🚀 System Architecture

A unified server architecture with multiple client connection options.

Network Ports

🔊 TX Subsystem

One shared transmit pipeline on the server. Clients never touch PTT or the soundcard directly. They remote-control the server’s TxEngine. Supports CW, RTTY, PSK31, voice, SSTV (19 modes), and WEFAX (IOC 576/288 at 60/120 LPM). All TX settings are server-owned and broadcast to every client via the TX:CONFIG protocol.

A single-owner lock ensures only one client keys the radio at a time. Text modes go through an encoder; voice pipes PCM straight into the same output buffer.

Digital-mode TX lifecycle

TX:SEND is the normal flush-and-stop path. TX:UNKEY aborts mid-character; TX:ABORT releases a stuck lock from any caller.

Voice TX — binary uplink handshake

The token pattern lets the voice uplink stream with no per-packet framing overhead while still enforcing “only the current lock owner writes audio.”

Lock state machine

Only one client can own the lock at a time. Non-owners get TX:ERROR:NOT_OWNER on any TX:TEXT/SEND/UNKEY/AUDIO attempt.

📷 Screenshots

See ShackLink in action across different platforms.

🛠 Installation

Get up and running in minutes with these simple steps.

Step 1: Install OmniRig v2.0 or later

OmniRig v2 is required for up to 4-rig support.

Step 2: Install ShackLink Server

1. Download the latest release from ditdots.com
2. Extract all files to a folder (e.g., C:\ShackLink)
3. Run ShackLinkServer.exe
4. When prompted by Windows Firewall, click Allow Access

Step 3: Configure Firewall (Tools → Configure Firewall)

Allow incoming connections on these ports:

• 8080 – TCP radio control
• 8081 – TCP audio streaming and UDP auto-discovery
• 8082 – Voice TX uplink (LAN)
• 8084 – Unified HTTP + WebSocket (web client, HiFi MP3, /ws) — only port needed for remote access
• 8085 – SDR spectrum stream (LAN)

Step 4: Connect Clients

Open a browser to http://[SERVER_IP]:8084 for web client, or install desktop/mobile client and enter server IP.

🔬 Lab Measurements (RF Workstation)

Professional multi-trace spectrum analysis built on up to 8 simultaneous SDR sources. Available on both the server and the desktop client.

Multi-Trace Overlay (8 Traces)

The Lab tab provides a SkiaSharp-rendered spectrum display with up to 8 independent traces, each bindable to a different SDR slot. Per-trace controls include:

• Trace modes: ClearWrite (live), MaxHold (peak envelope), MinHold (noise floor), Average (configurable N), RMS
• Math traces: A+B and A−B operations between any two traces for comparison and delta measurements
• Per-trace gain offset (−60 to +60 dB) and noise floor (−160 to −20 dB) via slider columns in the Traces grid
• Color customization: independent trace color and peak-hold color per trace, chosen via color picker

Zoom & Stitched Multi-Band View

The mouse wheel zooms at the cursor on both the analyzer and the waterfall (Ctrl+wheel = gain, Ctrl+Shift+wheel = floor), backed by a zoom history (Zoom Back via right-click or the mouse’s back button), a Zoom Out ×2 action, a log-scale toolbar Zoom slider, and draggable navigator edge grips. Stitch mode deletes the empty gaps between disjoint SDR bands and splices the covered ranges into one continuous axis: a 40 m slice directly beside a 20 m slice.

• Stitch segments (kept ranges): each trace can contribute just a chosen slice of its span to the stitched view, so you compose exactly the multi-band picture you want (e.g. only the FT8 windows of three bands, side by side).
• Segment lane editor: the Segments toolbar toggle opens one lane per visible SDR trace: a live mini-spectrum of the slot’s full span with the kept range as a draggable band (drag edges to resize, body to move, empty space to draw, double-click for full span). The plot re-stitches live while you drag.
• Quick actions: or simply zoom into a slice and right-click → Stitch Segment ▸ Keep Current View.
• Honest by design: segments are render-side only (markers, measurements, and exports still read the full trace at real frequencies), kept ranges survive retunes (they ghost while the slot is away and re-apply when it returns), and no LOs move, so it works on rig-tracking and IF-locked slots too.

