HDMI 2.1 New Feature: Enhanced Audio Return Channel (eARC) for the audio junkie in you

Hey! We’re back. We just had a very busy week last week.
Let’s talk about HDMI 2.1. As you might already know, HDMI 2.1 Specification was published last year in November 2017. If any of you have had the luck to read it you’ll know what features we will likely see from HDMI devices in the not-so-distant future. But since you’re already here let’s discuss those features and speculate what kind of devices will likely support those features. Since each new HDMI 2.1 function/feature deserves an article of their own we’ll try to write a series of blog posts about them in the coming weeks. Today, we’ll talk about a neat new feature called eARC. Ok, let’s go!


eARC is short for enhanced Audio Return Channel. ARC itself is an old tech — it was a new feature back when HDMI 1.4 came out around June 2009. ARC points to the technique where the sink, after receiving audio visual signal from any of its connected devices, “returns” or outputs the audio part of the signal through HDMI to an upstream amplifier (which supports ARC).

Usually the flow of the HDMI signal is from source to sink (upstream device to downstream device). ARC’s signal(audio) however flows from sink to source. That’s why they call it “audio return” channel. Sounds neat eh? But I guess it’s difficult to imagine it without a diagram. So let’s borrow HDMI Org.’s diagram on ARC to illustrate the difference.

Before without ARC function
As you can see, before when there was no ARC, the sink (or the TV if you prefer) sends its audio to an amplifier to render the audio there — most probably with an analog or digital S/PDIF. The idea is that since TVs do not have a good sound system those audio junkies who prefers pristine audio will want to use an AV amp to render audio just like you see in the diagram above. But adding another cabling just complicates the setup so HDMI Org. came up with a solution — the ARC solution.

HDMI setup with ARC function

Now look at what adding ARC function did to the setup. This setup looses the extra cable (S/PDIF). So how does the TV renders audio? The TV uses the HDMI cable to output audio to the connected AV amplifier. Since it can receive and output signal through this HDMI cable, the diagram shows that the flow is now bidirectional. So how does it work exactly? Well, there are several ways ARC can be used. See below:

Possible ARC Configurations

  1. The TV receives the audio/visual signal though an upstream AV amp (just like the illustration above). It decodes the signal. Takes the audio part of the signal and then “returns” it using the ARC channel in HDMI connection. ARC negotiation is done using CEC (Consumer Electronics Control) protocol also inside the HDMI connection.
  2. Imagine there is another source device (like for example a Playstation) directly connected to the TV above through a HDMI cable. The TV renders the Playstation’s video signal on its display but again takes the audio part and outputs it to the AV amplifier connected through another HDMI cable.
  3. The TV gets its audio visual contents from its built-in tuner, like a local TV program. It renders the video part on its display and again outputs the audio to the AV amplifier connected through a HDMI cable.

In all three setup, the TV uses the ARC channel to output the audio signal to an amplifier (which is usually a HDMI repeater) connected to one of its HDMI input port. For ARC to work, the TV and the amplifier device must both support ARC, the TV supports ARC Tx function while the amplifier supports the ARC Rx function.

Enhanced Audio Return Channel (eARC)

Now that’s all very good, but how about eARC? Don’t worry, we haven’t forgotten about it. We just wanted to make ARC clear to you first. The setup for eARC are mostly the same with that of ARC. What’s changed is the audio codecs supported. eARC now supports multichannel audio like 8ch.-192kHz LPCM audio, whereas before it could only support 2ch.-48kHz LPCM audio.

eARC also can now support the output for High Bit Rate audio codecs like Dolby True HD and DTS Master audio. What’s more and completely new to HDMI 2.1 is the support for the latest audio formats supporting three-dimensional object-based surround sound like Dolby Atmos and DTS:X. For head to head feature analysis see this table prepared by HDMI Org.:

ARC vs. eARC functionality

Looking at the table above, it is evident that the higher bandwidth of eARC enables the output of the new high bit rate audio codecs: 37 Mbits/second for eARC versus 1 Mbits/second when using traditional ARC link.

This added bandwidth together with the added complexity of implementing eARC data channel’s control and discovery will be tricky at first. This means adequate testing of eARC functionality will be crucial task for new developers. Test specification for HDMI 2.1 is currently being developed by HDMI Forum and is said to be scheduled for release in stages this year. When it is finally published the test spec will be an important guide for developers intending to develop eARC function.

Devices That May Probably Support eARC

As the ARC configuration diagram above shows, devices that will most probably support eARC functionality will include high end TVs (eARC Tx) and AV amplifiers (eARC Rx).  For the most part, the main reason for eARC is to enable support for high bitrate audio codecs. Taking advantage of high bitrate audio codecs like DTS Master, Dolby True HD, DTS:X and Dolby Atmos will most usually require multi-channel multi-speaker audio system setups.

That means that the target users has big living room and has advanced audio system installation. So either the user is pro sound user who demands the latest gadget in anything audio, or the user is very well off that he or she has cinema-level multimedia implementation at home. Whichever user it is, it is almost unmistakable that the devices that will support eARC will be targeting the high end users.

Ok, so that’s eARC.
Next week we’ll write about HDMI 2.1’s 8K resolution function and the technologies that drives it.


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