HDMI ARC and eARC: What They Are, How to Use Them, and Why They Matter

HDMI was born with a single purpose: to eradicate, once and for all, the analog and digital connectivity interfaces that preceded it in the consumer marketplace. And it didn’t take much effort to achieve this. By 2004, barely a year after its introduction, it was firmly entrenched in the A/V market. The end of SCART, the interface created to replace it, and DVI, which had been quite successful in A/V devices at the time, was a fact.

Since then, it's been implemented on a massive scale in audio and video devices and in many computer components. As such, we’re all more or less familiar with it. However, there are technologies associated with HDMI that haven’t yet caught on with users. Despite its practicality and relative ease of use, ARC is an excellent example of this. Let’s see what it is and what it can do for us.

Truly Bidirectional

This article aims to “poke the wound.” And in this case, the wound is the bidirectionality of the HDMI connection.

Unlike its predecessors, which were predictably more limited in functionality, HDMI allows the devices it connects, such as a Blu-ray player, a soundbar, and a television, to “talk” to each other. This means that data doesn’t just flow in one direction, as determined by the source and receiver elements, but rather, that information is transferred in both directions as needed.

This functionality enables the existence of protocols like CEC and, above all, ARC, which is the one we're most interested in. Its implementation relies on two channels in the HDMI interface, one for TMDS (Transition Minimized Differential Signaling) and the other for the CEC (Consumer Electronics Control) protocol.

The first of these channels, TMDS, is a high-speed data transfer technique over a serial connection. This channel isn’t unique to HDMI; DVI also uses it. Its importance lies in transporting the video, audio, and signaling data necessary for the content to flow from the source (the device that reads it) to the destination (the element that plays it).

What makes TMDS a robust technology, and therefore a good idea, is that, from the transmitter’s point of view, the data transmission uses an algorithm capable of minimizing the electromagnetic interference between the cable conductors. On the other hand, the receiver has a procedure for extracting temporary information from the received data stream, designed to decode audio and video information without error, even over long cable runs.

CEC, the other protocol that benefits from HDMI bidirectionality, allows you to use a single remote to control multiple devices. Of course, for it to work, all of the devices must be compatible with this technology—and most have been for a long time. In addition, you need to have it activated it from the configuration menus of the devices connected.

This Is How ARC Works

The acronym ARC stands for Audio Return Channel, a feature introduced in HDMI 1.4 that allows us to use one of our TV’s HDMI connections to extract its sound and send it to an audio device, such as a soundbar, an A/V receiver, or other gadgets.

Before the existence of this protocol, if we wanted the sound of our TV to go to our audio equipment when using a video source integrated into the TV itself, such as DTT or an app such as Netflix or YouTube, we had to connect them using a fiber optic cable anchored to the EIAJ/TosLink connectors of both devices.

ARC allows us to eliminate this cable and use the same HDMI connection to transfer video and audio from our A/V receiver to our TV, for example, or to transfer sound from a TV to our home sound system. Its usefulness is mainly practical, but there are other things to consider. The scalability of an HDMI connection is far superior to that of an optical digital connection.

The HDMI connection interface supports all the HD multichannel sound formats we use today, including Dolby Atmos and DTS: X. It will also support those that come in the future as new HDMI specifications become available.

An optical digital connection is much more limited in this scenario. It can only handle the sound formats that reigned in the DVD era (Dolby Digital, DTS, and derivatives) or other less demanding ones, but not those associated with Blu-ray and beyond (Dolby TrueHD, DTS-HD Master Audio, etc.).

However, the outlook for ARC isn't as bright as it seems. What I've just explained in the last two paragraphs has an important detail that we must be aware of: ARC protocol included in the HDMI 1.4 and 2.0 specifications is only capable of carrying PCM, Dolby Digital, and DTS audio, but not Dolby TrueHD, DTS-HD Master Audio, or the later multichannel digital sound formats.

In practice, this means that even if our TV has a video streaming application that allows us to access content with sound, such as Dolby Atmos, which Netflix has offered since the release of Okja, we won’t be able to enjoy it in that format with our sound equipment.

Fortunately, ARC is about to overcome the limitation in this sound return. It will do so through eARC, the latest and improved revision of the sound return channel.

HDMI 2.1 Bets on eARC

And it was about time. The companies involved in developing the latest HDMI specification, which, as we told you, finally hit the market in November 2017, took care to fix this fundamental limitation of ARC. The result is eARC, which simply means "Enhanced ARC."

Its goal is to enable TVs to send any current HD multichannel digital sound format to audio devices, such as soundbars, A/V receivers, or anything else. It can even handle Dolby Atmos and DTS:X, the most sophisticated multichannel sound formats available today.

And it doesn’t matter whether the HD sound comes from an app installed on the TV itself, DVB-T HD content, a video game console, or any other source connected to our TV. The audio transfer to our sound equipment will be hassle-free due to the enormous transfer speed enabled by HDMI 2.1 (up to 48 GB/s) and the new protocol used in the video and audio synchronization process. Let’s hope that eARC will finally end the annoying lip sync we’ve all suffered from at one time or another.

However, it’s essential to note that to take advantage of all that eARC has to offer, the devices involved in playing back the content must comply with the HDMI 2.1 specification. In practice, only the A/V devices that hit the shelves in recent years, and probably not all, will be able to do so. Let’s hope it doesn’t take much longer.

One Last Important Note

I have two brief notes before concluding this article, which may be useful for many users. First, we must remember that not all HDMI connectors on our TVs and sound devices are compatible with the ARC or eARC connection. The compatible ones are usually clearly marked with something like HDMI 1 IN (ARC) or similar. Those are the ones we should use.

On the other hand, we should remember to activate the ARC protocol in our TV's configuration menu and in the sound system.

Image | JESHOOTS.com

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