Klangbruecke

Windows tray app that bridges phone audio to the PC over the built-in Bluetooth radio — no Phone Link, no dongle.

• Music / notifications — the phone streams to the PC over A2DP; the PC renders it to any output you pick.
• Calls — cellular calls route to the PC's headset (speakers + mic) via the HFP hands-free role.

Status

Both halves and unattended reconnect work, validated on hardware with the packaged build.

• Music (Stage 0, 2026-08-04) — connects the A2DP sink, correlates the transport to the phone by Bluetooth address, routes to a chosen output.
• Calls (Stage 0, 2026-08-04) — claims the HFP hands-free role; a real call routes to the PC with audio in both directions.
• Reconnect (Stage 1, 2026-08-07) — a ConnectionManager state machine recovers unattended from a call ending, a range exit and return, sleep/resume, reboot, and a phone-initiated disconnect. It also restarts the route when the A2DP capture endpoint appears late — a case the connect path alone silently missed. This was the predecessor app's defining bug; it is fixed. FINDINGS §13, §14.

Not yet done: the full Stage 2 reconnect matrix (hand-verified scenarios) and tray selection of the call audio device.

Requires the packaged build. dotnet run is not a dev loop for the music half — AudioPlaybackConnection.TryCreateFromId kills an unpackaged process with an uncatchable access violation (FINDINGS §8).

Why this exists

Need	Existing option	Why it isn't enough
Music	AudioPlaybackConnector	Abandoned 2020, reconnect bug, music only
Calls	Thy Phone (Store, $3.69)	Works, but is not a tray app — must stay open
Both	Phone Link	Ruled out

Requirements

• Windows 10 build 19041+ (developed on 19045 / 22H2)
• .NET 8 SDK
• Windows SDK 10.0.19041.0 (for makeappx / signtool)
• A Bluetooth radio the Windows stack owns (no Zadig / WinUSB rebinding)

Install a release

Prebuilt, signed packages are on the Releases page — each carries the .msix and the public .cer it was signed with. The certificate is self-signed, so Windows refuses the package until that cert is trusted; unlike a downloaded .exe, an MSIX has no "install anyway" prompt, so this step is required (once per machine):

# elevated: trust the signing certificate
Import-Certificate -FilePath .\Klangbruecke.cer -CertStoreLocation Cert:\LocalMachine\TrustedPeople

# then from Windows PowerShell (not pwsh — the Appx module does not load there):
Add-AppxPackage -Path .\Klangbruecke-<version>.msix

Klangbruecke starts on sign-in and lives in the tray.

Build, package, install

MSIX packaging is load-bearing: package identity keeps TryCreateFromId from killing the process (§8) and carries the phoneLineTransportManagement capability the calls half needs. Sideloading needs no Microsoft approval.

dotnet build src/Klangbruecke/Klangbruecke.csproj -c Release
./packaging/New-DevCert.ps1      # once: create + trust a self-signed dev cert
./packaging/Build-Msix.ps1       # build, package, sign

Install the produced .msix with sideloading enabled (Settings → Update & Security → For developers → Sideload apps).

Cut a GitHub release (build, sign, push the commit, upload the .msix + .cer + install notes) with ./packaging/Publish-Release.ps1. The tray/app icons are generated from the brand mark by ./packaging/Generate-Icons.ps1.

Troubleshooting

• See what's happening. Right-click the tray icon → Diagnostics → Open Logs, or open %LOCALAPPDATA%\Klangbruecke\logs directly. Copy Diagnostics puts a paste-ready snapshot (version, OS, state, recent log lines) on the clipboard — review it before sharing.
• It won't connect. Check the pairing before suspecting the app. Klangbruecke shows "connected" for its own view of the connection; verify the real endpoint with PowerShell: Get-PnpDevice -Class AudioEndpoint | Where-Object FriendlyName -like '*A2DP*'. A stale pairing (the IRK trap) presents exactly like an app failure — look at BTHUSB events 35 / 16 / 24 in the System log first. See docs/FINDINGS.md §3.
• Music cuts out when a game goes fullscreen, or on screenshot overlays (ShareX, Snipping Tool), and the phone briefly switches to its own speaker. This comes from Windows, not Klangbruecke. The automatic Focus Assist rules disconnect the phone's hands-free link each time they turn on or off, and music skips while it reconnects. Fix: Settings → System → Focus assist → Automatic rules, turn off When I'm playing a game and When I'm using an app in full screen mode. See docs/FINDINGS.md §26.
• Force a reconnect. Diagnostics won't help if the app is deliberately dormant — use Connect Now to override a Disconnect or a switched-off auto-reconnect for one attempt.
• Reset configuration. Delete %LOCALAPPDATA%\Klangbruecke\settings.json and restart; the app starts from defaults (no phone selected).
• Check for a newer build. Diagnostics → Check for Updates, or see the Releases page.

Architecture

        Bluetooth (built-in radio, Windows stack)
                        |
    +-------------------+--------------------+
  A2DP sink                            HFP hands-free
  AudioPlaybackConnection              PhoneLineTransportDevice
    |                                        |
  "Line (<phone> A2DP SNK)"            call audio in/out (Windows owns the path)
  capture endpoint
    |
  WASAPI capture -> BufferedWaveProvider -> WASAPI render -> selected output

  ConnectionManager owns the lifecycle: a DeviceWatcher, an endpoint monitor,
  and a 30s reconcile loop drive connect / route / retry / reconnect.

Layout

src/Klangbruecke/
  Program.cs                         entry point, single-instance guard
  TrayContext.cs                     tray icon + menu (view only)
  Connection/ConnectionManager.cs    reconnect state machine
  Connection/                        LinkMachine, SuppressionLatch, MusicHalf,
                                     CallsHalf, ConnectionState, BackoffSchedule
  Bluetooth/AudioSinkService.cs      A2DP sink lifecycle
  Bluetooth/CallTransportService.cs  HFP call transport
  Bluetooth/LinkMonitor.cs           DeviceWatcher + ConnectionStatus
  Audio/AudioRouter.cs               WASAPI capture -> render bridge
  Audio/EndpointMonitor.cs           IMMNotificationClient: A2DP endpoint arrival
  Platform/                          scheduler, power notifier, package identity
tests/Klangbruecke.Tests/            4,200+ unit tests
packaging/                           AppxManifest, dev cert, MSIX build scripts
docs/FINDINGS.md                     research record; read before changing approach

Licence

Personal project, unlicensed.