A Windows 10/11 software implementation of "Dolby Digital Live": a virtual 5.1 audio device that accepts any multichannel PCM stream, encodes it to AC3 (Dolby Digital) in real time, and streams it as an IEC 61937 / S-PDIF bitstream out a chosen Toslink optical output to an AV receiver.

Also known as / what problem this solves: a free, open-source software Dolby Digital Live (DDL) and DTS Connect alternative for Windows — for PCs whose sound card can't encode surround to optical, so you can get true 5.1 surround over a single Toslink / S-PDIF (optical) cable to an AV receiver or soundbar. Essentially SoundPusher for Windows. Useful when "Dolby Digital Live" / "DTS Interactive" / "DTS Connect" isn't available on your motherboard or USB sound card (Realtek, ASUS Xonar, etc.).

Currently encodes AC3 (Dolby Digital, 5.1, 640 kbps). DTS Connect-style DTS output is a feasible extension — FFmpeg ships a DTS encoder; see the design notes in CLAUDE.md.

Keywords: Dolby Digital Live, DTS Connect, DTS Interactive, software AC3 encoder, real-time AC3/AC-3 encoder, 5.1 surround over optical/SPDIF/Toslink, IEC 61937 passthrough, WASAPI exclusive passthrough, virtual audio device/cable, FFmpeg AC3, SoundPusher for Windows, HTPC surround sound.

Get the latest Release:

• virtual-ac3-encoder-setup-x.y.z.exe — per-user installer (no admin); auto-starts hidden at logon, with Start-Menu shortcuts for config / log / device list.
• virtual-ac3-encoder-x.y.z-win64.zip — portable build; extract and run engine.exe (see QUICKSTART.txt).

You also need a virtual audio cable to capture surround audio — install VB-CABLE and set CABLE Input as your default 5.1 device — then connect the optical output to your receiver. Prefer to build from source? See Build below.

Optical (S-PDIF) can carry stereo PCM or a compressed 5.1 bitstream, never 5.1 PCM — so a surround stream has to be encoded to AC3 on the PC before it leaves the optical port. Many onboard sound chips don't offer this; this project does it in software.

Two components (the encoder is FFmpeg and cannot live in a kernel driver, so the work is split):

1. Kernel virtual audio driver (driver/) — a PortCls/WaveRT virtual cable (SYSVAD-derived) that publishes a 5.1 render endpoint plus a paired loopback-capture endpoint. Phase 3.
2. User-mode engine (engine/) — captures the 5.1 PCM, encodes AC3, wraps it as IEC 61937, and renders it to the optical endpoint via WASAPI exclusive passthrough.

The engine's real-time design (output clock = master, lock-free ring buffer between capture and output, drift correction by periodic latency-trim) is modeled on SoundPusher (MIT), the equivalent macOS tool. The AC3 encode + S-PDIF mux uses FFmpeg (libavcodec AC3 encoder + spdif muxer + libswresample), the same engine Kodi uses internally. See third_party/reference/ for the cloned references.

• Phase 1 — offline encoder core. SpdifEncoder (PCM → AC3 640 kbps 5.1 → IEC 61937) + lock-free RingBuffer + WAV test harness. Verified: output decodes as ac3, 48000 Hz, 5.1, 640 kb/s.
• Phase 2 — live WASAPI engine. Shared capture/loopback → ring buffer → exclusive IEC 61937 passthrough (output = master clock; SoundPusher-style 64-cycle drift-trim).
• TDD harness. doctest unit_tests (RingBuffer incl. SPSC stress + encoder contract).
• Phase 3 — kernel virtual audio driver, built & test-signed. 5.1 render-only virtual endpoint ("Virtual AC3 Encoder (5.1)"). Loads only with Secure Boot OFF (see Install).
• Working end-to-end (confirmed). Because Secure Boot is ON here, the live system uses VB-CABLE as the 5.1 source: engine --in "CABLE Output" --out "Realtek Digital Output" → receiver decodes Dolby Digital. (Our own driver is ready for when Secure Boot is off.)
• Phase 4 — packaging (config file + auto-start at logon; optional tray UI).

engine.exe:

• --list — list render + capture endpoints.
• --probe — which outputs accept AC3 passthrough (IsFormatSupported, non-intrusive).
• --mon — capture-only throughput diagnostic (non-intrusive).
• --loopback — treat --in as a render endpoint and capture it via WASAPI loopback (used with the virtual driver).
• --in <name> / --in-id <id> / --out <name> / --out-id <id> / --out-spdif
• --bitrate <bps> (default 640000) / --safe <frames> (drift target, default 1536)
• --config <path> (defaults to virtual-ac3-encoder.conf next to the exe) · --hidden (hide console) · --log <path> (log to file) · --duration <s> (auto-stop)
• --upmix surround|off — for stereo input, upmix to 5.1 via FFmpeg's surround filter (a free DTS Neo:PC / Pro Logic II-style matrix upmix). Default surround; use off for untouched stereo→front. Multichannel input is downmixed regardless.

Config precedence: built-in defaults < config file (key=value: in, out, in_id, out_id, bitrate, safe, loopback, out_spdif, upmix) < command-line flags.

Derived from Microsoft's SimpleAudioSample (third_party/reference/wds), trimmed to a single 5.1 render endpoint and rebranded. Build (WDK 10.0.26100 + VS2022):

& "C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\Bin\amd64\MSBuild.exe" `
  driver\SimpleAudioSample.sln /p:Configuration=Release /p:Platform=x64 /p:SpectreMitigation=false
# output: driver\x64\Release\package\  (SimpleAudioSample.sys / .inf / .cat)

# 1. Disable Secure Boot in your UEFI/BIOS (manual).
# 2. Elevated PowerShell:
scripts\install-driver.ps1 -EnableTestSigning   # enables test signing + trusts the test cert
# 3. Reboot.
scripts\install-driver.ps1                        # creates the ROOT\SimpleAudioSample device
# Uninstall: scripts\uninstall-driver.ps1 [-DisableTestSigning]

# With the virtual driver installed (render-only + loopback):
engine\build\Release\engine.exe --loopback --in "Virtual AC3 Encoder" --out "Realtek Digital Output"

# Or, without the driver, using an existing virtual cable (works with Secure Boot ON):
engine\build\Release\engine.exe --in "CABLE Output" --out "Realtek Digital Output"

Set the virtual device as the Windows default 5.1 output, play surround content, and switch the receiver to the matching optical input — it should report Dolby Digital.

Install the engine to a stable per-user location and have it start hidden at every logon, with restart-on-failure (no elevation, no Task Scheduler — a Startup-folder supervisor that runs in the real interactive session):

scripts\setup-autostart.ps1                                   # VB-CABLE -> Realtek (defaults)
scripts\setup-autostart.ps1 -In "CABLE Output" -Out "Realtek Digital Output" -Bitrate 640000
scripts\setup-autostart.ps1 -In "Virtual AC3 Encoder" -Loopback   # when using our own driver
scripts\remove-autostart.ps1 [-DeleteInstall]                 # undo

This stages engine.exe + DLLs to %LOCALAPPDATA%\virtual-ac3-encoder, writes virtual-ac3-encoder.conf there (edit it to change devices/bitrate), and drops a supervisor in the Startup folder that runs engine --hidden --log and relaunches it if it exits.

Requires Visual Studio 2022 (or Build Tools) and CMake. FFmpeg shared dev libs are fetched into third_party/ffmpeg/.

# 1. Fetch FFmpeg dev libraries (DLLs + import libs + headers)
scripts\fetch-ffmpeg.ps1

# 2. Configure + build
cmake -S engine -B engine\build -G "Visual Studio 17 2022" -A x64
cmake --build engine\build --config Release

scripts\make-test-wav.ps1                       # generates engine\test\test_5p1.wav (5.1 tones)
engine\build\Release\encode_wav.exe engine\test\test_5p1.wav engine\test\out.spdif

# Verify the result is real AC3 inside IEC 61937:
ffmpeg -f spdif -i engine\test\out.spdif -f null -

MIT — see LICENSE. The kernel driver is derived from Microsoft's MIT-licensed SimpleAudioSample, and the engine design is modeled on SoundPusher (MIT); the engine dynamically links FFmpeg (LGPL). Full attributions are in NOTICE.md. No third-party binaries (FFmpeg, VB-CABLE) are committed — they're fetched/installed separately.