Reverse engineer the Elgato HID protocol for noise reduction and save-to-flash

[copyleftdev]

Background

The Facecam has a USB HID interface (endpoint 0x89) alongside its UVC video interface. The Camera Hub software on Windows/Mac uses this HID channel for proprietary controls that aren't available via standard V4L2:

• Noise reduction (on/off toggle)
• Metering mode (average / center-weighted)
• Save settings to device flash
• USB transfer mode (bulk / isochronous)

The Extension Unit GUID is {a8e5782b-36e6-4fa1-87f8-83e32b323124} with 9 vendor-specific controls.

How to help

If you have access to Windows or macOS with Camera Hub:

1. Capture USB traffic with Wireshark + USBPcap (Windows) or Wireshark (macOS)
2. Toggle each control in Camera Hub while capturing
3. Share the .pcapng files

Alternatively, the Camera Hub binary itself could be analyzed with Ghidra. The related elgato4k-linux project found that Elgato capture cards use a 06 06 06 55 header pattern in HID packets.

Goal

Add facecam-ctl control set noise_reduction on and facecam-ctl save commands that talk directly to the device over HID — no Camera Hub needed.