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Perhaps it’s time to change the name. HDMI stands for “High-Definition Multimedia Interface”, but for some time it has supported Ultra High Definition, and now it’s going beyond even that. The HDMI Forum – an industry group that defines standards for HDMI – has released the specification for HDMI 2.1, with major enhancements for both home theatre and gamers.

The current version – HDMI 2.0 – bumped up capabilities from full HD to UltraHD: 3840 pixels across by 2160 pixels vertically. HDMI 2.1 goes to 8K, which is 7680 by 4320 pixels.

Actually, it even goes beyond that, to 10K. That’s 10240 by 4320 pixels. But that one’s in there primarily for commercial applications.

High Frame Rates

It’s not just higher resolution, it’s also higher frame rates. HDMI 2.0 can transmit UltraHD signals at up to 60 frames per second. Indeed, there’s at least one UltraHD Blu-ray movie shot in that format. (It’s Ang Lee’s Billy Lynn’s Long Halftime Walk and it’s almost unbelievably smooth and detailed.)

HDMI 2.1 pushes that to 120 frames per second at 4K. We’re unlikely to see a new version of Blu-ray supporting that any time soon. Probably never. But gamers are in for a treat. No longer will there necessarily be a trade-off between great resolution and the speed of the display.

And even 8K will go to 60 frames per second … and 120 frames per second with a feature new to HDMI 2.1: DSC (which stands for Display Stream Compression). The digital video on all forms of home entertainment is heavily compressed using “lossy” algorithms. Very heavily compressed. DVDs generally get something like a 20:1 compression ratio. That is, there is twenty times more video data produced when the signal is decoded. Some Blu-ray discs go as far as 33:1.

Until now, video has always been uncompressed on HDMI. That is, the player decodes the video to its full 20x or 33x size, and that’s what the cable carries. But even the new “Ultra High Speed HDMI Cable” can’t carry 120 frames per second at 8K, so version 2.1 has adopted Display Stream Compression from the Display Port specification. This applies what’s called “nearly lossless” or “visually lossless” compression to the signal, achieving a relatively mild compression ratio of up to 3:1.

Better pixels

In addition to providing many more pixels, HDMI 2.1 supports a new, improved form of HDR – High Dynamic Range. It’s called “Dynamic HDR” and from what I’ve been able to work out, it is much like the existing HDR standard except that it adds dynamic metadata. Huh?

Metadata is additional data that goes with a signal and tells the receiving device how to treat the signal. Dynamic HDR could have metadata which specifies different ranges of brightness for the main signal, even down to differences for every frame.

This should not be confused with Dolby Vision. It also does the same with dynamic metadata, but Dynamic HDR seems to use 10 bits for 1024 levels, while Dolby Vision uses 12 bits for 4096 levels.

Better sound via ARC

If you have a soundbar connected to your TV via HDMI, there’s a good chance you’re using a HDMI feature called ARC, for the Audio Return Channel. Most TVs have one of their HDMI inputs labelled “ARC”.

When it was first introduced, HDMI was a one way cable. The video and the sound would go down the cable from DVD player to home theatre receiver. The receiver would pull out the sound and decode and amplify it, and send the digital video on down the HDMI cable connecting it to the TV. ARC allows sound to go back the other way. So if you’re watching a free to air TV show, or Netflix on your smart TV, the digital audio can be sent back down the same cable to take advantage of the much better sound from the home theatre system or soundbar.

But it has so far been limited to standard stereo, or low bitrate surround sound formats like Dolby Digital. HDMI 2.1 introduces eARC, which offers two significant advances over ARC (and, no, I haven’t been able to fine out what the “e” stands for, although I’d guess “extended”).