Timecode Bridge — SMPTE LTC → Art-Net

Reads SMPTE linear timecode (LTC) from any Windows audio input — ASIO, WASAPI, or WDM/WaveIn — and broadcasts it as Art-Net ArtTimeCode (UDP 6454) with sub-frame latency. Built for show-critical use.

Ready-to-run build: dist\TimecodeBridge.exe (self-contained — no .NET install required on the target machine).

Quick start

1. Run dist\TimecodeBridge.exe — the bridge is always on: it starts listening immediately and translates the moment timecode appears. There is no start button.
2. Audio input — pick the driver (ASIO for lowest latency), the device, and the channel carrying LTC.
3. Art-Net output — pick the network interface facing your lighting network, and the target IP: 255.255.255.255 broadcasts to everything; use the console's IP for unicast (recommended on busy networks).

Any setting change is applied automatically about half a second after you make it. Settings persist in config.json next to the exe.

Updating: click check for updates at the bottom right of the window — the bridge downloads the newest release from GitHub, swaps the exe in place, and offers a one-click restart. (Disabled while settings are locked, so nothing can restart the app mid-show.)

Runs in the background: closing the window minimises to the system tray — the tray dot shows live status (grey no signal · green locked · amber freewheel) and its tooltip shows the current timecode. Double-click the tray icon (or launch the exe again) to reopen the window; right-click → Quit to actually exit. Drop a shortcut in shell:startup for unattended show machines.

Behaviour details

• Frame rate is auto-detected (24 / 25 / 29.97 drop-frame / 30, incl. the LTC drop-frame flag) and mapped to the matching ArtTimeCode type. Override it in the Rate box if you need to force one.
• Offset (frames) — default +1: an LTC frame is only fully readable at the end of the frame it labels, so +1 puts the output exactly on time. Adjustable ±10 frames for receiver-side alignment.
• Freewheel — on signal loss the bridge keeps generating consecutive frames at the measured rate for the configured number of frames (default 25), then goes silent and reports Signal lost.
• Lock rule — output only starts after two consecutive, range-valid, contiguous frames decode, so noise or a corrupted feed can never emit garbage timecode.
• Device resilience — if the audio device vanishes (USB unplugged, driver reset) the engine retries it every 2 s and freewheel covers the gap. Fatal events go to timecodebridge.log (capped at 1 MB, rotates mid-run).

Show safety features

• Art-Net discovery — the bridge answers ArtPoll, so it appears by name ("Timecode Bridge") in console and network-scanner device lists.
• Backup output — optional second target IP (and interface): every frame is sent to both consoles simultaneously. Leave the Backup IP empty to disable.
• Signal-loss alert — Windows notification (optionally with sound) the moment timecode output stops or falls back to the generator; the tray dot turns red while output is dead.
• Generator fallback — when LTC dies beyond freewheel, the bridge keeps generating gapless timecode from the PC clock indefinitely until LTC returns (display shows GENERATOR in amber). If no LTC ever arrives, it free-runs from 00:00:00:00 after 3 seconds.
• Crash watchdog — a companion process relaunches the bridge within ~5 seconds if it ever crashes. Clean quits (tray menu, Windows shutdown) do not trigger it.
• Run at startup — registers the app to launch at logon (straight to work, no interaction needed).
• Settings lock — LOCK SETTINGS freezes every control so nothing can be nudged mid-show; the lock state survives restarts.
• Signal tolerance — decoder is polarity-insensitive and copes with low level (−20 dB), DC offset, noise and ±10 % varispeed.

Latency

Stage	Typical
Audio input buffer	ASIO 1–10 ms · WASAPI ~10 ms · WDM ~10–20 ms
LTC decode + Art-Net send	< 0.1 ms (decoded the instant the sync word ends)

With the default +1 frame offset the output lands within the audio-buffer time of the true frame boundary — well under half a frame even via WDM.

Engineering notes

• Decode path is allocation-free: driver callback → lock-free SPSC ring buffer → dedicated highest-priority decode thread → pre-connected UDP socket.
• Process runs at High priority with 1 ms timer resolution and sustained-low-latency GC.
• Single-instance guard; UI polls engine state at 30 Hz (no cross-thread marshalling).
• Reverse-play LTC is not decoded (by design — resyncs on forward play).

Building from source

dotnet test TimecodeBridge.sln -c Release        # 52 tests: decoder, DF math, Art-Net, engine, soak/fuzz, features
dotnet publish src\TimecodeBridge\TimecodeBridge.csproj -c Release -r win-x64 `
  --self-contained true -p:PublishSingleFile=true -p:IncludeNativeLibrariesForSelfExtract=true -o dist

Layout: src/TimecodeBridge — app (Ltc/ decoder + timecode math, Audio/ inputs, ArtNet/ sender, Core/ engine + ring buffer, MainForm.cs UI). tests/TimecodeBridge.Tests — includes a reference LTC audio generator that exercises the decoder across all rates, sample rates and signal degradation.