USB-C Alternate Modes (or Alt-Modes) are optional features that hardware manufacturers can implement into their devices to support a wider range of data protocols for direct connection to peripherals (usually displays) without needing active adapters. USB itself carries packetized data which natively supports things like file transfers and keyboard activity. This packetized data could instead be a video signal such as HDMI or DisplayPort.
If a device supports a USB Type-C Alt-Mode, it will probably offer video out from a USB-C socket.
At the time of writing, USB Type-C has five Alternate Mode Partners, three of which should be familiar to most computer users, and only two of which we've been able to find 'in the wild':
Of these five, the top three are the most important to regular consumers, but despite its ubiquity on televisions, HDMI Alternate Mode is a specification without an implementation (as far as we can tell). Our research turned up no consumer hardware that supports HDMI Alt-Mode by name. Instead, HDMI compatibility over USB-C is generally offered via DisplayPort or Thunderbolt 3 Alt-Modes using adapters.
For computers, Intel released Alpine Ridge controllers in 2015 which were the first Thunderbolt 3 chipsets integrated into CPUs and offered DisplayPort 1.2 spec. These were followed up in 2018 by Titan Ridge, which improved performance and brought compatibility up to DisplayPort 1.4 spec. Not all motherboards or laptops support Thunderbolt 3, even if their CPU does.
For mobile phones, manufacturers originally required separate controllers to implement Alt-Mode support, and frequently didn't (except on high-end devices). Qualcomm added Alt-Mode directly into their system-on-a-chip Snapdragon 835 in 2017, greatly expanding the number of devices which could offer video out of the USB-C socket. Still, frustratingly, not all USB-C phones have Alt-Mode support.
USB-C Alternate Mode to a USB-C Monitor may offer Video, Audio, USB and Power Delivery.
Primarily, USB-C Alternate Modes provide single-cable connection to monitors and can offer up to 4K@60Hz/HDR video & audio directly to a USB-C enabled display. This connection can additionally offer USB-C Power Delivery and USB Hub (downstream) ports at the same time to keep your laptop/phone charging and your peripherals connected. Thunderbolt can do a bit more:
Thunderbolt Alt-Mode offers power, data, display, external graphics, Ethernet and USB all at the same time through a dynamically allocated link that can handle up to 40Gbps (but only on a 50cm cable). For example, it could deliver two 4K displays, USB 3.1 and Ethernet simultaneously over a single Thunderbolt USB-C cable when using compatible devices.
USB-C Alternate Mode to a HDMI or DisplayPort Monitor offers Video and Audio only.
DisplayPort Alt-Mode offers DP v1.2, v1.4 or v2.0 connectivity (depending on the hardware) for video connections anywhere from Full HD 1080p up to 8K@60Hz* over USB-C. Audio is usually also supported as a part of the protocol for screens with built-in speakers. Additionally, DisplayPort Alt-Mode can be used with DisplayPort, HDMI, DVI or VGA adapters for extended support to other monitor types.
HDMI Alt Mode currently offers HDMI v1.4b connectivity with support for 4K, ARC, HEC, CEC, Deep Color and a few other HDMI-specific features. HDMI Alt-Mode supports these HDMI-only features where DisplayPort Alt-Mode with a HDMI adapter does not.
*Display modes beyond 4K@60 require Display Stream Compression, a feature of DisplayPort v1.4 and up, and may additionally require DP8K certified cables for some high-bandwidth modes.
Your first challenge will be to confirm that both your source hardware and sink hardware (monitor/dock) both offer compatible Alt-Modes and inputs for the purpose you require. Sometimes it can be as easy as checking your Laptop sockets for the DP or Thunderbolt logo, but more often (especially for phones) it will require you to go through your user manual. Alt-Mode support should not be assumed of any hardware, even high-end stuff.
Also, you can't always expect maximum performance on all hardware, as different speeds (bandwidths) of USB-C may limit the maximum resolution available on your device - check those manuals!
Furthermore, not all USB-C ports on a device will have Alt-Mode support. A laptop with 3 USB Type-C sockets will probably only offer Alt-Mode on one of them. Identify it ahead of time so you know where to plug in your monitor, dongle or hub.
Remember that hardware manufacturers will advertise the highest performance of their USB-C Alt-Modes, but won't state how short the cables have to be to achieve those numbers. Things like 40Gbps Thunderbolt 3 and 100W Power Delivery are usually limited to 50cm cables!
Lastly, budget devices produced early-on in the commercial release of Thunderbolt 3 hardware (we're talking laptops from 2017 here) didn't always live up to their specifications, and likewise dongle adapters and cables produced around this time weren't always full-featured either. Try to stay on this side of 2018 if you can - we've had a lot of headaches with older tech simply working as advertised.
We've collected a list of popular devices which support a USB Alternate Mode. This is a mix of DisplayPort and Thunderbolt 3 Alt-Mode products. Generally Laptops are offering some level of Thunderbolt 3 functionality, while tablets, smartphones and hubs support DisplayPort Alt-Mode. Always check the product specs and/or manual to know for sure what your device is capable of connecting to.
This list is not complete, but we hope to update it from time to time as more products are released.
Feel free to contact us if we've missed an important feature of USB Alt-Mode, or know of a popular product that should be on our compatibility list!