Without an HDMI eARC connection the Sonos Arc can not deliver the best-quality sound – we show you how to maximise sound quality with your slightly older TV.
The Sonos Arc is the company’s first steps into the world of high-definition home cinema audio, delivering Dolby Atmos 3D sound.
Yet, as good as this move is, there’s potentially a problem for many people: the Sonos Arc only has a single eARC HDMI input, so what does this mean if you don’t have the same type of port on your TV? The answer depends on the type of TV you have, how old it is and the types of ports you have available.
Related: Sonos Arc review
What is HDMI eARC?
The HDMI Audio Return Channel (ARC) was designed to let audio flow in the opposite direction to a soundbar or home cinema amp. Designed as an upgrade to Optical S/PDIF, HDMI ARC has a maximum audio bandwidth of 1Mbps (S/PDIF is limited to around 384Kbps).
In addition to boosting bandwidth for higher-quality audio, HDMI ARC also supports HDMI CEC, which means your TV’s volume controls can adjust sound on the external device, and there’s built-in lip-sync correction, too, although this often needs manual adjustments to make it work properly.
The problem is that high-quality uncompressed audio, such as Dolby TrueHD, needs more bandwidth and won’t run over HDMI ARC. This is where HDMI eARC comes in, with an increased bandwidth of 37Mbps, and improved lip-sync.
HDMI eARC is backwards compatible with HDMI ARC, and can switch to the lower bandwidth option for compatibility with older devices. So, an HDMI ARC on a TV can be plugged into the HDMI eARC of the Sonos Arc.
Which TVs have HDMI eARC?
Although HDMI eARC can technically be implemented on HDMI 2.0b ports, the reality is it’s only TVs with HDMI 2.1 ports that support eARC. Effectively, unless you’ve bought a TV in the last 18 months or so, it’s unlikely that you’ll have HDMI eARC, which means you can’t run the highest-quality audio into the Sonos Arc.
If you are lucky enough to have HDMI eARC (the port on the back will be labelled as such), then the full high-quality signal can be passed through to the Sonos Arc.
Related: What is HDMI 2.1?
Can’t you connect a device directly to the Sonos Arc through an HDMI splitter?
HDMI eARC (and ARC before it) output sound on different pins in the HDMI cable than with a regular HDMI port. So, sound goes from a device (or app) into the TV, which then outputs the sounds on the right pins to the external device connected via eARC or ARC.
It’s more than just sound being on the right ports, as there’s a handshake that has to go along with the connection: this sorts out how sound is synchronised, plus the HDMI CEC control information.
Even if you buy an HDMI 2.1 splitter and send one cable into the back of the Sonos Arc, you won’t get any sound out of it at all. Unfortunately, there are currently no external sound boxes that can convert a signal into HDMI eARC, either, although some companies are looking at creating one.
If successful, you’d effectively have an external box with an audio output that would feed the Sonos Arc, and a video output that runs to your TV.
What happens if my TV only has HDMI ARC?
If you’ve got a TV with HDMI ARC only, then you can’t get full uncompressed audio into the Sonos Arc. Instead, some form of lossy, lower bandwidth codec has to be used. What happens, depends on the type of TV that you have.
There’s a bit of overlap, but the general rule is that if you have a TV built before 2016, it can use Dolby Digital, the oldest standard, for compressed 5.1 sound. If you’ve got a TV made after 2016, then it will most likely support Dolby Digital Plus, which is encoded in rates of up to 768Kbps, and supports 7.1 channels of audio.
Dolby Digital Plus is an advanced codec, designed to sound as close to the source material as possible, delivering higher-quality audio than Dolby Digital while using a lot less bandwidth than Dolby TrueHD. However, Dolby Digital Plus still uses lossy compression, so some detail is stripped out; Dolby TrueHD uses lossless compression.
What about Dolby Atmos?
If you’ve got HDMI eARC, then you can get Dolby Atmos at the highest quality with any Dolby TrueHD source, such as an Ultra HD Blu-ray player. This will move the full-quality sound through your TV to the Sonos ARC.
Streaming services that support Dolby Atmos don’t use Dolby TrueHD as it’s too bandwidth-heavy, and instead encode Atmos using Dolby Digital Plus, which is encoded at between 384Kbit/s and 768Kbit/s. So, if you watch Netflix or Disney Plus, then you’re still getting a compressed version of Dolby Atmos.