Marker System (8 Markers)

Up to 8 simultaneous markers with real-time frequency and amplitude readout:

• Normal: frequency and amplitude at the marker position
• Delta: frequency and amplitude difference between two markers
• Band Power: integrated power within a specified bandwidth
• Noise: noise density in dBFS/Hz
• Peak search / min search: jump to the highest or lowest point, navigate left/right between peaks
• N-dB bandwidth: find the −3/6/10/20/40/60 dB bandwidth around the current marker

The spectrum right-click menu adds Place Marker Here (drop a marker at the clicked frequency) and Clear All Markers (remove every placed marker and clear the marker grid in one action).

Automated Measurements

An eight-row Measurements grid (a label tab beside Markers) computes a chosen measurement per trace on every sweep, with blank-for-auto parameters and live Result/Detail columns; a quick readout also appears in the toolbar. Available measurements:

A Log to CSV toggle on the Measurements tab appends every enabled row to %LocalAppData%\ShackLink\Lab\measurements.csv about once per second for trend analysis. Readings are relative (dB/dBFS/dBc/%/Hz) by default.

Amplitude calibration (approximate dBm). There is no traceable power calibration, so the Lab is relative (dBFS) by default. You can assert an approximate reference per SDR slot: set the amplitude unit to dBm, then type a per-slot offset or use the Cal drop-down’s “Set from marker” helper (offset = known dBm − marker dBFS). Absolute readouts then show dBm*: the * marks it user-asserted, not traceable. Relative metrics are unaffected. Any gain change on the slot (MGC / LNA / AGC, conservatively IF gain) marks the calibration stale and reverts it to dBFS, so a stale offset never silently lies; the server also broadcasts a per-slot gain-change pulse so remote desktop clients invalidate their own calibration.

Display Modes

• DPX persistence: temporal density display showing signal persistence over time. Color palettes (unified with the SDR tab): Classic (blue), Classic (green), CoolWarm, Viridis, Plasma, Inferno, Grayscale, Neon (cyan)
• Spectrogram waterfall: time-frequency display with gain, contrast, floor, and speed controls. Synchronized frequency axis with the main spectrum. Toggles to a 3D perspective terrain view of the spectrogram history (GPU-accelerated, with a silent CPU fallback)
• Spectral CCDF: complementary cumulative distribution of spectrum-bin amplitude vs the mean (frequency-domain spectral peak-to-mean, not time-domain PAPR / crest factor)
• PSK constellation + EVM/MER: live I/Q scatter of recovered PSK31 symbols (2 points BPSK-31, 4 QPSK-31) with persistence, ideal-point overlay, lock indicator, and a true modulation-quality readout (Error Vector Magnitude %, Modulation Error Ratio dB). PSK31-only; honestly shows “—” when not locked. Computed server-side and streamed to the desktop client. A VSA summary adds peak EVM, a magnitude/phase-error split, residual carrier frequency error (after AFC), and I/Q offset, with a 60-second EVM trend
• Audio SINAD: true signal-to-noise-and-distortion ratio of a steady tone in the demodulated audio of the active SDR slot (a CW/SSB beat-note, an AM/FM tone, or a test tone). Measured from the actual demod audio, not the spectrum; a ratio, so no calibration needed; honestly shows “—” with no tone. Computed server-side and broadcast to the desktop client

Instruments (Dedicated Measurement Windows)

An Instruments toolbar dropdown opens focused, single-purpose measurement windows — each a popout running alongside the analyzer, modeled on professional spectrum-analyzer application firmware. All but the Zero-Span Scope and the recorder also run on the desktop client.

