USB Type C is meant to be the answer to all of our problems and be this magic, universal port right? Well in terms of charging things it’s pretty good, we’ve got the USB Type C PD (power delivery) spec that means my Apple charger will work on my MacBook Pro, my Samsung S9+, Samsung Gear Icon X, Nintendo switch, and practically anything else with a Type C port on it. Aside from the slightly complex variations of power output and compatibility with devices, I’ve had a really good experience with USB C being a truly universal solution for charging devices. However, getting a video signal out of a USB Type C port is another story.
I recently purchased a 2018 MacBook Pro (MBP) 15″ and I’ve been trying to work out how to setup my desk. I started investigating different docking stations, USB Type C adapters and cables, etc. I quickly learned that the world of USB Type C/Thunderbolt 3 docks and video adapters is complex and full of confusion. What’s the difference between Thunderbolt 3, USB3/3.1, “Thunderbolt 3 compatible” devices? Why do some only support mirroring on macOS but extended displays on windows? What is USB Type C alternate mode “alt mode”, etc.
I found myself asking so many questions and quickly fell into a rabbit hole of trying to understand the multitude of different options that are available on the market. I’m going to attempt to summarise everything I’ve learnt, so that you don’t have to go through that same pain.
Thunderbolt 3 vs USB 3/3.1 vs Thunderbolt 3 “Compatible”
I quickly discovered that there are two main types of docks, proper Thunderbolt 3 ones, USB 3/3.1 ones and Thunderbolt 3 compatible adapters/docking stations. The Thunderbolt 3 options seemed far more expensive than their USB 3.1 and “compatible” alternatives. So what gives? The main difference is the way they communicate with your device, whether that’s a laptop like my MBP or a phone like my S9+.
Thunderbolt 3 is a standard that’s been developed by Intel to allow you to connect high bandwidth peripherals such as displays and storage devices. However, because of the high amounts of available bandwidth, it’s also used in many docks or “port replicators”. In fact, with the 40Gbps of bandwidth it has, you can drive two 4k displays at 60hz and still have room leftover for other peripherals.
USB 3/3.1 on other other hand, is just the latest revision of the USB (Universal Serial Bus) protocol that has been around for a long time. Thunderbolt 3 “compatible” devices seem to be just a marketing ploy to get people to think they support Thunderbolt 3. Really, they just use the normal USB protocol that Thunderbolt 3 automatically falls back to. USB 3.1 only has 10Gbps of bandwidth compared to Thunderbolty 3’s 40Gbps which means it doesn’t event have enough for a single 4k 60hz display signal. However, USB 3.1 over Type C has a nice trick up its sleeve which I’ll explain later.
Thunderbolt 3 ports are often accompanied by a small lightning icon to signify the fact. However, my MBP and some other devices don’t always do this. Thunderbolt 3 ports will normally fallback to USB3/3.1 if that’s the only protocol the device (such as a dock or adapter) supports.
USB Type C Display Output Methods
There are many different ways that USB Type C devices (laptops and docks etc.) output and interpret display signals. I’ll explain the common ones below.
USB 3/3.1 Over Type C With DisplayLink Chip
USB Type 3/3.1 over Type C docks normally rely on a chip manufactured by a company called DisplayLink (or something similar). These chips use software to encode, compress and send a display signal over the lower bandwidth USB 3/3.1 protocol. However, these chips are software driven so they don’t perform well in demanding applications such as gaming or video editing. They might even struggle with playing some videos. Anything besides general office use is asking for trouble.
DisplayPort/HDMI Over Type C With Alternate Mode
Most cheap USB Type C dongles/adapters rely on on a neat trick called USB C alternative mode. Basically, a dongle/adapter/dock can ask a compatible device like a laptop or smartphone to output a non USB signal at the same time over some unused wires. Some examples of these non USB signals include HDMI and DisplayPort. Yep, the standard protocol that a HDMI or DisplayPort cable carries can also be carried by the humble USB Type C port.
