USB-C: Not as simple as it seems (when it come to video)

The latest ‘standard’ connector for computers and most things electronic that exchange data is the new USB-C connector. The plan for USB-C is for it to replace all those various previous versions of the USB (Universal Serial Bus) connector that we have at the moment, being:

·         USB 1.x (from memory there was 1.0, 1.1, and 1.2)

·         USB 2 Hi-speed

·         USB 2 Full-speed

·         USB 3

There are also the various types of USB connectors; including:

·         Type A

·         Type B

·         Mini-USB

·         Micro-USB

·         Micro-USB 3 (sometime known and Micro-B)

These will all be replaced with the simple standard USB-C, which, just to start the confusion, is also known as USB 3.1. Oh, no. Hang on. It’s not quite that simple. There is actually USB 3.1 Gen 1 and USB 3.1 Gen 2.

So now the confusion starts.

While USB-C actually comes as USB 3.1 Gen 1 and USB 3.1 Gen 2 you will find that the packaging labelling will generally not tell you which one you are getting. Does it matter? It does a little. USB 3.1 Gen 1 provides a data throughput of 5Gbps and USB 3.1 Gen 2 goes up to 10Gbps.

USB 3.1 Gen 1 is also known as SuperSpeed USB. USB 3.1 Gen 2 is, as you might have guessed, SuperSpeed+. But again, this is almost never indicated on packaging.

The other trick that USB C/USB 3.1 has is that it can carry video, and, if needed, it can also provide power to the video device—but you probably would not want to be doing that from a notebook PC that is not connect to mains power as it would drain your battery rather quickly.

USB-C can carry a number of video transmission formats, but the two that are most interesting are HDMI and DisplayPort. Oh. I should point out that it only carries the ‘old’ HDMI specification (which I think was known as v1.4b). It doesn’t carry the current 2013 HDMI 2.0/2.1 specification which allows for 4K resolution at 60 fps (and whole pile of other stuff).

But now we get to the really tricky bit when it comes to video from your computer.

If you have a notebook computer with discrete video (i.e., it has a separate video card and does not force video computations through the main processer or use main memory for video caching) then it is likely that if you connect a video device via USB-C that your discrete video capability will get bypassed. This is because now the main processor will have to do all the video rendering work in order to pass it via the USB-C connection to the screen.

Where this happens, there will also be an impact on the performance of your computer. Especially when you are doing things that require a lot of video rendering—such as playing games, watching movies, photo editing, or video editing. There will also be an impact on the amount of main memory you have because your video will now be needing to use main memory instead of (or as well as) the video memory on the discrete card.

However, if your computer firmware is smart enough and the discrete graphics card supports it, then the discrete video can be transmitted via the USB-C connection. In which case your main processor and main memory will not be impacted.

Right now this is not an issue that we need to be too concerned about because there are currently very few external computer screens that have USB-C input. But this is expected to change rapidly over the next year.

My advice today would be that if you are connecting an external monitor and you have the choice of DisplayPort, HDMI, or USB-C then don’t pick USB-C.