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Will It Work? · Checker

Will this USB-C hub or dock run my monitors?

By Bulan Sarkar, founder of ElectronicsInfoline (since 2000)

A USB-C hub that runs one monitor perfectly can drop a second one, cap 4K at 30 Hz, or slow the Ethernet. Which one happens is arithmetic: how many lanes the hub gives to video, what the computer's port carries, and what each monitor needs. Enter all three and the checker does the sums.

It works if every monitor's bandwidth fits on the video lanes the hub uses, the port supports MST (or Thunderbolt) for more than one screen, and each hub output lists the mode. Example: two 2560 × 1440 monitors at 60 Hz need 5.63 Gbit/s each; a hub with USB 3 ports on a DisplayPort 1.4 laptop carries 12.96 Gbit/s on two lanes, so both fit (11.26 Gbit/s), as VESA itself states for this setup.

1. The computer's USB-C port

From the computer maker's spec sheet for that port.

2. The hub or dock

USB 3 ports mean the hub keeps two lanes for data: half the video bandwidth.

W

As the hub maker states it.

W
W

3. The monitors

Monitor 1
Monitor 2

With limitsWorks, with limits

  • video: 2 different pictures on one link need MST support in the computer. Its spec sheet does not say, so check before you buy.
  • video: Bandwidth fits: 8.83 of 12.96 Gbit/s.
  • data: Ethernet at 1 Gbit/s fits on the hub's USB 3 data link.
  • power: The laptop gets up to 85 W, at least the 65 W its maker lists.
Show the working
  1. 2560×1440 at 60 Hz, 8-bit: CVT-RB2 total 2640 × 1481 → pixel clock 234.590 MHz × 3 × 8 bit = 5.63 Gbit/s
  2. 1920×1080 at 60 Hz, 8-bit: CVT-RB2 total 2000 × 1111 → pixel clock 133.320 MHz × 3 × 8 bit = 3.2 Gbit/s
  3. DisplayPort Alt Mode, 2 lanes (the other two carry USB 3 data) at HBR3: 2 × 8.1 × 0.8 (8b/10b coding) = 12.96 Gbit/s
  4. Total need 8.83 of 12.96 Gbit/s (68 %)
  5. Monitor 1 on HDMI 2.0: covered by the listed 3840×2160 at 60 Hz
  6. Monitor 2 on HDMI 2.0: covered by the listed 3840×2160 at 60 Hz
  7. Laptop power: min(charger 100 W, hub pass-through 85 W) = 85 W
Video bandwidth needed against what the link carriesMonitor 1: 5.63 Gbit/s; Monitor 2: 3.2 Gbit/s. Link carries: 12.96 Gbit/s.Monitor 15.63 Gbit/sMonitor 23.2 Gbit/sTotal8.83 Gbit/sLink carries: 12.96
Uncompressed need of each monitor against what the USB-C video link carries (dashed line).

Bandwidth uses VESA reduced-blanking timing and no compression (DSC). A monitor and computer that both support DSC can carry more.

How the checker works it out

Lanes. A USB-C port has four high-speed lanes. DisplayPort Alt Mode can put video on all four, leaving USB 2.0 for data, or on two, leaving two for USB 3 data (VESA). A hub with USB 3 ports takes the second way.

USB-C DisplayPort Alt Mode: four lanes or twoTop: all four high-speed lanes carry DisplayPort; USB data runs at USB 2.0 on its own pins. Bottom: two lanes carry DisplayPort and two carry USB 3 data. Power runs alongside in both.Four lanes of videoDPDPDPDP+ USB 2.0+ powerMost video; USB data at USB 2.0 onlyTwo lanes of videoDPDPUSB 3USB 3+ USB 2.0+ powerHalf the video bandwidth; USB 3 data on the other two
Figure 1. VESA: Alt Mode can use all four high-speed lanes for video, with USB 2.0 data on separate pins, or two lanes for video and two for SuperSpeed USB data. A hub with USB 3 ports usually takes the second way, which halves the video bandwidth.

Link capacity. Lanes × lane rate × coding efficiency. HBR2 runs 5.4 Gbit/s a lane and HBR3 8.1 (VESA); both lose 20 % to 8b/10b coding (VESA). So two lanes of HBR2 carry 8.64 Gbit/s and four carry 17.28.

