
  (then logout/login)
Step 4: Unplug and replug the USB cable
Step 5: Update USB-serial driver (FTDI, Silicon Labs CP210x, Prolific)

9.1.2 “Timeout reading from rig”

Symptom: CAT connects but then errors with Timeout.

Probable cause: wrong baud rate (radio says X, software says Y).

Solution: 1. Go to the radio menus and read the configured baud rate 2. In DECODIUM, Setup → Radio → Baud rate, set the same value 3. Press Test CAT

Common values: - Kenwood TS-590: 57600 (recommended) - Yaesu FT-991A: 38400 - Icom IC-7300: 19200

9.1.3 “Mode unexpectedly toggles to USB”

Symptom: you’re operating in DATA-U but on every TX the radio switches to USB.

Cause: known bug fixed in v1.0.261. If on previous version, upgrade.

Solution: download v1.0.262 (or at least v1.0.261). The fix ensures DATA-U/D1 preserved across TX/RX transitions.

9.1.4 HRD doesn’t reconnect after restart

Symptom: close DECODIUM and HRD, reopen DECODIUM, HRD not detected.

Cause: HRD must be started and connected BEFORE DECODIUM retries the bridge.

Solution (v1.0.261+): the new “last-successful CAT retry path” logic automatically attempts HRD at startup. If still failing: 1. Start HRD manually 2. In HRD, verify TCP server active (Tools → TCP Server → Status) 3. In DECODIUM, click Test HRD to force reconnection

9.2 Audio problems

9.2.1 No decodes visible

Symptom: everything seems OK, but no decodes appear for minutes.

Sequential diagnosis:

Check 1: Correct band?
  Standard FT8 bands:
    40m = 7.074 MHz
    20m = 14.074 MHz
    15m = 21.074 MHz
    10m = 28.074 MHz
  If radio is on 7.080 (FT2 band), no FT8 there.

Check 2: Audio IN VU meter moving?
  Open `S:` indicator at bottom-left.
  Bar still → audio IN problem
  Bar in red zone → audio too high (clipping)
  Bar in green zone → audio OK, problem elsewhere

Check 3: TIME SYNC white or red?
  If red, sync NTP. Without sync, no FT8 decoding.

Check 4: Coherent mode?
  Radio display says USB-DATA? Top-left DECODIUM says FT8?
  If radio in CW or LSB, no digital audio decoding.

Check 5: Filters active?
  Setup → Filters: aggressive FDR? Disable and retry.

9.2.2 Audio too loud (clipping)

Symptom: VU meter in red zone, sporadic decodes.

Solution: 1. On the radio: lower USB IN level (Kenwood Menu 64, Yaesu Menu 113, Icom USB AF Output Level) 2. On the PC: Audio control panel → USB CODEC microphone → reduce level 3. Target: VU in green zone, peaks not above -10 dB

9.2.3 PTT doesn’t fire

Symptom: you press TX or TUNE but the radio doesn’t go into transmit.

Diagnosis:

Check 1: Coherent PTT method?
  Setup → PTT → method. If CAT, must match CAT port.
  If RTS/DTR, you must have a PTT cable connected to the radio side.

Check 2: Test PTT works?
  Setup → PTT → Test PTT
  If yes → problem in TX message or SEQ
  If no → wiring/PTT configuration problem

Check 3: Correct RTS/DTR semantics?
  Empty = DECODIUM doesn't touch the line
  ON = forces high (line always active)
  OFF = forces low (line always inactive)
  If you have a PTT cable wanting "high = TX", set ON.

9.3 Live Map problems

9.3.1 Empty map

Symptom: Live Map shows no stations.

Causes: 1. Internet not connected → DECODIUM downloads map tiles from OpenStreetMap 2. Firewall blocking DECODIUM 3. Decodes still 0 → Live Map shows only after valid decodes

Solutions: 1. Check internet connection (ping tile.openstreetmap.org) 2. Add DECODIUM to firewall exceptions (Windows Defender, ufw on Linux) 3. Wait at least 30 seconds after the first decode

9.3.2 Slow or “jerky” map

Symptom: map lags, zoom is jerky.

Cause: GPU rendering not accelerated.

Solution (Windows): 1. NVIDIA / AMD Control Panel → 3D settings 2. Add decodium.exe with “High performance” profile 3. Restart DECODIUM

Solution (Linux):

export QSG_RENDER_LOOP=threaded
./Decodium-1.0.262-x86_64.AppImage

9.4 Application startup problems

9.4.1 “QML import failed”

Symptom: at startup, error with failed QML import.

Cause: Qt 6.11 version not available on system (Linux), or corrupted QML cache.

Solution: 1. Delete QML cache: ~/.cache/Decodium/qmlcache/ 2. Restart

On Linux, if persistent, verify the AppImage is not on noexec filesystem:

mount | grep noexec  # if current folder is noexec, move the AppImage

9.4.2 Crash on first launch (macOS)

Symptom: click DECODIUM, window appears and disappears.

