The Arcane Mess of Cables

Photo by Jakub Żerdzicki on Unsplash

If you do a fair bit of fiddling with computer systems or electronic equipment (👋🏻 Hi, fellow audiophiles), you must have encountered cables of different forms and sizes. Considering the horrors we had in the past, the modern-day mantra of USB-for-everything, with some HDMI and whatever the weird contraption Apple throws at us, is usually enough to connect everything. Except it isn’t.

The evolution(?) of cables 🔌

The great thing about the cables of yesteryears was that they either worked or did not work. And when they failed, they failed spectacularly due to their analogue signalling. If you ever had a loose mouse connector that skipped constant white noise from speakers or a bad VGA cable producing an occasional rainbow on your monitor, you know what I’m referring to.

However, for good or bad, we have moved to a digital world where we can fail and gracefully degrade. You only know when you try to push the limits, and then you wonder whether it’s a faulty cable, connector, or software!

A few years ago at work, we had these Thunderbolt 3-enabled monitors that also supported USB C Displayport Alt mode + data (I will explain these later). Thunderbolt uses the same interface as USB C but only works if you plug it in a particular orientation. However, the monitor works virtually identically in both modes, with the refresh rate being lower and USB ports dropping to Gen 2 speeds (480 MBps) if you have connected it to the USB C mode by plugging the cable wrong. There is no other indication unless you notice the jankyness in the mouse (I wasn’t used to 120Hz screens then, so I couldn’t see much) or slowness in data transfer speeds over USB. How confusing it was!

One cable to rule them all

USB Type C is everywhere! It charges devices, transfers data, connects peripherals, and has a powerful alt-mode that allows you to use some of the pins for transmitting signals other than USB. It will enable you to connect displays and even transmit analogue audio (which is how many Androids removed the headphone jack). It’s the holy grail of connectors. However, it’s a super convoluted mess!

The USB-IF messed up big time. With so many optional capabilities, it resulted in maddening permutations and finding cables that can support them is a task. Also, let’s not talk about naming now!

As it turns out, USB C is simply the connector form factor. USB 1–4 are generations that determine speed, signalling, power delivery, and other capabilities. A USB C cable can be of any generation from 2 to 4 and support any power delivery mode, which gets electronically marked with a marker chip. If the chip fails, it gracefully degrades to USB 2 capabilities. The Thunderbolt standard also uses the USB C connector, another high-speed connectivity solution by Apple and Intel.

Here are some combinations that I know exist:

1. A USB C cable, by default, supports 5V/1.5A (7W) current, but most cables usually support 15W of power. If your phone charges slowly, try a different cable, like a laptop’s power cable. Those things are rated at 100W (20V/5A). Have a look at the USB PD Wikipedia article for more combinations of power delivery support. Your cable may support any power level.
2. USB C cables can support USB 2/3/4. The thin and long ones that come for charging are usually USB 2. They only support transmitting data @ 480 Mbps and don’t support audio, video, or any other protocol in alt mode.
3. USB C cable “certified” for USB 3, which was first renamed USB 3.1 and then renamed USB 3.2 Gen 1, supports 5 Gbps data transfer and transmitting audio or video. USB 3.2 Gen 2 supports 10 Gbps, USB 3.2 Gen 2 x2 supports 20 Gbps, and USB4 supports 40 Gbps in addition to alt-mode. (What the hell is this USB-IF!?) During alt-mode, the cables either fall back to USB 2 or 3 speeds depending on the generation and the devices.
4. Thunderbolt is a different beast. It looks like a USB cable but only supports Thunderbolt at 40/80 Gbps for Gen 3 and Gen 4, respectively. If connected to non-Thunderbolt ports, it falls back to USB 2. Thunderbolt supports audio and video output, but you cannot use a Thunderbolt cable to connect to displays over DisplayPort alt mode.

These are the combinations I have personally verified with different hardware and a box full of USB C cables. Want more madness? See the Wikipedia article for USB C.

Display cables

You can connect displays using HDMI or DisplayPort (DP). These are much more robust than analog-based VGA or component connectors. Currently, DP 2.0 is faster, with 80 Gpbs supporting 8k displays at a 120 Hz refresh rate. HDMI is widely used with media equipment and can transfer audio or video. DP is more popular on computers. Both standards are always playing catch-up in terms of capabilities. But simply connecting two devices with a compatible cable is not enough.

For HDMI and DP, devices typically advertise the version of the connector they support, each being backwards compatible. For example, DP 1.4 supports 4K@120 Hz at 10-bit colour, and HDMI 2.0 supports 4K@60 Hz at 8-bit colour. However, for some reason, god knows what, the HDMI and the VESA alliance explicitly forbid cable manufacturers from stating the maximum version the cable is certified to work with. You must now tally the speed or designator to find the corresponding version.

1. A high-speed HDMI cable or an HBR DP cable supports 4K@30 Hz using HDMI 1.4 or DP 1.2
2. A premium high-speed HDMI cable or an HBR2 DP cable supports 4K@60Hz using HDMI 2 or DP 1.4
3. An HBR3 DP cable supports 4K@120 Hz using DP 1.4
4. A UHBR DP cable or an ultra high-speed HDMI cable supports 8K@60 Hz using DP 2.0 or HDMI 2.1
5. You can connect a DP output to an HDMI input. The HDMI version would be the nearest compatible version that the port support.

Also, both HDMI and DP can work in DisplayPort Alt-mode, adding another layer of compatibility complexity.

The harbringing of USB Type C and the evolution of HDMI and DisplayPort have simplified the cable conundrum by allowing a single connector to handle multiple functions. However, this advancement has also introduced a new layer of complexity with multiple modes of operation, power delivery support, and varying data transfer speeds. While these modern connectivity solutions offer incredible versatility, users need to clearly understand these cables’ diverse capabilities and limitations to ensure seamless and optimal device connectivity.

As technology advances, we’ll likely see further refinements in these standards to streamline the user experience and reduce cable compatibility confusion. Things are already in motion. The Thunderbolt and USB4 specifications merged, so there is one less permutation and some good news to cheer on!

Originally published at https://recursivefunction.blog.