DisplayPort link payload by rate and lane countHBR2 × 2 lanes: 8.64 Gbit/s; HBR2 × 4 lanes: 17.28 Gbit/s; HBR3 × 2 lanes: 12.96 Gbit/s; HBR3 × 4 lanes: 25.92 Gbit/s; UHBR20 × 4 lanes: 77.37 Gbit/sHBR2 × 2 lanes8.64 Gbit/sHBR2 × 4 lanes17.28 Gbit/sHBR3 × 2 lanes12.96 Gbit/sHBR3 × 4 lanes25.92 Gbit/sUHBR20 × 4 lanes77.37 Gbit/s
Figure 2. Payload after coding: HBR2 and HBR3 lose 20 % to 8b/10b (× 0.8); UHBR20 carries 19.34 Gbit/s per lane (VESA). Two lanes is the usual USB-C hub with USB 3 ports.

Monitor need. Pixel clock × 24 bits (or 30 for 10-bit colour). The pixel clock includes the blank border, from VESA's reduced-blanking timing: 4K at 60 Hz comes to 522.614 MHz, so 12.54 Gbit/s. Our timing code reproduces VESA's published pixel clocks for 5K and 8K exactly (VESA).

Several monitors. With MST the streams share one link, so their needs add up (VESA).

One DisplayPort link split into two streamsThe laptop sends two independent video streams over one DisplayPort link; the hub splits them to two monitors. Both streams share the link's bandwidth.Laptopone link, two streamsHubScreen AScreen B
Figure 3. With Multi-Stream Transport (MST) one DisplayPort link carries separate streams for each monitor, and they share its bandwidth (VESA). Without MST, a hub can only show the same picture on both.

HDMI outputs. HDMI.org lists the modes each version carries: HDMI 1.4 runs 4K at 24, 25 and 30 Hz (HDMI Licensing Administrator), HDMI 2.0 4K at 50/60 Hz (HDMI Licensing Administrator), HDMI 2.1 8K60 and 4K120 (HDMI Licensing Administrator). A monitor mode outside those lists gets a “with limits”: the hub maker has to list it.

Worked example: two 1440p screens on one port

Each 2560 × 1440 at 60 Hz screen: 2640 × 1481 × 60 = 234.590 MHz × 24 bits = 5.63 Gbit/s. Two of them: 11.26 Gbit/s. Two lanes of HBR3: 2 × 8.1 × 0.8 = 12.96 Gbit/s. It fits, which matches VESA's own example of two 1440p screens on two HBR3 lanes (VESA).

Video bandwidth needed against what the link carriesScreen 1 (1440p60): 5.63 Gbit/s; Screen 2 (1440p60): 5.63 Gbit/s. HBR3 × 2 lanes: 12.96 Gbit/s.Screen 1 (1440p60)5.63 Gbit/sScreen 2 (1440p60)5.63 Gbit/sTotal11.26 Gbit/sHBR3 × 2 lanes: 12.96
Figure 4. Two 1440p60 screens need 11.26 Gbit/s; two HBR3 lanes carry 12.96. The same pair on two HBR2 lanes (8.64) would not fit.

What each kind of port promises

The connector looks the same on every laptop, but what it carries varies. Thunderbolt versions come with Intel's minimums (Intel); for a plain USB-C port, the computer maker's spec sheet is the only source.

What each kind of USB-C port promises for video and chargingUSB-C, data only: video no video; charging see maker. USB-C + DP Alt Mode: video DP 1.2 / 1.4 / 2.1; charging see maker. Thunderbolt 3: video one 5K or two 4K; charging up to 100 W, optional. Thunderbolt 4: video two 4K60 or one 8K60; charging required, under 100 W. Thunderbolt 5: video two 8K60 (dock); charging up to 240 WPortVideoLaptop chargingUSB-C, data onlyno videosee makerUSB-C + DP Alt ModeDP 1.2 / 1.4 / 2.1see makerThunderbolt 3one 5K or two 4Kup to 100 W, optionalThunderbolt 4two 4K60 or one 8K60required, under 100 WThunderbolt 5two 8K60 (dock)up to 240 W
Figure 5. Same connector, different promises. Thunderbolt figures are Intel's minimums; for a plain USB-C port the computer maker's spec sheet is the only source.

Two monitors are covered in more depth in two monitors from one USB-C port, and the per-monitor maths in how much bandwidth a monitor needs.

