[noggin]

I have seen conversations regarding tvOS 12.x - that Apple might be using Dolby Atmos in Dolby MAT (Metadata-enhanced Audio Transmissions) to add tvOS system sounds to the LPCM audio stream.

See the Dolby Atmos for Home Theatre (click) discussion paper page 12

Interesting. References to Dolby MAT suggest that it is the way of carrying a variable bitrate Dolby True HD signal over a fixed bitrate HDMI link.

https://www.dolby.com/uploadedFiles/wwwd...ations.pdf

Seems to suggest that a MAT decoder is used to generate a True HD bitstream from an HDMI source?

However it looks as if Atmos has added a MAT 2.0 option to allow Atmos metadata to accompany PCM (or far more simply compressed?) streams rather than just accompanying DD+ or True HD audio? This would allow for games consoles to encode Atmos audio on-the-fly without having to encode to True HD or DD+?  I wonder if Apple TV 4K Atmos compatibility will require some form of decoding in the ATV4K to generate a MAT 2.0 stream, rather than simply bitstreaming out a DD+ Atmos stream?

Dolby Atmos in Dolby MAT
A Dolby MAT encoder resides in a Blu-ray player to pack the variable bit-rate Dolby TrueHD bitstreams for transmission over the fixed bit-rate HDMI. A Dolby MAT decoder is concurrently employed in the Dolby TrueHD decoder in the sound bar to unpack the Dolby TrueHD bitstreams. With the introduction of Dolby Atmos, we have expanded the Dolby MAT technology to support encoding and decoding of Dolby Atmos metadata incorporated in lossless pulse-code modulation (PCM) audio.
A key benefit of Dolby MAT 2.0 is that Dolby Atmos object-based audio can be live encoded and transmitted from a source device with limited latency and processing complexity. Among the likely sources are broadcast set-top boxes and game consoles. The Dolby MAT 2.0 decoder in the Dolby Atmos enabled sound bar outputs the object-based audio and its metadata for further processing inside the device. The Dolby MAT 2.0 container is scalable and leverages the full potential of the HDMI audio pipeline.

I think this refers to carrying a MAT (1.0?) Dolby True HD encoded bitstream with Atmos extensions rather than a PCM MAT 2.0 one - but may also be of interest.

https://forum.kodi.tv/showthread.php?tid...pid2726825

Nevcairiel (developer of LAV Filters):

"Atmos bitstreaming is not fully understood. If the bitrate exceeds a certain limit, it breaks apart, because its not known how to handle that. This was never a problem with ordinary TrueHD since the bitrate doesn’t get high enough. Unfortunately the specifications for that are not available anywhere online (not even to buy), since its not part of the IEC standard that usually controls HDMI bitstreaming. So if anyone knows how Dolby MAT (Metadata-enhanced Audio Transport) works - the format used to wrap TrueHD and Atmos for HDMI transmission, please let me know. "

Incidentally it also allowed me to stumbled on this interesting bit of information. It suggests that there are two implementation options in the ARC standard (not including eARC) - and this may explain why some TVs output Atmos to their AVRs and others don't ?

http://community.cedia.net/blogs/david-m...r-hdmi-arc

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;

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.
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.

If correct it suggests that current non-eARC Atmos streaming may be compromised (but this may not be an issue for the bitrates and quality levels of OTT services?)? It may, of course, be incorrect. (Not related directly to the ATV4K Atmos output discussion - but there have been discussions about ARC and Atmos in the past ISTR)

Looks as if single mode uses Pin 14 + Ground to carry SPDIF, common mode uses Pin 14 + Pin 19 + ground to carry a differential signal (which may also be linked to Ethernet over HDMI)