I was recently discussing with Australian integrator, Hemant Daya the use of HDMI ARC to deliver Dolby® Atmos sound from the Netflix app in a TV back to an AVR. But ARC doesn't support Atmos, right? Well, not until the next generation eARC, that is. So why then does it appear to work (sometimes)? The answer is a little complex, and I felt well worth sharing...
Firstly, a little background. HDMI Audio Return Channel (ARC) is based on the IEC 60958-1 specification, which is essentially the S/PDIF audio spec. There's two types of ARC in HDMI; Single Mode and Common Mode, but unfortunately we don't get informed as to which type any given product supports.
Single Mode ARC uses a single wire in the HDMI link, with performance limited to around 3Mbps. This enables support for 2.0 LPCM and "lossy" compressed surround formats including Dolby Digital and DTS 5.1, but nothing more. Cable length is a huge factor with Single mode, with reliability typically getting flaky beyond about 7-8m in a passive HDMI cable.
Common Mode ARC is superior, but less common (excusing the pun) in terms of deployment. It's still based on the same S/PDIF spec, but can theoretically support up to around 12Mbps. This is enough to carry 24-bit 192kHz 2-channel audio, or — to the point of this blog — a Dolby format called E-AC-3, which we know better as Dolby Digital Plus. This can also carry Metadata-enhanced Audio Transmission (MAT), being audio objects; aka Atmos.
So existing HDMI ARC can potentially support Dolby Atmos by maxing out the Common mode capability with an MAT stream. But there's a huge catch. Two, actually;
HDMI 2.1 will change things considerably. It introduces enhanced ARC, or eARC for short, which can support up to twelve times the bandwidth of ARC, supporting up to 32 channels of 24-bit 192kHz audio! That is, eARC will support all of the same audio formats upstream as what we can already get in a downstream HDMI link.
In summary, a basic, lower resolution form of Dolby Atmos may work through existing HDMI ARC, but it is highly system and link dependent. This is why we tend to generalise that ARC doesn't support Atmos, as it's out of spec and can't be relied upon. If you are designing this capability into systems, sticking with proven product combinations and short connectivity can give your systems an edge until eARC comes along and opens up our options.
David Meyer, has worked in the home technologies industry for around 30 years and now works as a full-time technical consultant to CEDIA in Technology & Standards. He is a professional member of the Institute of Electrical and Electronics Engineers (IEEE), contributor to the CEDIA/CTA joint R10 Standards Working Group, is a prolific educator and subject matter expert in video and connectivity, and author of many internationally published articles and white papers.