HDMI, DisplayPort, USB-C: which cable carries what

An image is a data rate

Every second, the computer sends the monitor the color of each pixel of each image. The uncompressed rate is a simple product:

data rate = (pixels per image + blanking) × images per second × bits per pixel

Blanking is a short pause between lines and between images, a legacy of cathode-ray tubes reduced to a few percent in current timings. Bits per pixel are 24 for 8-bit color and 30 for 10-bit color.

A 4K image at 60 Hz in 8-bit thus requires about 12.5 Gbit/s. Raise the rate to 144 Hz and the depth to 10 bits, and the requirement reaches 39 Gbit/s, three times more.

Uncompressed data rate of common modes (reduced blanking) and the least capable link able to carry each.
Image	Data rate	First link that carries it uncompressed
1920 × 1080, 60 Hz, 8 bit	3.2 Gbit/s	HDMI 1.4
1920 × 1080, 240 Hz, 8 bit	14.0 Gbit/s	HDMI 2.0
2560 × 1440, 60 Hz, 8 bit	5.6 Gbit/s	HDMI 1.4
2560 × 1440, 165 Hz, 8 bit	16.3 Gbit/s	DisplayPort 1.2
2560 × 1440, 240 Hz, 10 bit	30.8 Gbit/s	HDMI 2.1
3440 × 1440, 144 Hz, 10 bit	23.5 Gbit/s	DisplayPort 1.4
3840 × 2160, 60 Hz, 8 bit	12.5 Gbit/s	HDMI 2.0
3840 × 2160, 60 Hz, 10 bit	15.7 Gbit/s	DisplayPort 1.2
3840 × 2160, 144 Hz, 10 bit	39.2 Gbit/s	HDMI 2.1
3840 × 2160, 240 Hz, 10 bit	68.6 Gbit/s	DisplayPort 2.1 UHBR20
5120 × 2880, 60 Hz, 10 bit	27.7 Gbit/s	HDMI 2.1
6016 × 3384, 60 Hz, 10 bit	38.2 Gbit/s	HDMI 2.1

What each link carries

A link is defined by its raw bit rate and by the share of it that remains for the image once the line coding is deducted, the payload.

Fig. 1. Usable capacity of each video link, and what four common modes require. A mode to the right of a bar does not pass without compression.

HDMI

HDMI 2.0 (18 Gbit/s, payload 14.4) carries 4K at 60 Hz or 1440p at 144 Hz, in 8-bit. It is the port of most office monitors, of older graphics cards and of many docking stations.

HDMI 2.1 (up to 48 Gbit/s, payload 42.7) carries 4K at 120 Hz and at 144 Hz in 10-bit without compression, and adds variable refresh rate. Caution: the label “2.1” does not guarantee the full rate. The standard allows ports limited to 24 or 40 Gbit/s, and some monitors marked HDMI 2.1 accept no more than an HDMI 2.0 signal. Look for the rate in Gbit/s, or for the explicit mention of 4K at 120 Hz.

DisplayPort

DisplayPort 1.2 (21.6 Gbit/s, payload 17.3) carries 4K at 60 Hz and 1440p at 165 Hz.

DisplayPort 1.4 (32.4 Gbit/s, payload 25.9) is the most widespread on current monitors and graphics cards. It carries 4K at 120 Hz in 8-bit, 1440p at 240 Hz, and well beyond with DSC compression.

DisplayPort 2.1 defines three speeds, UHBR10, UHBR13.5 and UHBR20 (40, 54 and 80 Gbit/s). At the highest, 4K at 240 Hz in 10-bit passes uncompressed. Here too, a “2.1” port may be limited to the lowest speed: read the UHBR level.

DisplayPort also allows several monitors to be chained on one output (MST), each connected to the previous one. Windows and Linux support it; macOS does not extend the desktop over MST and shows the same image on all the chained screens.

DSC: compression that does not show

Display Stream Compression reduces the data rate by a factor of up to three with a loss that VESA qualifies as visually lossless, and which controlled tests confirm: observers do not distinguish the compressed image from the original. It is DSC that allows 4K at 144 or 240 Hz on DisplayPort 1.4, or 5K and 6K screens on a single cable.