Cause: Gatekeeper blocking execution.

Solution:

xattr -cr /Applications/DECODIUM.app
open /Applications/DECODIUM.app

9.5 Log files and debug mode

9.5.1 Where are the logs

Three main files:

• decodium.log — general application log
• cat.log — CAT communications (only with CAT debug active)
• decoder.log — decoder log (rarely useful)

9.5.2 Enable debug mode

In Setup → Advanced → Debug logging, enable:

• CAT debug — logs every CAT command sent/received
• Audio debug — logs audio levels per slot
• Decoder debug — logs multi-pass details

Important: debug mode generates huge files (hundreds of MB per hour). Enable only for active troubleshooting, then disable.

9.5.3 Bug report

To report reproducible bugs:

1. Reproduce the bug with debug mode ON
2. Open Help → Report bug
3. Logs are automatically attached
4. Description: what you were doing, what happened, what you expected

Bug reports with logs → high priority in community tracking.

Chapter 10 — Appendices

A. Keyboard shortcuts

B. decodium.ini configuration file

The main configuration file resides in:

B.1 Main sections

[Station]
MyCall=IU8LMC
MyGrid=JN71DC
Operator=
DXCC=auto

[Radio]
RigType=KenwoodTS590
RigPort=COM3
RigBaud=57600
StopBits=1
HRDBridge=false

[Audio]
AudioIn=USB Audio CODEC
AudioOut=USB Audio CODEC
SampleRate=48000
BufferSize=1024

[PTT]
Method=CAT
PortShared=true

[FT2]
EnableASYMX=false
EnableQQ=true
EnableDEEP=true

[Filters]
FDR=true
SpectralMask=true
SlidingAGC=false

[UI]
Theme=ShannonDark
WaterfallPalette=SDRClassic
FontScale=100
Language=en

B.2 Manual edits

Edit decodium.ini with a text editor (DECODIUM must be closed).

Automatic backup: at every startup, DECODIUM creates decodium.ini.bak. If you modify and something goes wrong, restore from .bak.

C. FT2 technical specifications

For those wanting to understand or implement the protocol:

C.1 Modulation

• Type: 4-GFSK (4-tone Gaussian Frequency Shift Keying)
• Nominal tone: audio center (e.g. 1500 Hz)
• Spacing between tones: 41.667 Hz
• Total RF bandwidth: 167 Hz
• Gauss BT filter: 1.0 (inter-symbol smoothing)

C.2 Timing

• Symbol duration: 24 ms (1/41.667 baud)
• Total frame: 79 symbols = ~1.896 s
• Costas sync: 16 symbols at edges (7+9)
• Payload: 174 raw symbols (= 87 effective symbols with FEC, 77-bit payload)
• T/R cycle: 3.75-3.8 s (guard time included)

C.3 FEC

• Code: LDPC (174, 91) — rate ~0.52
• Decode algorithm: Normalized Min-Sum, 20 iterations max
• CRC: CRC-14 (polynomial 0x6757)
• Correction capacity: ~10 errored bits / 174

C.4 Payload

• Useful length: 77 bits
• Supported formats:
  • Standard QSO (callsign1, callsign2, grid/report)
  • Telemetry (numeric sequence)
  • Free text (max 13 characters)
  • ARRL field day, DXpedition mode

C.5 ASYMX

• Specification: 100% compatible extension with the FT2 standard physical layer
• Implementation: free TX without even/odd slot wait
• Synchronization: lives in the signal’s Costas sequence, not in the station

D. Credits and license

D.1 Development

• Martino Merola IU8LMC — protocol designer, project lead
• Salvatore Raccampo 9H1SR — lead C++ developer, macOS/Linux builds
• LU7DID — Raspberry Pi port

D.2 Technical heritage

• Joe Taylor K1JT — FT8/FT4/JT65/WSPR protocols (WSJT-X), LDPC mathematics
• The Qt Company — UI framework (Qt 6.11)
• OpenStreetMap — Live Map tile data
• Hamlib team — CAT library

D.3 License

DECODIUM 4.0 is released under GPL v3. Complete source code:

• Main repository: https://github.com/iu8lmc/Decodium-4.0-Core-Shannon
• Salvatore mirror: https://github.com/elisir80/Decodium-4.0-Core-Shannon

D.4 Standards

• ADIF 3.1.7 — unanimous 22:0 vote of March 22, 2026 for SUBMODE=FT2
• Graham G3ZOD — ADIF secretary, certification mediator

E. Changelog v1.0.0 → v1.0.262

End of the Complete User Manual. For advanced technical documentation (full API reference, complete troubleshooting, decoder internals, complete FT2 protocol specs), see the Reference Manual.

73 de Martino IU8LMC & Salvatore 9H1SR DECODIUM / FT2 Team — ARI Caserta · Italy · GPLv3