The good news here is that if you have a TV with HDMI ARC that supports Dolby Digital Plus and Dolby Atmos, then you’ll get the same quality from streaming services as on a TV with HDMI eARC.
Things get a bit more complicated when it comes to external devices. Blu-ray and Ultra HD Blu-ray discs only carry Dolby Atmos as part of a Dolby TrueHD soundtrack, which as we know can’t be passed through to the Sonos Arc, as there’s not enough bandwidth. There is a workaround: if your TV supports Dolby Atmos decoding natively, such as LG’s 2018 OLED TVs, then the TV can decode the high-resolution Atmos input and re-encode it as Dolby Digital Plus to send to the Sonos Arc.
You get a slightly compressed version of the sound, but in practice, it’s hard to hear the difference as the change is subtle.
Related: What is Dolby Atmos
Does Dolby Atmos encoding work with all inputs?
Unfortunately, whether or not a TV can encode Atmos to Dolby Digital Plus still depends on the device and what it’s capable of. For example, Sky Q has Dolby Atmos on some of its programmes and films, but when I fired up the integrated Netflix app, there was no Dolby Atmos content available; using the LG Netflix app, I could send Atmos to the Arc.
What about DTS sound?
Sonos doesn’t support any DTS sound format at all. In fact, if you try and play a disc using any form of DTS sound (original DTS, DTS-HD MA, or DTS:X) you won’t get any sound on your Sonos Arc at all. That’s rather frustrating, as many discs only have a surround soundtrack in DTS audio, and don’t offer an alternative Dolby Digital one.
Currently, there are two options for what to do. First, if you have a Blu-ray or Ultra HD Blu-ray player that can convert the audio to Dolby Digital then you can use this option. This will maintain the surround sound. Secondly, you can get your player to output DTS sound in PCM. With this option, your player converts the soundtrack to an uncompressed form. This sounds good, but currently the Sonos Arc only supports stereo PCM, so you will lose surround sound using this method. Sonos has said that it’s working on multi-channel PCM support, but this will only work over eARC not ARC.
What if I don’t have ARC at all?
If you don’t have a TV with any type of HDMI ARC connection, then you can use the optical adaptor that Sonos provides. As noted above, this will limit you to Dolby Digital with more compression, and therefore lower-quality sound. And, you won’t be able to get Dolby Atmos.
There has to be a workaround, right?
If you don’t have a TV with HDMI eARC and don’t want to have to upgrade, then HDFury has something that could work for you. The HDFury 4K Arcana 18GBps is an eARC adaptor. It has one input, which you use to connect your external devices via a switch (Sky Q, Ultra HD Blu-ray player, etc), and two HDMI outputs: one sends the picture to your TV, the other sends sound in an eARC compatible format to your Sonos Arc.
With this solution, you’ll get full uncompressed audio sent to the Sonos Arc, and you don’t have to upgrade your TV. It could also reduce lipsync problems, which can occur when everything runs through your TV.
The project is currently in development, with pre-order products expected to ship by 1 October 2020. The original goal has been reached, but the product will continue to be developed with new features. HDFury is promising to try and add extra features before products are received if 800 pre-orders are launched. These additional features are the ability to forward CEC volume control from source to Arc or from TV to Arc, and the ability to forward TV ARC input and send it to the soundbar.
The company is confident, but can’t guarantee, that these features can be implemented. We’ll bring you more information and a review of the box as soon as we can.
Can I use the Sonos Arc with a projector?
HDMI ARC ports aren’t that popular on projectors, although there are some models that have these. If your projector has HDMI ARC, you need to connect your playback device to the HDMI in port, and then connect the HDMI ARC output port to the Sonos Arc. As far as we can tell, there are no projectors that support Dolby Atmos via this method, and we can’t find any projectors that have an HDMI eARC port for the highest quality audio.
For projectors that don’t have HDMI ARC, you currently need a device that can output via S/PDIF, although you can also buy an HDMI audio extractor, such as the Lindy 4K Audio Extractor that can take an HDMI input, pass the video onto a screen, such as a projector, and output the sound via S/PDIF to the Sonos Arc. In the future, the HDFury Arcana (see above) could do the job.
However, doing this limits you to 5.1 Dolby Digital with no surround sound. Given the limitations of using a projector, largely the lack of Dolby Atmos support, you’ll be better off with an alternative sound system.