How to Use the USB Video Capture Dongle

Symptom:

Your headless machine gets totally hosed and you need to see the video to fix it, e.g. in BIOS setup. But something prevents you from connecting a monitor to it such as all monitors being in use, or a very un-ergonomic location (my case).

How to fix:

I bought a HDMI video capture dongle for $18 on Amazon, sold by Amazon affiliate, fulfilled by Amazon. Brand name is Extenuating Threads (creative, uninformative). It expects HDMI input (can't tell what revision) at up to 4K (3840x2160px @60FPS) (or lower resolution and/or frame rate) on a full size connector. Output is by USB-2.0 up to 1080p @60FPS (1920x1080px). The chip will downscale the video if necessary. Video standard is YUV. Audio on HDMI (if any) is also passed to the USB side. Vendor:product ID is 534d:2109; as text, manufacturer is MACROSILICON, product is USB Video. The uvcvideo driver creates a device /dev/video0 (or whatever minor number).

VLC (on Linux) did a good job displaying the image. Start it up. Under Media, pick Open Capture Device, set the capture mode to Video Camera (the default), then pick your /dev/video* device. If you have multiple ones, e.g. a builtin webcam, you may have to try different ones to find the video capture dongle. The rest of the settings can be left at defaults, unless you also need sound (not for BIOS setup). Finish by hitting Play (bottom row).

When VLC downscales the source's native 1920x1080px resolution to provide borders and controls on the 1920x1080px laptop screen, blur and scaling artifacts make an appearance, and latency is introduced: be leisurely when moving the mouse. My fonts are pretty small and you need to know what the text is about, to figure it out. At a reduced resolution the text is more readable.

Hit the stop button on VLC's player controls, when you're done.

A companion issue is to connect a compact folding keyboard to the host by USB rather than Bluetooth, which won't be working in the pre-boot context. Some such keyboards will talk on the USB charging cable, but most are Bluetooth only.