Skimmer for Linux

A native GTK4 multi-channel CW/RTTY skimmer for Linux — decode every signal in a band segment at once, and spot it. The free-software counterpart of CW Skimmer / SDC, built as a TCI client for sdr-for-linux.

It connects to an ExpertSDR-compatible TCI server, pulls a wideband IQ stream straight from the radio, splits it into hundreds of narrow channels and decodes them in parallel. Callsigns that validate are pushed back as spots onto the radio's panadapter (click one to tune) and served to local loggers over a CW-Skimmer-dialect telnet cluster feed.

sdr-for-linux (TCI server) ──IQ──► skimmer-for-linux ──► channelizer ──► CW / RTTY decode
        ▲                                                                       │
        └──────────────────────── SPOT (callsign @ freq) ◄──────────────────────┘
                                   also ──► local telnet cluster feed (loggers)

Twenty metres during SAC CW: 1536 channels of 125 Hz off a 192 kHz IQ stream, 59 stations tracked and 1987 spots sent. The waterfall carries a callsign column — a dot on each station's frequency, "CQ" where the station is calling — and the decode pane below follows the tuned one. Callsigns that validate are underlined; the tuned station's call is bold in the column.

What it does

• Decodes the whole segment at once. A 192 kHz IQ stream becomes 1536 channels at 125 Hz spacing in CW, or 768 at 250 Hz in RTTY; every channel runs its own decoder, squelch and frequency lock.
• CW, with a choice of engine: the soft-decision Viterbi decoder (default) or the neural DeepCW model when an ONNX Runtime is installed. The older classical decoder is still in the binary, one environment variable away.
• RTTY, 45.45 Bd / 170 Hz shift Baudot: matched filters with automatic threshold correction, automatic polarity, unshift-on-space.
• Spots to the radio. A validated callsign goes back over TCI and appears on the sdr-for-linux panadapter; clicking it there tunes the VFO.
• A local telnet cluster feed in the CW Skimmer dialect, so a logger on the same machine sees the spots as an ordinary cluster.
• A waterfall with a callsign column: frequency vertical, time flowing sideways, a dot and a call on every tracked station, click to tune.
• Logbook integration with log-for-linux: stations you have already worked are spotted in gray, and clicking a call pre-fills the logbook's entry row.
• The Super Check Partial call list keeps itself current in the background.

PSK (BPSK31/63) is planned, not implemented.

The skimmer is read-only towards the radio: it never keys and never changes radio state on its own. The one exception is tuning the VFO when you click a callsign. The telnet feed is a local source for loggers — there is no uplink to the RBN network.

Requirements

• A running TCI server with an IQ stream. sdr-for-linux is the tested one (Preferences → Radio → TCI). The client is written to the TCI specification rather than to that server's habits, so other servers should work in principle; one SunSDR / ExpertSDR3 run has been reported (#2), which is not yet a supported target.
• Linux with GTK4, libadwaita, GLib/GIO, libwebsockets, libcurl ≥ 7.85 and FFTW (single and double precision).
• To build: meson and ninja.
• Optional, for the DeepCW engine only: an ONNX Runtime shared library. It is opened at run time, never linked, so the app runs without it.

Install

Every release ships prebuilt packages on the Releases page:

• AppImage (any current distribution, glibc 2.43+): download, chmod +x Skimmer_for_Linux-*.AppImage, run.
• Ubuntu 26.04+: sudo apt install ./skimmer-for-linux_*.deb
• Fedora 44+: sudo dnf install ./skimmer-for-linux-*.rpm
• Arch Linux (AUR) — skimmer-for-linux: paru -S skimmer-for-linux. Without a helper: git clone https://aur.archlinux.org/skimmer-for-linux.git && cd skimmer-for-linux && makepkg -si. The same recipe is in this repo as packaging/PKGBUILD.

The newest release is 0.5.0.

Build from source

meson setup build
meson compile -C build
meson test -C build             # 14 offline gates, no radio needed
./build/skimmer-for-linux

To install into your user prefix, with the desktop entry, icon and AppStream metainfo, so the app appears in the app grid:

meson setup builddir --prefix=$HOME/.local
meson compile -C builddir
meson install -C builddir

skimmer-for-linux --version prints the version and exits.

