Confuse HDMI, HDMI ARC, and HDMI eARC at Your Own Risk

Building a home theater or designing home theater equipment? I can tell you it isn’t easy.

After buying our first house, my wife and I wanted the full experience: a dark room with Dolby Atmos surround sound and a projector shining onto a white wall. Though a theater nerd, and tech writer of over 10 years, nothing could prepare me for the HDMI port compatibility issues I was about to experience.

And yes, there are different kinds of HDMI ports — it’s USB all over again.

Figure 1: What ‘universal’ serial bus did you want: USB, micro-USB or USB-C? (Image: StarTech.com)

Building my dream basement theater

The plan was as follows: make the theater as wireless and simple as possible. So, naturally, I became drawn to Sonos. They have wireless capabilities and the ability to mimic 7.1.4 surround sound with only four units and without a receiver. Typically, that sound setup requires seven speakers at seating level, one subwoofer, four ceiling speakers, and one receiver controlling them all. But Sonos approximates this experience by bouncing sound off your walls.

The Sonos Arc soundbar connects wirelessly to its subwoofer and the rear speakers — no problem. However, I soon found it requires a wired HDMI connection from a TV or projector to perform any theater functions. So much for it being wireless — but that’s a story for another day.

I didn’t have, nor did I want, an HDMI cord traveling across the room from my projector to the soundbar, but it wasn’t the end of the world. However, after this mistake, I thought it best to test out this setup before buying anything new. I hooked up the units with a smaller cord, but instead of epic pictures and sound, I got silence and wavy lines on my wall. Apparently, HDMI wasn’t good enough.

A tale of three — make that six, maybe more — HDMIs

The Sonos Arc required an HDMI ARC (audio return channel) or HDMI eARC (enhanced audio control channel) connection. Meanwhile, my projector only had HDMI 2.0 which has neither.

Figure 2: A SiI9437 HDMI eARC audio/video evaluation board. (Image: Lattice Semiconductor Corp.)

HDMI ARC enables a TV or projector to send an audio signal directly to a receiver or soundbar. The idea is to reduce the number of cables overall. Instead of having your Blu-ray, game system, and smart TV box having two output wires (each connecting to the screen and sound systems) they only need to connect to the TV or projector. The HDMI ARC port on your TV or projector does the rest.

As for eARC, it works much the same way, but it can handle higher signal quality, speed, and bandwidths. And before you look it up, eARC is not backward compatible with ARC. However, some home theater equipment designers can choose to make this possible. In other words, read labels if a consumer — or ‘be kind and please rewind’ if you are a designer.

Sadly, the rabbit hole gets more complicated from here. This isn’t a good thing for either the consumer or the home theater equipment designers. For instance, HDMI ports can come in various shapes and sizes. Most are familiar with a standard HDMI connector, but they also come in mini and micro forms — like its USB cousin. So, with the above three options, this makes six different HDMI ports to look out for.

Then there are features that are available in some HDMI version releases and not others. This includes features like source-based tone mapping (SBTM), variable refresh rate (VRR), auto low latency mode (ALLM), quick frame transport (QFT), and quick media switching (QMS). For those interested in what features are compatible with which HDMI they have, the version codes you will need to look up range from 1.0 to 1.4 and 2.0 to 2.1b.

So, how did I solve my theater problem? I investigated HDMI splitters that could take a signal and separate the video and the audio. The result works perfectly. I now have a functional projector connecting to a 7.1.4 surround sound system — via a very long cable — with full Dolby Atmos compatibility.

How to prevent HDMI compatibility issues

While designing my perfect theater and buying my audio and projector systems, I fell for the oldest engineering error in the book: compatibility issues. Though this time the square peg could fit into the round hole — as all the ports were standard HDMI sized — they could not speak the same language.

Therefore, don’t make the same mistake. If you are designing your own home theater, or you are designing the equipment yourself and are ready to buy parts, do yourself a favor and visit the HDMI Licensing Administrator website first.