How does a USB-C dock obtain a video signal from the GPU in order to provide a port for an additional monitor?
Got a tip for us?
Let us know
Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Source of the video signal that a USB-C dock feeds to its HDMI port?
- Thread starter milleron
- Start date
- Sort by reaction score
If it's a USB-C dock and doesn’t use DisplayLink technology, it works like this: USB-C ports provide DisplayPort video output via DisplayPort Alternate Mode. This is then fed to an active DisplayPort-to-HDMI converter to provide a HDMI port.
If it's a Thunderbolt 3/4 dock, it works like this: two DisplayPort signals from the host's Thunderbolt 3/4 controller are going into it, one of which is then fed to an active DisplayPort-to-HDMI converter to provide a HDMI port.
Last edited:
In the USB-C dock case, if it supports USB 3.x then there's only two lanes of DisplayPort instead of the usual 4 since two of the 4 SuperSpeed lines of a USB-C cable are used for USB 3.x. USB data is 10 Gbps + 480 Mb/s and DisplayPort is up to 12.96 Gbps (USB 3.x and USB 2.0/1.1 are completely separate from each other).
Some USB-C docks only support USB 2.0/1.1 - those will have 4 lanes of DisplayPort. In that case, USB data is 480 Mb/s and DisplayPort is up to 25.92 Gbps.
Thunderbolt has the equivalent of 8 lanes of DisplayPort (two separate connections of 4 lanes each). It combines data (PCIe and/or USB) and video (DisplayPort) into a single Thunderbolt signal which makes more efficient use of the bandwidth (up to 40 Gbps). DisplayPort is up to 34.56 Gbps (38.6 Gbps in some cases for the Apple Pro Display XDR). Data can use whatever is not being used by DisplayPort (up to ≈24 Gbps). The host Thunderbolt controller tunnels the PCIe/USB/DisplayPort data over Thunderbolt. The peripheral Thunderbolt controller converts the tunnelled signals back to PCIe/USB/DisplayPort. An HDMI adapter might be connected to the DisplayPort output of the Thunderbolt controller.
To identify a DisplayPort to HDMI adapter (such as those being used by a USB-C or Thunderbolt dock), you can use a utility like AllRez to get the DPCD info. The DPCD info will include an OUI and an ID.
Here's some examples: #21
A USB-C or Thunderbolt Dock may include a DisplayPort MST Hub. An MST hub may have multiple outputs. Some outputs might be a HDMI adapter. AllRez should be able to dump the DPCD info for all the MST hubs and all devices connected to the MST hub. macOS doesn't support multiple displays from an MST hub, but can use the MST hub to convert 2 lanes of DisplayPort at a high link rate (such as HBR3) to 4 lanes of HBR2 which is useful for some adapters such as DisplayPort to Dual Link DVI adapters which require 4 lanes of HBR for full 2560x1600 60Hz output.
Some USB-C docks only support USB 2.0/1.1 - those will have 4 lanes of DisplayPort. In that case, USB data is 480 Mb/s and DisplayPort is up to 25.92 Gbps.
Thunderbolt has the equivalent of 8 lanes of DisplayPort (two separate connections of 4 lanes each). It combines data (PCIe and/or USB) and video (DisplayPort) into a single Thunderbolt signal which makes more efficient use of the bandwidth (up to 40 Gbps). DisplayPort is up to 34.56 Gbps (38.6 Gbps in some cases for the Apple Pro Display XDR). Data can use whatever is not being used by DisplayPort (up to ≈24 Gbps). The host Thunderbolt controller tunnels the PCIe/USB/DisplayPort data over Thunderbolt. The peripheral Thunderbolt controller converts the tunnelled signals back to PCIe/USB/DisplayPort. An HDMI adapter might be connected to the DisplayPort output of the Thunderbolt controller.
To identify a DisplayPort to HDMI adapter (such as those being used by a USB-C or Thunderbolt dock), you can use a utility like AllRez to get the DPCD info. The DPCD info will include an OUI and an ID.
Here's some examples: #21
A USB-C or Thunderbolt Dock may include a DisplayPort MST Hub. An MST hub may have multiple outputs. Some outputs might be a HDMI adapter. AllRez should be able to dump the DPCD info for all the MST hubs and all devices connected to the MST hub. macOS doesn't support multiple displays from an MST hub, but can use the MST hub to convert 2 lanes of DisplayPort at a high link rate (such as HBR3) to 4 lanes of HBR2 which is useful for some adapters such as DisplayPort to Dual Link DVI adapters which require 4 lanes of HBR for full 2560x1600 60Hz output.
Thanks very much; that furthers my understanding considerably.
My problem now is that I cannot tell if my 2020-vintage motherboard's USB C Gen 2x2 port has DisplayPort Alternate Mode. The manual doesn't say anything about it, and the online support person whom I contacted knows nothing about computers -- no help whatsoever. There's nothing left to do but experiment, and I was hoping to figure it out before spending money on a hub with a DP and an HDMI port.
My problem now is that I cannot tell if my 2020-vintage motherboard's USB C Gen 2x2 port has DisplayPort Alternate Mode. The manual doesn't say anything about it, and the online support person whom I contacted knows nothing about computers -- no help whatsoever. There's nothing left to do but experiment, and I was hoping to figure it out before spending money on a hub with a DP and an HDMI port.
Magnificent explanation. Thanks very!