The way this works is the dongle/dock will ask the output device if it’s able to support HDMI/DisplayPort etc. via alternative mode. If it can, the device starts to output a native HDMI/DisplayPort signal straight from the GPU – no software to get in the way like a DisplayLink chip. These cheap adapters are completely passive, basically just joining the correct wires from the Type C connector to the right places ono the HDMI/DisplayPort connector. They don’t manipulate or process the signal.
A Side Note On DisplayPort MST
Part of the DisplayPort standard includes MST – Multi Stream Transport. This handy feature allows you to daisy chain displays, use multiple outputs to drive a high res/refresh rate display, or carry multiple signals to different monitors as a “splitter” from a hub. A lot of docking stations and adapters that support more than one display out rely on MST, which is fine for the most part. However, Apple does not properly support MST in macOS. The only part of MST that’s supported is driving one larger screen from two DisplayPort outputs.
Unfortunately this means a lot of docking stations that work flawlessly in Windows or Linux show a “mirrored” image on both outputs instead of separate images for each. There’s nothing that can be done as a workaround as the problem is macOS fundamentally not supporting it. What this practically means is that some docking stations with multiple display outputs will only show up as a single one in macOS and output the same image on each one.
A Mixture of the Above
You’d think that adapters and dongles would probably pick one of the above methods and stick with it. However, from what I’ve seen most docks that advertise 2 or more outputs rely on some crazy combination of the methods above. Some will have one DisplayPort driven via USB Type C alt mode, and another two with a DisplayLink chip, or two with DisplayPort and MST via USB C Alt mode. This crazy mishmash of implementations and lack of information on product data sheets means it’s difficult for even a tech savvy consumer to work out if something is compatible with their device.
For example, I found this great looking Dell dock for the reasonable price of $200. I was about to buy it when I saw a review saying it only supports one display output on macOS. After looking into this I figured out it was due to the lack of MST in macOS. I then found a more expensive one for $300 from Lenovo, and thought sweet, this is it. Apparently it uses DisplayPort via alt mode for one connector and a DisplayLink chip over USB for the other two. This means you get one output with “good” performance and the other two a severely restricted in comparison.
Passive Dongles/Adapters and Cables
I didn’t spend too much time researching this, but there are still a few problems here. Whilst not good practice, someone should be able to plug a USB C to HDMI into a HDMI to DisplayPort adapter then use it with their screen, right? Well not quite, because of the way USB C video outputs are so varied and inconsistent, it’s unlikely you’ll be able to find the right combination of adapters that will work. It ends up just being easier to buy a new USB Type C adapter for every single type of output you need rather then chaining old ones onto a single Type C to HDMI adapter.
You’d also think that all USB Type C cables are the same right? Well, only certain cables support Thunderbolt 3, and only some cables are rated for higher amounts of power. How do you know? It’s impossible to tell. USB Type C enabled devices are developing into an ecosystem where you have to plug something in and cross your fingers that it all works. This isn’t the way it was meant to be.
Most manufacturers don’t tell you what ungodly mess they’ve got going on inside their products. Because of this complete mishmash, some display outputs will be severely limited in their performance, while the one next to it might be fine. Some docks and adapters may work fine with windows machines but not with macOS. On top of that, sometimes you can’t tell if a USB Type C port, cable or device, is USB3/3.1, Thunderbolt 3, DisplayPort/HDMI over alt mode compatible, etc. It used to be if a cable fit, the device and cable were compatible, but that’s no longer the case.
Consumers shouldn’t need to spend hours researching how an adapter or dock is implemented to work out if it’s going to be compatible with their use case and performance needs. This inconsistency and lack of information from manufactures is a massive problem and is dragging down an otherwise great standard that should be universal and consistent.
P.s. if I’ve left anything out or made any mistakes please let me know in the comments. My head is still spinning from the huge amount of information I’ve processed over the last day while trying to write this.