SoundBlaster Multichannel S/PDIF with Volume Sync

Restore native Windows volume control for Creative Sound Blaster cards that output 5.1 surround over a single S/PDIF (optical/coax) cable using the real-time Dolby Digital Live or DTS Connect encoder.

The problem

S/PDIF can only carry 2-channel uncompressed PCM. To get 5.1 over it, Creative cards encode the multichannel mix into a Dolby Digital / DTS bitstream in real time. To enable that you must set the card's "Speakers" endpoint (5.1) as the Windows default and turn on the encoder.

The catch: with the encoder active, the driver claims hardware volume control, so Windows hands volume to the card - and the card ignores volume for the encoded digital stream. The result is that the Windows volume slider, media keys, and mute do nothing. Only the receiver's volume works.

The fix

Volume on an encoded bitstream can't be changed after encoding. The only place to apply it is on the PCM before the encoder. Equalizer APO inserts a preamp into the audio engine at exactly that point, so its gain is applied to the audio before Creative's encoder compresses it.

This project is a tiny background app, DdlVolumeSync, that reads the Windows master volume/mute of the default playback device and writes it into Equalizer APO's preamp (volume.txt). Net effect: the normal Windows volume controls work again - slider, media keys, mute, and the on-screen volume OSD - while you keep full 5.1 over S/PDIF.

Apps -> Speakers (5.1) -> Equalizer APO preamp -> DDL/DTS encoder -> S/PDIF -> Receiver (5.1)
                                ^
                                |
                      DdlVolumeSync writes the dB here,
                      mirrored from the Windows master volume

Requirements

• Windows 10/11, 64-bit
• A Creative sound card with the Dolby Digital Live and/or DTS Connect encoder (e.g. Sound BlasterX AE-5 / AE-5 Plus and many others)
• Equalizer APO installed
• .NET 8 runtime (or build with the .NET 8 SDK)

Setup

1. Configure the card (one time)

• In Sound Blaster Command (or your card's control panel): set output to Speakers / 5.1, and enable the Dolby Digital Live (or DTS Connect) encoder.
• In Windows Sound: set "Speakers (your Creative card)" as the default playback device and configure it as 5.1 Surround.
• Connect the receiver via S/PDIF. Confirm a 5.1 test plays on all speakers and the receiver shows "Dolby Digital".

2. Attach Equalizer APO to that endpoint (one time)

• Run Equalizer APO's DeviceSelector.exe (older builds: "Configurator").
• Tick the Speakers endpoint of your Creative card. For Creative cards, use the "Install as SFX/EFX" troubleshooting option so the effect runs ahead of the encoder.
• Reboot when prompted.
• Verify: temporarily set Preamp: -20 dB in config.txt and confirm the output gets quieter while still showing Dolby Digital 5.1. (Then revert.)

3. Install DdlVolumeSync

From a PowerShell prompt in the repo root:

# builds the app, wires Equalizer APO's config to read volume.txt, and adds a login startup entry
./scripts/install.ps1

That's it - move the Windows volume and the 5.1 S/PDIF output level follows.

Uninstall

./scripts/uninstall.ps1

How it works (details)

See docs/how-it-works.md for the full technical write-up, including the exclusive-mode pitfall (ASIO bridges / pro-audio apps grabbing the device) and the volume-curve mapping.

Notes / known good behavior

• The receiver's own volume still works and composes with this.
• The app follows the default playback device, so the preamp only matters while your Creative 5.1 endpoint is the default - exactly when you want it.
• Volume is quantized to 0.5 dB to avoid needless config rewrites during slider drags.
• Set DDLVOLUMESYNC_CONFIG_DIR to override the Equalizer APO config path if it is not at the default %ProgramFiles%\EqualizerAPO\config.

License

MIT - see LICENSE.

Disclaimer

Not affiliated with or endorsed by Creative Technology, Dolby, DTS, or the Equalizer APO project. "Dolby Digital Live" and "DTS Connect" are trademarks of their respective owners.