Why won't my USB-C cable show video?
AnswerIt depends
Usually it's a USB 2.0-only "charging" cable, or a USB-C port that doesn't output video. Video over USB-C uses the cable's high-speed wire pairs, and USB 2.0 cables don't have them. Try a cable marked 10Gbps or higher, or Thunderbolt, and check that the port has a DisplayPort or Thunderbolt icon.
4K 60 Hz over USB-C, 8-bit
| Link | Needs | Has | Result |
|---|---|---|---|
| USB-C DP 1.2 ×2 | 12.54 Gbps | 8.64 Gbps | Too much |
| USB-C DP 1.4 ×2 | 12.54 Gbps | 12.96 Gbps | Fits |
| USB-C DP 1.4 ×4 | 12.54 Gbps | 25.92 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.
"Has" is the link's usable rate after line coding. DSC is modelled as up to 3:1 compression. Full method.
Work through it in order
- Cable: USB 2.0 USB-C cables, including most phone cables and many 240 W charging cables, carry no video. A 10Gbps+ or Thunderbolt cable fixes most cases.
- Port: phones, tablets and budget laptops often have USB-C ports without DisplayPort Alt Mode. Some laptops have video on only one of their USB-C ports.
- Length: passive full-featured cables are 1–2 m. Longer ones need to be active or built for video.
- Adapter: a USB-C to HDMI adapter converts DisplayPort to HDMI. A cheap HDMI 1.4 adapter caps 4K at 30 Hz.
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
USB 2.0
The baseline every USB port still speaks. Fine for keyboards, mice and charging; slow for files.
- data: 480 Mbps
- video: No
- power: 2.5 W (port); more via USB-C/PD