dual 4K@30fps on USB 5Gbps. How is that possible?

[lowieken]

Hello,

I have a rather general question on DisplayLink docks and bandwidth usage.
For example, the HP Universal USB C/A dock (https://www8.hp.com/h20195/v2/GetPDF.aspx/4AA7-5549ENW.pdf). This supports 2 x 4K display (even 3 on TB hosts). However this dock has a USB 3.2 Gen 1 host interface (5 Gbps).
In my calculations I find that one 4K@30fps display takes up around 6 Gbps (https://toolstud.io/video/bitrate.php?imagewidth=3840&imageheight=2160&colordepth=24&framerate=29.97) uncompressed.
So my question is: how is it possible then to have 2 x 4K@30fps over a 5Gbps host interface?
What am I missing?

Cheers

Kris

[AlbanRampon]

Hello Kris,

I'm afraid we wouldn't explain how in details.
But that dock is dual 4Kp60 + single 4Kp30 over alternate mode. Thunderbolt not required or useful here. The third display does not use USB traffic so can't be added in the same bandwidth budget.
Dock does drive the displays directly, so the bandwidth calculation doesn't really work that way. It does on the monitor link, but not on the USB link.

Kind regards,
Alban

[lowieken]

Hi Alban,

Thanks for your reply. I think I understand why my calculation does not make sense. It only makes sense on the DP/HDMI links between the dock and the displays...

Will the dual 4Kp60 + single 4Kp30 also work over a type-A USB3 port?

Cheers

Kris

[AlbanRampon]

Hello Kris,

No, you lose one because alternate mode is a USB-C feature. Alternate mode means non USB traffic over the USB-C plug.
Over the type A connector, you will get USB features of that dock: 2x 4Kp60 DisplayLink video, ethernet and audio.
You can however daisy-chain products with DisplayLink technology to add a USB graphics adapter to get a third display over type A.

Kind regards,
Alban

[lowieken]

That's great.

Many thanks for your clarifying and intelligible answers, Alban.

Cheers