Advanced Tools

• Transfer function (S21): two-SDR stimulus/response measurement with calibration and limit-mask overlays. Export as Touchstone S1P/S2P
• Limit masks: regulatory and user-defined spectral masks for compliance visualization (spurious emission, harmonic suppression, bandpass)
• C# scripting: Roslyn-powered scripting engine with a MeasurementApi providing programmatic access to traces, markers, and measurements. Editor in a separate popup window with example scripts
• Cable loss compensation: predefined cable types and custom entry for length-based loss correction
• Export: CSV (all traces with frequency/amplitude columns), Touchstone S1P/S2P, PNG screenshot, and measurement log

Tune From Spectrum

Enabled from the spectrum right-click menu (Tune From Spectrum), this turns the passive Lab display into an interactive listening surface that drives the SDR back-end:

• Select a trace: click a trace to highlight it; audio-follow switches to that trace’s SDR slot, so you now hear that device.
• Tune within the band: click again inside the selected trace’s band to move that slot’s demodulator (the listen point) to the clicked frequency.
• No re-center: tuning moves the listen point only; the SDR LO is not re-centered, so the trace stays put instead of jumping (previously the LO auto-centered on the click, which shifted the whole trace by megahertz on wide / FM-band spans). You tune within the currently captured span; to listen outside it, use Band Browse to move the window.
• Overlays: the selected trace can show a VFO frequency readout (Show Frequency) and a passband box (Show Passband).
• Peak markers: mark the top-N peaks on the selected trace. The Peak Markers submenu configures count (3–10), color, size (2–8 px), style (filled circle / outline / square / diamond), and optional frequency labels.

Band Browse

Active while Tune From Spectrum is on, Band Browse shifts a slot’s SDR LO so you can sweep across the band:

• Grab and drag a trace line horizontally, or click the plot then press Left/Right arrow keys, to shift that slot’s LO.
• Held listen frequency: if the slot is the audio-active (listening) one, the absolute listen frequency is held by compensating the demod offset. Audio mutes when the held frequency leaves the captured span and resumes when a later shift brings it back into span.
• Step size: with arrow keys, Shift = ×10 coarse step, Ctrl = fine step.
• Refused for rig-tracking slots and IF-panadapter mode, with a brief on-display “Locked” notice.

Shift Station to Edge

Also part of Tune From Spectrum, Shift Station to Edge parks a slot’s tuned signal at the left or right edge of its span (the server computes the LO from the listen frequency and demod mode). Pair a Right shift on a lower trace with a Left shift on a higher trace to butt two bands together for a wide, gap-free side-by-side view.

Popout Independence (Link Toggle)

The Lab popout window carries a Link toggle. Linked, it mirrors the tab: mode, settings, and data stay in lock-step. Unlinked, the popout becomes fully independent, with its own zoom, amplitude scale, palette, 3D camera, and pane layout (all persisted separately), so you can keep a wide overview on the tab and a zoomed detail view in the popout at the same time.

No-Data Legend Status

A checked (visible) trace whose SDR source is not delivering data still appears in the spectrum legend, its name shown dimmed with a short status word (Not started, No data, or Unavailable) in place of a frequency range, so you can see the trace is selected but waiting on its device.

Desktop Client Support

The full Lab tab is available on the desktop client with identical functionality. All UI code is shared via <Compile Include> links. The client’s SdrStreamPlayer feeds the same TraceEngine with SDL1 frames from the server’s spectrum stream (TCP 8085). Persistence, popout, scripting, transfer function, and export all work identically on both server and client.

Architecture

The Lab tab is a passive overlay on existing SDR spectrum data by default. When Tune From Spectrum is enabled it can select the audio slot (which device you hear), tune a slot’s demodulator listen point, and (for Band Browse) shift a slot’s LO; it still never alters the SDR capture pipeline or FFT. The desktop client uses dedicated commands for these actions (SDR:<n>:DEMODTUNE:<absHz> / SDR:<n>:LOSHIFT:<Hz> over TCP, sdrDemodTune / sdrLoShift over WebSocket); the server’s own SDR-tab panadapter click still re-centers as before, so only the Lab tune is no-re-center. The engine library (ShackLinkLab, netstandard2.0) provides TraceEngine, MarkerEngine, MeasurementEngine, PersistenceEngine, TransferFunctionEngine, LimitMaskEngine, CcdfEngine, and ScriptHost. UI rendering uses SkiaSharp with double-buffered offscreen compositing and GDI+ blit. Total CPU budget at full load (8 traces, 8 markers, persistence, spectrogram): ~15–20 ms per 33 ms frame.

