One of the more common questions we are asked by readers concerns LG’s implementation of Dolby Atmos, Digital Digital+ and Dolby TrueHD surround sound decoding in select LG premium 4K OLED and, more recently, Super UHD LED LCD TVs.

Part of the confusion surrounding some of these high-quality surround sound implementations concerns compatibility with external soundbars, AV receivers, stand-alone decoders and other devices that need to receive the signal from the TV over the HDMI Audio Return Channel (ARC) connection.

Since the release of HDMI v.1.4, ARC was made available over one channel, and a number of advancements have occurred that have caught up to and surpassed the initial capabilities of the specification. First, the bandwidth requirements/benefits for streaming audio has gone up, enabling many over-the-top streaming services to move to a more robust Dolby Digital+ codec for online streaming. This has stretched the bandwidth limits of the original ARC specification.

The current ARC was designed to pass a maximum of approximately 1 Mbps (it was originally intended to be 384 Kbps), which is just big enough for DD+ but is insufficient to handle newer and more robust lossless surround sound formats that arrived later, including Dolby TrueHD, DTS Master Audio and others.

On top of these object-based audio formats including Dolby Atmos and DTS:X were introduced, bringing over-head height channels and other 3D sound effects to home theaters.

Dolby Atmos can be encoded as part of either a Dolby TrueHD stream or a Dolby Digital+ stream, depending on capacity/bandwidth requirements. When encoded in either version, the stream containing Atmos carries essentially metadata instructions for extracting and placing the location(s) of the sound of objects in relevant positions around the audience. Dolby TrueHD/DTS-HD Master Audio, which are both lossless variable bit-rate codecs, can take the same object-based information instructions (for Atmos or DTS:X respectively) but each requires significantly more bandwidth to do so.

In the same way, an Atmos encoded Dolby Digital+ stream contains metadata to extract and place object sounds around the room, except that format is lossy. (So the Atmos or similarly DTS:X information is compressed to fit in the limited HDMI-ARC space, but it lacks some of the clarity, depth and dimensionality of the lossless surround sound codecs transporting it).

Dolby Digital+ uses a more efficient compression technique than Dolby Digital at data rates from 96Kbps to 6 Mbps. It can also support up to 7.1 discrete channels, instead of just 5.1 for Dolby Digital.

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On last year’s LG 7 and 2018 8 Series OLED TVs and some Super UHD LED-LCD TV models with Dolby Atmos you can send the decoded Atmos surround information in the TV out over the HDMI ARC connector, but only when the signal is carried in a lossy Dolby Digital+ surround sound stream.

LG TVs also now support the lossless Dolby TrueHD decoding format internally, but this must be delivered into the television over a non-ARC HDMI input for decoding internally by the television.

One of the things that has made this whole issue so complicated is that when LG first introduced its Dolby Atmos OLED TVs, none of those sets had Dolby TrueHD support built in. That came later through a firmware update. And while a number of the brand’s premium TV models now support TrueHD, that is only the case if a TrueHD signal is input over a non-ARC HDMI input from a supporting device like a Blu-ray player. There is no way to get the decoded Atmos/True HD signal back out of the TV over the old ARC version. The sound can only be enjoyed through LG’s built-in sound system or integrated soundbar (like the one in the LG “wallpaper” 4K OLED models).

Further, the best way to enjoy Dolby Atmos or DTS:X presentations is when they are carried as part of lossless Dolby TrueHD or DTS-HD Master Audio soundtracks. Ideally, these should be played through a true multi-channel home theater speaker system, complete with overhead front and rear-channel channel speakers.

Again, this is because a television, even a good one like LG’s W7 or W8 4K OLEDs from 2017 and 2018 with the supplied external soundbar (connected to the screen by a flat ribbon cable), is going to miss some of the dimensionality of the full Dolby Atmos/TrueHD experience with the smaller, more constricted speaker layouts.

To explain further, Dolby TrueHD is a lossless codec that uses less space than a Pulse Code Modulation (PCM) track – an analog signal converted to an uncompressed digital form — but identical in sound to the original master. Dolby TrueHD supports up to eight full-range channels (with room for expansion) of 24-bit/96 kHz audio (at the discretion of the studio) up to a maximum of 18 Mbps bitrate.

So, while you can get an LG TV’s decoded Atmos signal to work carried in a DD+ soundtrack over ARC, you will have to decide if the clarity of sound from that compressed format is good enough for you. If it’s not, you are better off listening to the Ultra HD Blu-ray version with the Dolby Atmos or DTS:X content connected directly into a supporting AV receiver, which is then connected to the television via and HDMI output carrying the video information.

For those who buy a higher-end LG TV in 2019, things get much more flexible using the new Enhanced Audio Return Channel eARC feature specified for voluntary support in the new HDMI 2.1 specification. With that implementation carrying eARC ample bandwidth is supported to enable carrying the full Dolby Atmos/Dolby TrueHD (or DTS:X/DTS-HD Master Audio) experience.

It was also designed to get rid of the many device recognition and handshake issues that plagued the original ARC implementation. Unfortunately, the first HDMI 2.1 with eARC devices aren’t likely to appear until sometime next year. We are still waiting for the HDMI Group to put out testing specifications and for supporting SoC’s to emerage, before anything is going to happen. That isn’t expected to begin into some time in the fall.

LG has said this year’s TVs will not be upgradeable to HDMI 2.1 inputs, and it is unlikely any other 2018 televisions will be either.

But before you hold off the purchase of a TV this year, you also must consider this: most of the OTT streaming services carrying Atmos surround sound today – these include: Netflix, Amazon, Vudu etc. — are using the lossy Dolby Digital+ implementation of Dolby Atmos, leaving Ultra HD Blu-ray of HD Blu-ray players as the primary way to listen to Atmos-supporting soundtracks in a lossless Dolby TrueHD environment.

So if you don’t have a TrueHD supporting NAS device, Blu-ray or Ultra HD Blu-ray player and plan not to get one, the chances are you won’t have any way of getting any Dolby Atmos/TrueHD content any way. Streaming Atmos/Dolby Digital+ is pretty munch the only available alternative.

We can’t recommend highly enough the benefit of getting a full Dolby Atmos or DTS:X surround sound experience over both a full object-based audio surround sound speaker system playing a Dolby TrueHD sound track and then over one of LG’s Dolby Atmos-supporting 4K OLED or Super UHD LED LCD TVs.

Both experiences are worth the effort and should help any listener understand the differences in the benefits of each approach.

By Greg Tarr

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