How can they work?

tms9918 · September 12, 2023

USB 2.0 VGA graphics cards, like this one http://pcdeacitec.com/archiveros/179041_datasheet.pdf

claim to provide resolutions like 2048 x 1280; 16 bits

Now, usb 2.0 is 480Mbit/s. This gives

480E6/(2048*1280*16)= 11.44Hz.

which appears to be too low

How do they actually work? Do they apply some compression in the transfer protocol?

Jurrunio · September 12, 2023

could be something like this?

https://www.anandtech.com/Show/Index/8191?cPage=2&all=False&sort=0&page=1&slug=nvidia-kepler-cards-get-hdmi-4k60hz-support-kind-of

mariushm · September 12, 2023

There's a driver, the driver then takes 30-60 snapshots of the virtual monitor and sends the information like a video, think of it like encoding to mpeg2 or maybe there's something proprietary that compresses only the differences between previous frame and current frame and then compresses, using some video codec that uses little cpu to compress.

For example this chip : https://www.display-solution.com/wp-content/uploads/displaylink-usb-multi-multiple-monitor-chips-DL-1x5.pdf

says it uses DisplayLink DL2 video compression and this page https://bjorn3d.com/2010/10/his-multi-view-ii/ says the software arranges the screen in small regions, like maybe 256 x 256 pixels or something like that, and is smart enough to detect if that region is static or movie and compresses those regions with different methods depending on what type of content is detected.

The limitation is the 16 MB of memory on the chip, this limits you to 2048 x 1280 x 2 bytes per pixel = 5,242,880 bytes, but the chip may need space for at least 2 uncompressed frames plus some memory for the stream coming from the pc.

you couldn't do 480 mbps on usb 2.0 even if you'd try, that's theoretical maximum... real throughput possible due to arranging data in packets/frames is more like 400 mbps (around 50 MB/s)

see answer here : https://superuser.com/questions/317217/whats-the-maximum-typical-speed-possible-with-a-usb2-0-drive