🔢 Digital Decoders

The server includes built-in decoders for eight digital modes: CW (Morse), RTTY, PSK31, FT8/FT4 (relayed from WSJT-X), SSTV, WEFAX, NAVTEX, and APRS. Select the decoder mode from the dropdown to switch between them: the choice is broadcast to every connected client.

Morse (CW) Decoder

1. Select CW (Morse) from the decoder mode dropdown
2. Enable the decoder checkbox
3. Choose decoder algorithm: Speed Priority or Accuracy Priority
4. Decoded text appears in the display and streams to clients

RTTY Decoder

1. Select RTTY from the decoder mode dropdown
2. Enable the decoder checkbox
3. The decoder auto-detects baud rate (45.45 or 50 baud) and shift (170/425/850 Hz)
4. Monitor signal quality and lock status indicators
5. Decoded text appears in the display and streams to clients

PSK31 Decoder

1. Select PSK31 from the decoder mode dropdown
2. Pick a modulation mode (9 variants):
   • BPSK-31 / BPSK-63 / BPSK-125: binary PSK at 31.25, 62.5, or 125 baud
   • QPSK-L-31/63/125: quadrature PSK with Viterbi FEC, lower sideband
   • QPSK-U-31/63/125: quadrature PSK with Viterbi FEC, upper sideband
3. Set the center frequency (200–3000 Hz) to match the signal in the audio passband
4. Enable AFC to automatically track frequency drift
5. Enable FEC for Viterbi forward error correction on QPSK modes
6. Adjust squelch to filter noise when no signal is present
7. Decoded text appears in the display and streams to clients

SSTV Decoder

Receives Slow Scan Television images on voice modes (typically USB). The SSTV panel embeds a split view directly in the Decoder tab: decoded image on the left, live waterfall of the SSTV audio band (1200–2300 Hz) on the right, with a draggable splitter between the two so you can rebalance their sizes. A thin status strip at the bottom shows detected mode, current line, and progress.

1. Select SSTV from the decoder mode dropdown
2. Tune to a known SSTV frequency (e.g., 14.230 MHz USB on 20 m)
3. The decoder auto-detects the mode from the VIS header and renders the image line-by-line alongside the waterfall
4. For weak signals where VIS detection fails, pick a Forced Mode (e.g., Robot36). Choose AUTO to return to detection.
5. Right-click the image for Save / Clear / “Expand to Window” (the last opens a detachable full-size decoder window with the same layout).
6. Clients that connect mid-decode receive a catch-up stream so their display syncs with the server.

Supported modes (19 total): Robot 36 / 72 / 8BW / 24BW, Martin M1 / M2, Scottie S1 / S2 / DX, PD 90 / 120 / 160 / 180 / 240 / 290, Wraase SC2-120 / SC2-180, Pasokon P3 / P5 / P7.

FT8 / FT4 (via WSJT-X)

The server does not decode FT8/FT4 directly. It listens on UDP port 2237 for decode messages from an WSJT-X instance running on the same machine, and forwards each decode to connected clients.

1. Run WSJT-X configured for your radio as usual
2. In WSJT-X: File → Settings → Reporting → enable UDP Server on port 2237
3. On the server, select FT8/FT4 from the decoder dropdown
4. Decodes (time, SNR, DT, Δf, message) appear in the list and are broadcast to clients with SNR-based color coding

WEFAX Decoder (Weather Facsimile)

Receives grayscale weather charts on HF marine bands. The WEFAX panel uses the same split layout as SSTV: image on the left, live waterfall of the WEFAX audio band on the right, draggable splitter, status strip showing line / progress at the bottom. Slant / shift sliders correct any residual alignment drift.

