LDACast

A user-mode LDAC A2DP source for Windows 10/11: stream system audio to Sony (and other LDAC-capable) Bluetooth headphones in LDAC quality, with a Fluent desktop app in the style of DLSS Swapper.

Windows ships no LDAC codec for A2DP and does not expose L2CAP to user mode, so this program takes a generic CSR8510-class USB Bluetooth dongle for itself (via WinUSB) and runs the whole HCI/L2CAP/AVDTP/A2DP path with BTstack, encoding the WASAPI loopback mix with Sony's libldac.

Features

• LDAC on Windows: HQ (909/990 kbps), SQ (606/660), MQ (303/330), plus experimental ABR that steers quality from buffer depth.
• DLSS Swapper-style app (ui/LDACast, C# / WinUI 3): device cards with per-device quality + ABR, one-click radio switch, capture-source dropdown (default device or virtual cable), live stream health (realtime ratio, wire rate, buffer, over/underruns, sink caps, negotiated config) with an automatic drop-a-rung hint, safe-parameters button, and a stability guide page.
• One-command radio switch (tools/ldacmode.ps1 status|bt|ldac): hand the dongle between Windows and LDACast in seconds, no replug.
• Dedicated capture source (--capture NAME, --list-capture): capture a virtual cable (e.g. VB-CABLE) instead of the default device, so music routes silently to the headphones instead of blaring from the speakers. --check-capture reports a device's mix format and exits without touching the radio, and --help lists every flag.
• Automatic pairing: first connect bonds over SSP Just Works; link keys persist per radio, reconnects are silent after that.
• Measured, not assumed: the design decisions were checked against packet traces from this radio — see ARCHITECTURE.md. What has not been verified is listed in LIMITATIONS.md.

Quick start

Prerequisites: Rust (1.98+, x86_64-pc-windows-msvc), Visual Studio 2022 with the C++ workload, LLVM, .NET 8 SDK (for the app), Zadig (once, to bind the dongle to WinUSB), and a CSR8510-class Bluetooth dongle (e.g. TP-Link UB400).

crates/ldac-sys and crates/btstack-sys run bindgen, which needs libclang. It is normally picked up from PATH; only if the build cannot find it, set $env:LIBCLANG_PATH = 'C:\Program Files\LLVM\bin' first.

git clone --recursive https://github.com/YoshKoz/LDACast.git
cd LDACast
cargo build --release
cargo test --workspace

Give the dongle to the app (one-time Zadig step below), then either click Stream in the desktop app or run:

.\target\release\ldacsrc.exe --addr 14:3F:A6:35:D0:AA --quality sq

The sink must be in pairing mode the first time; link keys are stored next to the working directory. Every HCI packet goes to ldacsrc.pklg (readable with python third_party\btstack\tool\dump_pklg.py); the program's own log lines go to stdout. A status line every 5 seconds reports buffer depth, packet/frame counts, measured wire rate, encoder bitrate/EQMID, ring over/underruns and errors.

Switching the radio between Windows and LDACast

Once WinUSB has been installed once (below), ownership is a single command and needs no Zadig and no replug:

.\tools\ldacmode.ps1 status   # who owns the radio right now
.\tools\ldacmode.ps1 bt       # give it back to Windows
.\tools\ldacmode.ps1 ldac     # hand it to this program
.\tools\ldacmode.ps1 ldac -Run   # ...and start streaming

It refuses to switch while ldacsrc is running, elevates itself, and reports the driver and child nodes it ended up with rather than assuming the switch worked. Measured 3-11 seconds per switch.

Give the dongle to this program

1. Start Zadig as administrator
2. Options -> List All Devices
3. Pick the device whose USB ID is your Bluetooth radio (here: 0A12 0001)
4. Choose WinUSB as the target driver, then Replace Driver
5. Unplug and replug the dongle. A driver swap alone leaves the old Bluetooth stack loaded and WinUsb_Initialize fails with error 50

Check it took:

Get-PnpDevice -InstanceId 'USB\VID_0A12&PID_0001*' |
  Get-PnpDeviceProperty -KeyName DEVPKEY_Device_Service,DEVPKEY_Device_Children

Service must be WinUSB and Children must be empty.

Set the playback format

LDAC carries 44.1, 48, 88.2 or 96 kHz, mono or stereo. WASAPI loopback delivers the shared mix format of the captured playback device, and there is no resampler in this program — if the device runs at any other rate the program refuses to start and says so. Set the device to stereo at one of those rates in Sound -> device -> Properties -> Advanced -> Default Format.

Tip: install VB-CABLE (free), set CABLE Input as the default playback device, and pass --capture CABLE (or type CABLE in the app's capture box). Music then routes silently to the headphones; --list-capture shows the exact endpoint names.

Choosing a quality

• hq — maximum LDAC quality (909 kbps at 44.1 kHz, 990 at 48 kHz).
• sq — the balanced default (606/660 kbps).
• mq — most robust (303/330 kbps).

The ceiling is the radio link, not the software. A2DP retransmits damaged packets, so a link that cannot keep up shows up as gaps in the audio; the status line's rt ratio and its overrun counter are what to watch.

Measured here — CSR8510 (TP-Link UB400 class) to a WH-1000XM3, 48 kHz stereo, hq: rt 1.00x sustained at 375 frame/s and 330 B/frame, wire 988-996 kbps against a 990 kbps target, over 0 under 0 encerr 0 senderr 0, no dropouts, over a short indoor link. That is the only configuration that has been listened to; the other rates and modes are implemented but unverified, as LIMITATIONS.md records.

If you do hear dropouts, drop one rung — the desktop app's Guide page walks through the same checklist. --abr lets the encoder walk EQMID from buffer depth instead, but it is off by default and unproven.

Give the dongle back to Windows

.\tools\ldacmode.ps1 bt, or Device Manager -> the device under "Universal Serial Bus devices" -> Update driver -> Browse -> Let me pick -> Generic Bluetooth Radio (bth.inf). The original binding is recorded in docs/radio-original-driver.txt. Link keys are per stack: a headset last paired through LDACast may need re-pairing in Windows, and vice versa.

Layout

Path	What
crates/ldacsrc	A2DP/AVDTP logic, encode, packetise, reconnect
crates/capture	WASAPI loopback thread, endpoint selection, SPSC ring
crates/ldac-sys	libldac built from source, bindgen FFI
crates/btstack-sys	BTstack built from source, bindgen FFI
crates/ldacgui	Minimal egui control panel (same engine)
ui/LDACast	WinUI 3 desktop app (devices, stream health, settings)
tools/ldacmode.ps1	Radio ownership switch Windows <-> LDAC
third_party/btstack	BTstack submodule (pinned)
third_party/ldacBT	libldac snapshot (Apache 2.0)

Limitations

See LIMITATIONS.md for the full list. Headlines: Windows loses Bluetooth entirely while this runs (one radio, one owner); no microphone/AVRCP; loopback-only capture; no resampling; verified against one sink (WH-1000XM3). A kernel A2DP driver coexisting with the Microsoft stack is intentionally out of scope (it requires Microsoft signing; Secure Boot stays on).

Licensing

• Own code in crates/ and ui/: MIT, see LICENSE.
• third_party/btstack: BlueKitchen, personal non-commercial use only (commercial use needs a BlueKitchen license).
• third_party/ldacBT (libldac): Apache 2.0, Sony.
• LDAC is a trademark of Sony. This project is not affiliated with Sony.