Why is my USB-C hub stuck at 4K 30 Hz?
AnswerIt depends
Most likely the hub converts to HDMI 1.4, or uses only 2 lanes of DisplayPort 1.2, and neither can carry 4K 60 Hz. 4K 60 needs 12.54 Gbps. Two DisplayPort 1.2 lanes give 8.64 and HDMI 1.4 gives 8.16, so the highest refresh that fits is about 41 Hz, which the hub rounds down to 30. Look for a hub that says "4K 60 Hz" and "HDMI 2.0", or one with DisplayPort 1.4.
4K 60 Hz, 8-bit
| Link | Needs | Has | Result |
|---|---|---|---|
| USB-C DP 1.2 ×2 | 12.54 Gbps | 8.64 Gbps | Too much |
| HDMI 1.4TV timing | 14.26 Gbps | 8.16 Gbps | Too much |
| USB-C DP 1.4 ×2 | 12.54 Gbps | 12.96 Gbps | Fits |
| HDMI 2.0TV timing | 14.26 Gbps | 14.40 Gbps | Fits |
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.
HDMI 1.4/2.0, standard TV timing (CTA-861)
3840×2160 active → 4400 × 2250 total pixels per frame
4400 × 2250 × 60 Hz = 594.00 MHz pixel clock
594.00 MHz × 24.0 bits per pixel = 14.26 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.
4K 30 Hz, 8-bit
| Link | Needs | Has | Result |
|---|---|---|---|
| USB-C DP 1.2 ×2 | 6.18 Gbps | 8.64 Gbps | Fits |
| HDMI 1.4TV timing | 7.13 Gbps | 8.16 Gbps | Fits |
Some hubs let you trade USB speed for video: in 4-lane mode they keep only USB 2.0 for their ports and pass full DisplayPort bandwidth through, enough for 4K 60. The setting is usually a switch or an app, if it exists at all.
The standards involved
DisplayPort over USB-C (Alt Mode)
How most USB-C ports send video. The USB-C pairs are handed over to DisplayPort.
- data: USB 2.0 or USB 3 alongside
- video: DP 1.4 (or 2.1) over USB-C
- power: USB PD at the same time
HDMI 1.4
1080p at any common refresh rate, 4K only at 24–30 Hz. Introduced ARC.
- data: 10.2 Gbps
- video: 4K 30 Hz / 1080p 120 Hz
DisplayPort 1.2
4K at 60 Hz since 2010, plus daisy-chaining (MST).
- data: 21.6 Gbps
- video: 4K 60 Hz / 1440p 144 Hz