Connecting to the radio

Start the TCI server first, then the skimmer; the connection is automatic. With nothing set in Preferences → Radio the skimmer looks for sdr-for-linux on this machine — over its Unix socket ($XDG_RUNTIME_DIR/sdr-for-linux/tci.sock) first, then over TCP at ws://127.0.0.1:40001. A radio elsewhere, or another TCI server, goes into the Server row as ws://host:port (40001 unless your server says otherwise); a socket moved in sdr-for-linux's preferences as unix:/path. The IQ sample rate is radio state: the server announces it and the skimmer follows, so a segment is as wide as the radio's IQ stream (48, 96, 192 or 384 kHz with sdr-for-linux).

No radio at hand? SKIM_IQ_FILE=<capture.cf32> replays a recorded IQ file through the whole pipeline at real-time pace, looping, and draws it in the window.

Using the window

Waterfall — frequency runs vertically, time flows sideways, the kHz scale sits between the picture and the callsign column, and a marker shows where the radio is tuned. Retuning the radio moves the marker, not the window.

Action	Effect
Wheel over the waterfall	Pan up and down the band
Ctrl + wheel	Zoom in and out around the pointer — the signal under it stays put
Drag the kHz scale strip	Pan
Click a callsign in the column	Tune the radio to that station, fix the decode pane on it, pre-fill the logbook
Click a callsign in the decode pane	The same
Hover a callsign in the column	Tooltip: frequency, speed, SNR, how often heard, age

A station calling CQ carries a CQ prefix in the column. A call the logbook reports as already worked — or as not scoring in the current contest — is drawn in gray instead of green, both in the column and on the panadapter.

Decode pane — below the waterfall, it follows the tuned station and shows its text as it is decoded, with the station's speed and SNR in the header. Calls the Super Check Partial list knows are underlined green (gray once worked), and the protocol words take a colour by their role: calling (CQ, TEST, QRZ) amber, DE blue, the report (5NN, 599, ENN) violet, closing (TU, 73, K, BK, KN, SK, AR) coral. Only whole words are coloured, and only once the text has firmed up — a draft tail stays gray. Preferences → Display switches the word colours off.

Preferences

Four tabs. Everything is stored in ~/.config/skimmer-for-linux/settings.ini; the key for each row is given here so the file can be edited directly.

Radio

Row	Key	Notes
Server	[tci] server	unix:/path, ws://host:port or host:port; empty (the default) = sdr-for-linux's socket, then ws://127.0.0.1:40001. A pre-0.6 host + port pair still reads: the default 127.0.0.1 / 40001 as empty, anything else as ws://host:port

Decoding

Row	Key	Notes
Mode	[decode] mode	cw or rtty — the whole segment decodes as one mode; changing it reconnects
CW engine	[decode] engine	v2 — "Classical (v2)", the soft-decision Viterbi decoder, default — or deepcw — "DeepCW (neural)"
Device	[decode] device	cpu or cuda, shown for DeepCW only
Keep MASTER.SCP current	[scp] auto_update	Default on; see below
Loaded list	—	Read-only: release and call count of the list in use

Spots

Row	Key	Notes
CQ only	[spots] cq_only	Default on: only stations calling CQ are spotted, an S&P answer does not own the frequency
Frequency step	[spots] round_hz	0 exact, or 10 / 20 / 50 / 100 Hz — snaps outgoing spots to a grid, the measured value stays exact inside the app
Telnet feed → Enable	[rbn] enabled	Default off
Telnet feed → Operator call	[rbn] call	Announced in the feed's spot lines
Telnet feed → Telnet port	[rbn] port	Default 7300

Display

Row	Key	Notes
Decode pane font size	[ui] decode_font_pt
Colour protocol words	[ui] keyword_colours	On by default; off leaves the decode text plain, validated calls stay marked. Applies at once
Colour scheme	[ui] palette	Waterfall palette: Classic, Mono white, Mono green, Mono amber, Inferno, Turbo

[ui] view remembers whether the waterfall is shown (waterfall or none).

The telnet spot feed

Enable it in Preferences → Spots, then point a logger at the machine:

$ telnet localhost 7300
skimmer-for-linux telnet feed
Please enter your call: OK1BR
OK1BR-#: Hello OK1BR, spots follow.
DX de OK1BR-#:   14043.0  UN8PT        CW    20 dB   0 WPM  CQ      0928Z
DX de OK1BR-#:   14025.6  OH3KAV       CW    31 dB  35 WPM  CQ      0928Z

The dB is SNR in 500 Hz — the scale CW Skimmer reports and RBN lists, so the figure stands beside the other skimmers' for the same station.