The hub I bought is a low-tier Plugable that apparently contains DisplayLink technology. The USB 3.2 Gen2 ports (Some USB-A and one USB-C 2x2) on the computer do not have either DisplayLink or DisplayPort Alternate Mode. Will it be possible for the Plugable dock with DisplayLink to supply a video signal to one HDMI port for connection to a small 1080 monitor with no refresh rate need higher than 30 Hz ?
It's a Gigabyte X570 Aorus Master. After repeatedly poking their tech support for an answer to my question, they finally replied that it has "no video"
Perhaps installing the DisplayLink drivers will suffice. Thanks very much for that link.
It's not going to have DisplayPort Alternate Mode unless the CPU has an integrated GPU or the motherboard has a DisplayPort input.
Some AMD processors have integrated GPU. Which AMD processor did you get? Integrated GPU won't be useful if there's no DisplayPort or HDMI or VGA or USB-C DisplayPort Alt Mode port.
USB-C DisplayPort Alternate Mode requires a connection to a GPU.
DisplayLink is a USB device that you connect to a USB port. It may be part of a USB dock. It just requires a DisplayLink driver to transmit graphics data over USB to the DisplayLink device. DisplayLink doesn't require a GPU. All the work is done by the CPU. DisplayLink should be fine for small displays and low refresh rate. It can do 4K up to 60Hz depending on the DisplayLink adapter.
Which revision?
I don't see where it says it supports gen 2x2.
gen2 means 10 Gbps.
x2 means two lanes.
10 Gbps x2 = 20 Gbps.
USB 20 Gbps is USB 3.2 gen 2x2. This is ≈19.4 Gbps before 128b/132b encoding.
Normal USB 10 Gbps mode is USB 3.1 gen 2 or USB 3.2 gen 2x1. This is ≈9.7 Gbps before 128b/132b encoding.
Normal USB 5 Gbps mode is USB 3.0 or USB 3.1 gen 1 or USB 3.2 gen 1x1. This is ≈4 Gbps before 8b/10b encoding.
There's a USB 3.2 gen 1x2 mode. This is ≈8 Gbps before 8b/10b encoding.
When there's two lanes of USB, then there's no room for DisplayPort Alt Mode. DisplayPort Alt Mode can be 2 lanes or 4 lanes. A USB-C cable has 4 lines. USB 3.x uses 2 lines (one for receive and one for transmit). USB 3.2 x2 uses 4 lines - two for receive and two for transmit - similar to Thunderbolt or USB4 except Thunderbolt and USB4 can do 20 Gbps per line (they also have a 10 Gbps per line mode). Thunderbolt is actually 10.3125 or 20.625 Gbps per line on the wire after 64b/66b encoding. USB4 is 9.7 Gbps per line before 64b/66b encoding or 19.4 Gbps per line before 128b/132b encoding (aka gen 3).
USB4 2.0 adds a gen 4 mode and a gen 4 asymmetric mode. Thunderbolt 5 will have similar.
https://en.wikipedia.org/wiki/USB4
https://en.wikipedia.org/wiki/Thunderbolt_(interface)
Thanks for that. I apologize for this not being a Mac problem. I posted here because this is the most reliable tech forum I've ever found on the Internet, and, sure enough, you all were gracious enough to enlighten me sufficiently to proceed.
1. The USB-C port is not labeled 2x2. It apparently is just a 10Gbs Gen2. This board is Rev1.0; I might have gotten that incorrect information from the manual on a later revision.
2. The CPU is a 5800X3D w/ no APU. If I had an integrated APU, I'd get the 5th monitor that way and not have to fuss about the mystery (to my naive mind) of video over USB. 😀
3. Plugable has the DisplayLink drivers for download on their support pages. They're probably identical to the ones at displaylink.com, but just in case they've modified them for their hardware, I'll use the Plugable one.
4. The marvelous help I've received here allows me now to purchase a 1080 monitor without worrying about whether it'll work in this setup.
Thanks again. Mine is an Apple household. My Mac hardware is all functioning perfectly, but ya can't run Flight Simulator on a Mac, not even one with Windows on Boot Camp. And the ultimate setup, other than VR, requires 5 monitors.
1. The USB-C port is not labeled 2x2. It apparently is just a 10Gbs Gen2. This board is Rev1.0; I might have gotten that incorrect information from the manual on a later revision.
2. The CPU is a 5800X3D w/ no APU. If I had an integrated APU, I'd get the 5th monitor that way and not have to fuss about the mystery (to my naive mind) of video over USB. 😀
3. Plugable has the DisplayLink drivers for download on their support pages. They're probably identical to the ones at displaylink.com, but just in case they've modified them for their hardware, I'll use the Plugable one.
4. The marvelous help I've received here allows me now to purchase a 1080 monitor without worrying about whether it'll work in this setup.
Thanks again. Mine is an Apple household. My Mac hardware is all functioning perfectly, but ya can't run Flight Simulator on a Mac, not even one with Windows on Boot Camp. And the ultimate setup, other than VR, requires 5 monitors.
Last edited:
I don’t see your mainboard having any video output (and the manual doesn’t mention anything), so having one wouldn’t do you any good.
True! So obvious, but I didn't think of it myself! One thinks of upper tier boards having everything you could ever need, but there are some features present on lower-priced boards that are dropped on enthusiast-level ones. Something to remember.