It requires support at both ends, the graphics card and the monitor. Its only notable side effects are practical: some combinations of DSC, several screens and variable refresh rate are refused by certain graphics drivers. For a single monitor it can be used without a second thought.

USB-C: one connector, several realities

The oval USB-C connector can carry data, video and power together, which allows a laptop to be connected to its monitor, keyboard, network and charger with a single cable. But the connector guarantees nothing: each function is optional.

Video passes through DisplayPort Alt Mode: the USB-C port lends its high-speed wires to a DisplayPort signal. With all four pairs assigned to video, the link has the full capacity of DisplayPort; USB data then falls back to the old USB 2.0 speed. With two pairs for video and two for fast USB, the usual setting of monitors with a built-in hub, the video capacity is halved: enough for 4K at 60 Hz with DisplayPort 1.4, limited to 4K at 30 Hz with DisplayPort 1.2. A 4K monitor stuck at 30 Hz over USB-C almost always comes from this; a setting in the monitor’s menu (“USB-C Prioritization,” “high resolution”) switches between the two modes.

Power follows the Power Delivery standard. A monitor announces the wattage it can supply to the laptop: 65 W suits thin laptops, 90 to 100 W most 14 to 16-inch machines, and a few recent monitors reach 140 W. A laptop that demands more than the monitor supplies charges slowly, or discharges under load.

Thunderbolt 3 and 4, and USB4, use the same connector with a guaranteed capacity of 40 Gbit/s shared between video and data: two 4K screens at 60 Hz, or one 5K or 6K screen, plus fast peripherals. Thunderbolt 5 doubles the rate to 80 Gbit/s and can go up to 120 for displays. A Thunderbolt monitor works on a plain USB-C port with reduced functions; the reverse is always true.

Before counting on a single cable, check three things: that the USB-C port of the computer outputs video (a DisplayPort logo, a lightning bolt, or the specification sheet), that the monitor supplies enough power, and that the cable is rated for video and for the wattage. The cable supplied with the monitor always is.

Cables

A cable has a ceiling too, and it is the usual culprit when a screen flickers, goes black for a second or refuses a refresh rate.

HDMI: “Premium High Speed” for 18 Gbit/s, “Ultra High Speed” for 48 Gbit/s. The certification label carries a hologram and a QR code.
DisplayPort: “DP8K” certified cables carry DisplayPort 1.4 at full rate; “DP40” and “DP80” correspond to the speeds of DisplayPort 2.1.
USB-C: video over long distances requires active or Thunderbolt-certified cables. A passive cable for full-rate video rarely exceeds 3 ft (1 m) at 40 Gbit/s, 6.5 ft (2 m) at lower rates. Charging cables supplied with phones often carry neither video nor fast data.

Keep cables as short as the installation allows: 6.5 ft (2 m) is safe for everything, 10 ft (3 m) already calls for certified quality at high data rates.

Useful extras on the monitor

A USB hub for keyboard, mouse and webcam, so that one cable to the laptop connects everything.
A KVM switch, which moves those peripherals from one computer to another along with the video input: valuable when a work laptop and a personal machine share the screen.
An Ethernet port, which turns the monitor into a complete docking station.
Picture-by-picture, which displays two inputs side by side, mostly on ultrawide screens.

Checking the chain

The image obtained is the best that all the links allow: the output of the computer, any dock or adapter, the cable, the input of the monitor. To diagnose a limitation, open the display settings of the system and read the active signal: resolution, refresh rate, bit depth. If the rate offered stops at 60 Hz on a 144 Hz monitor, or at 30 Hz on a 4K screen, one link is below the others. Test with the supplied cable, plugged directly into the computer, on the DisplayPort input: it is the configuration with the fewest unknowns.

The selector indicates, for each recommendation, the least capable link that carries it uncompressed at the suggested refresh rate.

Connections: HDMI, DisplayPort, USB-C and Thunderbolt