HDMI has a built-in Audio Return Channel (ARC). The system can be used to output TV audio but it has a number of limitations that have forced manufacturers to make receivers part of the video chain. An enhanced version called eARC (enhanced ARC) promises to fix everything. We explain the differences.
What is HDMI ARC?
Once upon a time, TVs were capable of outputting audio via analog phono ports. You may remember them as the circular red and white connectors for right and left stereo, respectively. Modern TVs no longer offer analog audio outputs as these were phased out in favor of digital output ports such as optical (S/PDIF). Two cables became one.
With the introduction of the HDMI 1.4 standard in 2009, ARC (Audio Return Channel) arrived. As the name implies, it can transfer audio over an HDMI cable from the TV’s HDMI ARC port to a soundbar or receiver. This is practical when you want to output TV audio to an external device and became even more relevant when streaming apps found their way into TVs.
Here is the HDMI organization’s original pitch:
HDMI ARC is based on the IEC 60958-1 specification, which is basically S/PDIF that is also used as the foundation for optical audio. The flexibility of the HDMI standard (plus copy protection), however, allows additional audio formats to be transmitted over HDMI ARC than via optical but bandwidth is still very limited at 1Mb/s at the base level, although it can be pushed to pass more data.
The limited bandwidth allows HDMI ARC to deliver stereo audio and compressed 5.1 surround. It can be pushed to transmit Dolby Atmos streams containing metadata but it requires an extension called Common mode that is not part of the base specification and must be actively supported in both TV (i.e. LG OLED) and receiver. However, bandwidth of standard HDMI ARC is too limited for compressed 7.1 surround and even more so for lossless audio (Dolby TrueHD or DTS-HD MA). Lip sync correction is also optional under ARC.
Other elements of ARC have been optional, too, which has led to a situation where frustrated TV owners have had to determine themselves exactly which elements are supported. For example, some ARC implementations in TVs are limited to stereo sound, while others that support surround are highly inconsistent. It is not uncommon to find TVs that are happy to pass 5.1 surround from the internal TV tuner via ARC but refuse to handle anything above stereo from other sources.
HDMI ARC has changed the industry
The limitations of ARC have had some unintended consequences and as a result manufacturers of receivers and amplifiers have been forced to take more responsibility.
Back in the analog age, we had separate cables for video and audio but HDMI encompasses both. As explained in the previous section, it is not possible to output 7.1 surround or lossless audio from the TV via ARC (upstream). However, it is possible to carry 7.1 surround and lossless audio the other way (downstream) via HDMI, meaning from a Blu-ray player to a receiver.
This limitation has led to a situation where viewers who seek to optimal audio experience are forced to loop the HDMI signal through a receiver or soundbar before it reaches the TV. This approach is far from unproblematic as it requires the receiver/soundbar to extract the audio signal while leaving the video signal untouched (pass-through). An alternative way is to have a Blu-ray player with two HDMI outputs (one for video and one for audio), which is typically a feature reserved for high-end players.
During the transition to 4K resolution, many owners realized that their receiver or soundbar was incapable of letting a 4K video signal pass through. Even receivers with HDMI 2.0 capabilities sometimes failed as they lacked support for the crucial HDCP 2.2 copy protection protocol that Hollywood has employed as a requirement for getting a 4K video signal to flow between devices.
The transition to HDR (High Dynamic Range) video has been an even bigger headache. It is not sufficient that a receiver supports HDR video pass-through. HDMI 2.0 is required for 4K, HDMI 2.0a is required for HDR10, and HDMI 2.0b is required for HLG. You must also make that the receiver supports Dolby Vision pass-through. And we may encounter additional HDR formats in the near future.
The receiver has become part of the video chain
The receiver has become part of the video chain – to some degree unintentional – but it has hardly solved the problem. Instead, it has increased production costs and forced manufacturers to dedicate additional manpower and resources to development and firmware updates. The same can be said for soundbars; at least if the manufacturer wants to support Atmos / lossless audio formats. A modern soundbar comes equipped with a full panel of HDMI inputs and outputs. And for what purpose? So that it can let a video signal pass-through without altering it, which is a little absurd, but there has been no other choice. HDMI ARC in TVs has simply been too limited.
It also means that even if services such as Amazon and Netflix wanted to go beyond Dolby Digital Plus for streaming audio, they would bump into the wall that is HDMI ARC – at least for built-in apps.
What is HDMI eARC?
If you have made it this far, you have probably already guessed that the industry has a solution. They call it eARC (enhanced Audio Return Channel) and it is a big step forward for TV audio.
eARC was introduced as part of the HDMI 2.1 standard but as reported by FlatpanelsHD in January 2018, the HDMI Forum has confirmed that eARC is one feature of HDMI 2.1 that can potentially be added to HDMI 2.0 based devices via a firmware update, although it is not entirely clear what is required on the hardware side. Bandwidth will be increased from approx. 1-3 Mb/s in standard ARC to 37 Mb/s in eARC. It is enough to push through 7.1 surround as well as lossless HD audio. It also offers other advantages.
With eARC, the HDMI Forum has also made ‘Lip Sync Correction’ mandatory, which means that picture and audio should remain in perfect sync throughout the video/audio chain. Another change is that eARC no longer relies on HDMI CEC (control protocol) for discovery and pairing. eARC has its own dedicated data channel, which allows connected devices to exchange information about which audio formats are supported – and not supported – at each end. The system will automatically select and use the optimal format.
eARC is a big step forward in technological terms but it may also solve the dilemma that we touched on in the previous section. A receivers or soundbar will no longer have to be the control center and will no longer have to handle video signals. Of course, it still can if you want it to but eARC changes the dynamics and offers an alternative. In the near future, the same audio quality and formats that can be sent downstream via HDMI, can be sent upstream via HDMI eARC.
Here is a diagram that the HDMI Forum published when eARC was announced.
You may be wondering where Dolby Atmos (and DTS:X) fits into all of this. It is important to remember that the object-based Dolby Atmos audio format can be carried in one of two ways:
Compressed in the Dolby Digital Plus container
Lossless in the Dolby TrueHD container.
In the future, the TV can become a control center for both picture and audio
As mentioned earlier, it is already possible to output Dolby Atmos in DD+ via HDMI ARC, although with several limitations for quality and the number of speaker channels. The built-in Netflix app on select LG OLED TVs can already do it. eARC will make it possible for TVs to consistently output high-quality Dolby Atmos, and make it possible for TV to do lossless Dolby Atmos pass-through.
In the future, the TV can become a control center for both picture and audio. A soundbar will no longer be required to have a full panel of HDMI inputs. This will affect how products are designed and hopefully simplify TV audio at home.
It will take some time to get there and eARC will need to be supported in both TV and connected audio system but there are encouraging signs of progress. New receivers with eARC are starting to come out but soundbars are a little further behind. In addition, several manufactures have announced that eARC will be added to some existing receivers via firmware. You can use your existing ‘HDMI with Ethernet’ cables for eARC. A certified ‘Ultra High Speed’ HDMI cable will only be required if you want to take advantage of the full HDMI 2.1 package (8K video etc.).
eARC solves several issues. Time will tell whether that is enough. The protocol is currently tied to the HDMI standard so it obviously requires a physical cable connection. Audio is increasingly being transmitted wirelessly – and then it may start all over again.