Tips and tricks

  • Check the HDMI version on the hub’s spec sheet. A hub that says “4K” with HDMI 1.4 means 4K at 30 Hz at most.
  • A hub with USB 3 ports keeps two lanes for data. If you need 4K at 60 Hz on a DisplayPort 1.2 laptop, a video-first hub (USB 2.0 ports) or a different port is the fix.
  • Look for “MST” or “multi-display” on the laptop’s spec sheet before buying a dual-screen hub. Without it you get the same picture twice.
  • Pass-through charging can be lower than the charger plugged into the hub. The hub maker states the figure; compare it with your laptop’s charger rating.
  • If the laptop has more than one USB-C port, check on its spec sheet which one carries video. They are not always the same.

What we would do

One monitor at 1080p or 1440p and 60 Hz: a USB-C hub with an HDMI 2.0 port works on any port with DisplayPort Alt Mode. A 1440p60 screen needs 5.63 Gbit/s, and even two lanes of DisplayPort 1.2 carry 8.64.

Two monitors, or 4K at more than 60 Hz: on a Thunderbolt 4 or 5 laptop, buy a Thunderbolt dock, since Intel guarantees two 4K60 screens on Thunderbolt 4. On a plain USB-C port, confirm MST on the laptop's spec sheet first and check the bandwidth here before you buy.

Questions people ask

Why does my USB-C hub show 4K at only 30 Hz? Two usual reasons. The hub's HDMI port may be HDMI 1.4, which HDMI.org lists for 4K at 24, 25 and 30 Hz only. Or the hub keeps two of the four USB-C lanes for USB 3 data: on a DisplayPort 1.2 laptop two lanes carry 8.64 Gbit/s, and 4K at 60 Hz needs 12.54.

Can one USB-C port run two monitors? Yes, if the port supports DisplayPort Multi-Stream Transport (MST) or is Thunderbolt, and the bandwidth covers both screens. Without MST, a hub shows the same picture on both screens or uses only one.

Is a Thunderbolt dock better than a USB-C hub? On a Thunderbolt port, yes for monitors: Intel says Thunderbolt 4 drives two 4K displays at 60 Hz or one 8K display at 60 Hz. On a plain USB-C port a Thunderbolt dock may not work fully; check the dock maker’s list of supported computers.

Does the hub reduce my laptop’s charging power? The laptop gets the lower of the charger’s output and the hub’s stated pass-through power. If that is below the laptop’s rating, it charges, but below its full rate.

What is DSC and does it change the answer? Display Stream Compression lets a monitor and computer that both support it squeeze the picture, up to 3:1 according to VESA. This checker counts uncompressed bandwidth only, so with DSC on both ends more can fit than it says.

Sources

Each figure on this page is quoted from the source below or worked out from them. Sources checked , each quote read again against the source in a second pass.