1. Select WEFAX from the decoder dropdown
2. Tune to a known WEFAX broadcast (e.g. NOAA: 4346 / 8503.9 / 12789.9 kHz USB; DWD Hamburg: 3855 / 7880 / 13882.5 kHz USB)
3. The decoder runs a phasing-lock detector first: looks for the white phasing pulse pattern that precedes the image and aligns the line phase from it. Mid-stream joins fall back to the legacy fold/cluster sync path automatically.
4. Use the slant / shift sliders on noisy captures with broadcaster clock drift
5. Save / Clear / Expand to Window via the toolbar or right-click on the image

NAVTEX Decoder (Maritime Safety)

Receives NAVTEX maritime safety broadcasts on 490 / 518 / 4209.5 kHz. The decoder implements the SITOR-B forward error correction protocol (CCIR 476, 100 baud, 170 Hz shift).

1. Select NAVTEX from the decoder dropdown
2. Tune to a NAVTEX broadcast frequency (e.g. 518 kHz for international English, 490 kHz for national language)
3. Decoded characters appear in the text display and stream to all connected clients

APRS Decoder (Packet Radio)

Decodes AX.25 1200 baud Bell 202 AFSK packets. Displays callsign, position, and message data from APRS beacons.

1. Select APRS from the decoder dropdown
2. Tune to 144.390 MHz (North America) or 144.800 MHz (Europe/most regions) on FM
3. Decoded packets appear as one-line summaries showing source callsign, position coordinates, and beacon text

Image TX (SSTV + WEFAX)

The TX tab supports sending images directly from the server (or any client). When you switch the TX-mode picker to SSTV or WEFAX, the chat panel swaps in an ImageTxPanel:

1. Pick mode (SSTV: 18 sub-modes; WEFAX: IOC 576/288, LPM 60/120)
2. Choose Image: the panel resamples and shows a preview
3. Click Send: the encoder runs, PTT keys via OmniRig, the live build-up appears in the preview as the encoder transmits
4. Test (no RF) mode: encoder runs but PTT stays low, for offline rehearsal. A WAV is saved next to the source image automatically.
5. Non-transmitting clients see TX:STATUS and TX:PROGRESS so they can follow the line counter

From a remote client (web / mobile), the image is uploaded chunked over the WebSocket binary channel 0x04, SHA-256 verified, then staged server-side (per-client cap of 4, 10-min idle GC, 5 MB max). After staging, TX:KEY:SSTV:<imageId>:<mode> or TX:KEY:WEFAX:<imageId>:<ioc>:<lpm> kicks off the encoder.

Saving Decoded Text

Click the Save… button to save decoded text to a file. The default filename includes the decoder mode (cw/rtty/psk31) and timestamp. SSTV images can be saved via the Save button above the split, by right-clicking the image, or from the detached “Expand to Window” view.

📍 Band Plan (Frequency Table)

Built-in searchable reference of amateur allocations and common operating frequencies.

Click Freq. Table in the Audio Streaming panel to open the Band Plan window. It stays open alongside the main UI so you can hop around the band quickly.

Using the Band Plan

• Filter by band category (HF, VHF, UHF) and by mode with the quick-filter buttons
• Search by frequency, band name, or description
• Select a row and click Tune (or double-click) to set the radio to that frequency and mode
• The current row auto-highlights as the radio frequency changes
• Calling frequencies are visually distinguished

🔧 Troubleshooting

OmniRig Won’t Connect

• Check if OmniRig is running and configured properly
• Verify COM port settings match your radio
• Restart OmniRig and the server application

Clients Cannot Connect

• Verify the server is running and accessible
• Check firewall allows ports 8080, 8081, 8082, 8084, 8085 (and UDP 8081 for service discovery) — Tools → Configure Firewall does this for you
• Ensure client has correct server IP address
• Try pinging the server IP from the client machine

No Audio

• Select correct audio input device on server
• Check sound card levels in Windows Mixer
• Verify audio cables are connected
• Ensure audio streaming is started on server
• Check client volume is not muted

Digital Decoder Not Working

• Enable the decoder checkbox on server
• For CW: Adjust decoder mode (Speed/Accuracy) based on signal conditions
• For RTTY: Ensure the signal uses standard amateur radio parameters (45.45/50 baud, 170/425/850 Hz shift)
• Ensure audio streaming is active
• Check audio levels (too quiet or too loud affects decoding)
• Monitor the signal scope and quality indicators