- Connecting
- Need
- Answer
Two 4K monitors from one laptop
Get this
A Thunderbolt 4 (or USB4 40Gbps) cable to a Thunderbolt dock with two video outputs
Look for
- Laptop port: Thunderbolt 4/5 or USB4 with two displays listed
- Dock: Thunderbolt 4 with 2× HDMI 2.0 or 2× DisplayPort
- Mac: needs a Thunderbolt dock (no MST), and check how many displays the chip supports
Avoid
- USB-C (non-Thunderbolt) hubs with two HDMI ports often mirror, or drop to 4K 30 Hz
- DisplayPort MST hubs on a Mac
The numbers: 4K 60 Hz 8-bit
| Link | Needs | Has | Result |
|---|---|---|---|
| Thunderbolt 4 | 12.54 Gbps | 25.92 Gbps | Fits |
| USB-C DP 1.4 ×2 | 12.54 Gbps | 12.96 Gbps | Fits |
| USB-C DP 1.2 ×2 | 12.54 Gbps | 8.64 Gbps | Too much |
How the numbers are worked out
Reduced-blanking timing (VESA CVT-R2)
3840×2160 active → 3920 × 2222 total pixels per frame
3920 × 2222 × 60 Hz = 522.61 MHz pixel clock
522.61 MHz × 24.0 bits per pixel = 12.54 Gbps
8-bit colour, full RGB/4:4:4: 3 × 8 = 24.0 bits per pixel.
"Has" is the link's usable rate after line coding. DSC is modelled as up to 3:1 compression. Full method.
Standards behind this answer
2020
Thunderbolt 4
The same 40 Gbps as Thunderbolt 3, with the minimums raised so every port behaves the same.
- data: 40 Gbps (PCIe 32 Gbps)
- video: two 4K 60 Hz or one 8K
- power: up to 100 W (USB PD)
USB4 (20 and 40 Gbps)
Thunderbolt 3's protocol, opened up as a USB standard. Tunnels USB, DisplayPort and PCIe over one cable.
- data: 20 or 40 Gbps
- video: DisplayPort tunnelled
- power: USB PD required
DisplayPort 1.2
4K at 60 Hz since 2010, plus daisy-chaining (MST).
- data: 21.6 Gbps
- video: 4K 60 Hz / 1440p 144 Hz