Dolby Atmos over HDMI ARC

By David Meyer posted 06-25-2018 23:46

  
OTT Atmos via HDMI ARC

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; 

  1. Channel count & resolution — while E-AC-3 can support up to 15.1 channels of audio, it's well beyond the HDMI ARC spec. Even getting 7.1 channels to pass is a stretch, so even if the stream contains object metadata, it will lack the resolution and height speakers. 
  2. System support — getting Atmos to work through HDMI ARC requires Common mode support in both the TV and AVR, and a high integrity link in-between. Even then, performance will be marginal.
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.

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