The feed is stricter than the panadapter: a callsign reaches the panadapter at a confidence of 0.70, the feed at 0.85, and the feed always sends CQ callers only. The score alone is not enough: one copy keyed after "CQ DE" already scores 0.90, and so does a garble of it. So a call goes out only once it was read at least twice on its channel (a call MASTER.SCP knows passes on one reading), and only after a hold of 8 s, during which the station table folds a torn reading into the full call — the line is sent from the table as it stands after the hold, and a telnet line cannot be taken back. The readings and the hold are settings in Preferences → Spots (policy after OK1HRA's measurements against RBN). A frequency outside the amateur bands (2200 m to 6 m, the widest allocation any region has) never goes out: a wide receiver span hears past the band edge, the panadapter shows it, the feed does not. A station is repeated no more often than every 600 s unless it moves. The server is app-owned, so client sessions survive a reconnect to the radio.

Logbook integration

log-for-linux answers a read-only UDP service on 127.0.0.1:2238. The skimmer asks it about every call it is about to spot and colours the spot by the answer: new stations green, stations already in the log — or invalid for the running contest — gray. Logging a QSO recolours the call within seconds, without a re-spot.

Clicking a callsign, in the column or in the pane, sends the call to the logbook as well, which pre-fills its entry row.

Nothing here is required: if the logbook is not running, every answer is "unknown", every spot is green and spotting is unaffected.

The callsign dictionary

The extractor uses the Super Check Partial list at ~/.config/skimmer-for-linux/master.scp as a dictionary boost — a decoded call found in the list gains confidence.

With Keep MASTER.SCP current on (the default), the app asks supercheckpartial.com in the background, at most once a day, whether a newer list is out; it downloads only a file that changed, checks it (checksum, call count, how much of it looks like callsigns) and swaps it into the running dictionary without a reconnect. A missing network, a slow server or a bad file changes nothing — the copy on disk stays and the app carries on. Switch it off to keep a list of your own: with the switch on, a file copied in by hand is replaced at the next check.

The DeepCW engine

decode_deepcw runs the published DeepCW model (a Conformer + CTC network, AGPL-3.0-only, not part of this repository) through ONNX Runtime. To use it:

1. Install an ONNX Runtime with a C API — on Arch, onnxruntime or onnxruntime-cuda for the GPU. SKIM_ORT_LIB=<path> points at a library that is not on the loader path.
2. Put the model at ~/.local/share/skimmer-for-linux/models/deepcw/model.onnx.
3. Preferences → Decoding → CW engine → DeepCW. The subtitle of that row says whether the engine is available on this machine.

The model reads whole words a second or two behind the keying, so its text arrives later than the classical decoder's; the uncommitted tail is shown in gray until it firms up. Without the runtime or the model the app says so in the log and keeps decoding with v2.

Files

Path	What
~/.config/skimmer-for-linux/settings.ini	All settings
~/.config/skimmer-for-linux/master.scp	Super Check Partial call list (.state records the update rate limits)
~/.local/share/skimmer-for-linux/	Decode logs, one file per day
~/.local/share/skimmer-for-linux/models/deepcw/	DeepCW model, if installed

Environment variables

Everything that matters day to day is in Preferences. These are the few switches that are not:

Variable	Effect
SKIM_IQ_FILE=<file.cf32>	Replay a recorded IQ file into the window instead of connecting (SKIM_IQ_RATE, SKIM_IQ_CENTER override the sidecar)
SKIM_CW_V1=1	Use the classical CW decoder
SKIM_CW_ENGINE=v1|v2|deepcw	Pick the CW engine for this run
SKIM_TONE_SPLIT=1	Decode two stations in one channel separately — opt-in, awaiting its live validation
SKIM_TONE_FOCUS=1	Narrow the channel filter onto a lone carrier (implies the splitter) — same status
SKIM_ORT_LIB=<path>	ONNX Runtime library for DeepCW
SKIM_SCP_URL=<url>	Take the call list from somewhere else

Licence

GPL-3.0-or-later. See LICENSE.

Author

Richard Fakenberg — OK1BR — rifak.cz