  1. VESA, Welcome VESA Workshop Tokyo, Japan 2024 (VESA PDF, page 10, 'DP Standard Version Number...'): • DPv1.1 = HBR (2.7 Gbps/lane) • DPv1.2 = HBR2 (5.4 Gbps/lane) • DPv1.4 = HBR3 (8.1 Gbps/lane) • DPv2.x = UHBR rates (10/13.5/20 Gbps/lane) vesa.org/wp-content/uploads/2024/10/VESA-Tokyo-Workshop_Octo…
  2. VESA, DP 1.3 Overview for VESA v1 (VESA/DisplayPort.org PDF, page 4, 'DP 1.3 Link Rate Increase'): 8.1 Gbps link rate, per lane x 0.8 to account for 8b/10b transport coding overhead x 4 maximum number of available lanes 25.92 Gbps total usable data transfer rate displayport.org/wp-content/uploads/2014/09/DP-1.3-Overview-f…
  3. VESA, Why DisplayPort - VESA - Interface Standards for The Display Industry: Up to 8.1 Gbps per lane–32.4 Gbps over 4 lanes Allowing for overhead — 25.92 Gbps over 4 lanes vesa.org/displayport-developer/why-displayport/
  4. VESA, VESA Releases Updated DisplayPort™ Alt Mode Spec to Bring DisplayPort 2.0 Performance to USB4™ and New USB Type-C® Devices - VESA - Interface Standards for The Display Industry: Featuring the highly efficient 128b/132b channel coding shared with USB4, DisplayPort 2.0 delivers a maximum payload of 77.37 Gbps across four lanes (up to 19.34 Gbps per lane) vesa.org/featured-articles/vesa-releases-updated-displayport…
  5. VESA, VESA® Brings DisplayPort™ to New USB Type-C Connector - VESA - Interface Standards for The Display Industry: By leveraging USB Type-C’s flexibility, the DisplayPort Alt Mode can choose to transmit on just one or two of the four available lanes, so that the other two lanes can be used for SuperSpeed USB data at the same time. vesa.org/featured-articles/vesa-brings-displayport-to-new-us…
  6. VESA, VESA® Brings DisplayPort™ to New USB Type-C Connector - VESA - Interface Standards for The Display Industry: When using all four lanes for DisplayPort Alt Mode, which could drive a monitor with up to 5K (5120 x 2880) resolution, USB 2.0 data can still be carried across the USB Type-C connection using separate pins dedicated for that function. vesa.org/featured-articles/vesa-brings-displayport-to-new-us…
  7. VESA, VESA Rolls Out Early Certification Program for Video Source and Display Products Using HBR3 High-Bandwidth Link Rate - VESA - Interface Standards for The Display Industry: By using the DisplayPort Multi-Stream feature, this bandwidth can also be shared among multiple monitors connected to a dock. vesa.org/featured-articles/vesa-rolls-out-early-certificatio…
  8. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 2 lane at HBR3 can support 2 x 2560x1440@60Hz using CVT Reduced Blank 1 vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  9. VESA, Welcome VESA Workshop Tokyo, Japan 2024 (VESA PDF, page 68, 'Minimal Vblank and Hblank requirements'): VBLANK period way shorter than 300 uSec, let alone 460 uSec listed as the minimum VBLANK period in VESA CVT Standard vesa.org/wp-content/uploads/2024/10/VESA-Tokyo-Workshop_Octo…
  10. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 5120 x 2880 60 Hz IT No DP 1.3 VESA CVT RB 2 924.144 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  11. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 7680 x 4320 60 Hz IT VESA CVT RB 2 2068.660 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  12. HDMI Licensing Administrator, HDMI Specification 1.4b: This includes support for 4096×2160 at 24 Hz, 3840×2160 at 24, 25, and 30 Hz, and 1920×1080 at 120 Hz. All resolutions and refresh rates can be support with the High Speed HDMI Cable. hdmi.org/spec/hdmi1_4b
  13. HDMI Licensing Administrator, HDMI Licensing Administrator, Inc. (press release: HDMI FORUM RELEASES VERSION 2.0 OF THE HDMI SPECIFICATION, 4 Sep 2013): This latest HDMI Specification, the first to be developed by the HDMI Forum, offers a significant increase in bandwidth (up to 18Gbps) to support new features such as 4K@50/60 (2160p), which is 4 times the clarity of 1080p/60 video resolution; 32 audio channels; as well as dynamic auto lip-sync and extensions to CEC. hdmi.org/press/bodydetails/24
  14. HDMI Licensing Administrator, HDMI FORUM RELEASES VERSION 2.1 OF THE HDMI SPECIFICATION: This latest HDMI Specification supports a range of higher video resolutions and refresh rates including 8K60 and 4K120, and resolutions up to 10K. Dynamic HDR formats are also supported, and bandwidth capability is increased up to 48Gbps. hdmi.org/announce/detail/172
  15. Intel, Thunderbolt™ 3 brings Thunderbolt to USB-C (Thunderbolt 3 Overview slides): Single-cable connection for one 5K or two 4K displays thunderbolttechnology.net/sites/default/files/thunderbolt_3_…
  16. Intel, What Is Thunderbolt™ 4 and How Is It Different from USB-C? – Intel: Up to two 4K 60 Hz displays or one 8K 60 Hz display on a single connection intel.com/content/www/us/en/architecture-and-technology/thun…
  17. Intel, Thunderbolt™ 5 for Gaming – Intel: One Thunderbolt 5 port can connect to up to two 8K 60Hz DisplayPort or HDMI monitors through a compatible dock or adapter. intel.com/content/www/us/en/learn/thunderbolt-5-for-gaming.h…
  18. VESA, FAQ - DisplayPort: Developed in collaboration with the MIPI Alliance, VDC-M provides a higher level of compression ratio (up to 5:1) at the same visually lossless quality level as VESA’s Display Stream Compression (DSC) standard (which offers up to 3:1 compression), with the trade-off of higher circuit complexity. displayport.org/faq/

Author: Bulan Sarkar · Report an error

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