The biggest new feature is a new cable to support the new features, and it is capable of supporting video streams of ~50Gbps without compression. (Note: The modes taking 48.11Gbps do not require compression, those that support 50.11Gbps do require DSC so we will call the break point 50Gbps) With this new hardware which is backwards compatible with the current A/C/D connectors, you can do a lot more neat things.
High rez? We got your high rez.
The one 99% of people care about is above, the resolutions. You can do up to 8K/60 in high bit depths with the new cable, above that you need DSC compression. DP took the opposite route, requiring compression to get to 8K but they took the same cable. For the next step to >8K, DP will have to go to new cables, HDMI won’t. Which is better? Neither, they are just different routes and some prefer one to the other. In any case HDMI 2.1 supports up to 10K120 with 12b color 4:2:0 encodings or 10K60 12b in 4:4:4. Both of these two require 120.29Gbps data rates so DSC is obviously needed.
As you can see from the above, 120Hz is baked in as are variable refresh rates. On top of that BT.2020 color is part of HDMI 2.1 as well. For those interested in HDR, there is another big change here as well. Normal HDR data used descriptor data once per stream, 2.1 adds dynamic HDR descriptors that can be sent on a per-frame basis if needed. This should go a long way to improving visual quality in new silly-rez panels.
There is a lot more non-video data passed along a HDMI 2.1 cable too, more than the 100Mbps ethernet channel now supported. The biggest advance in this area come in with audio data and a feature called eARC. Instead of 7.1-channel sound, eARC supports 32-channel high-bitrate uncompressed audio. For those readers pining for object-based audio, your cable spec has arrived.
Then there are a few more technical features that reduce perceived latency in HDMI 2.1 the first of which is called Quick Frame Transport (QFT). “Each video frame travels faster from the source even though the source does not increase its frame rate and results in deceasing latency.” SemiAccurate can’t say exactly how this works but we would bet it starts sending the frame before it is fully buffered like several other video and network protocols do, think cut-through routing on a video level. Why? “This reduces lag for gaming, real-time interactive virtual reality, and enables more responsive karaoke.” Anything that makes karaoke more responsive is… is… someone stop karaoke before it spreads more, please.
Then there is an interesting feature called Quick Media Switching (QMS) that is long overdue. What QMS does is allow resolution, frame rate, and other basic video spec changes to occur without that annoying black screen pause. It probably works by keeping the highest rez available on the panel and just using the scaler to compensate rather than switching panel modes. That said we don’t know for sure about this one either. Anyone want to bet this one was pushed by the broadcasters for high-rez commercials? This one probably makes karaoke better too, not good but better.
Last up is ALLM or Audio Low Latency Mode. From the sound of it this mode decouples audio and video streams so slowdown on the video side doesn’t delay sounds. The official documentation explains this is useful for switching from movies to gaming and VR, something that people do on the fly all the time or something. Why not just keep audio at low latency all the time, I can’t think of a good reason why not. At least it’s not Karaoke.
Those are the high points the new HDMI 2.1 spec brings to the table. Of the lot, the new cable is the big bang, it allows support of up to 10K120 12b 4:2:0, with compression, or 10K60 12b 4:4:4. Without DSC you are stuck with ‘only’ 8K60 12b, oh the horror of karaoke in such low rez! On top of that variable refresh, low latency features, 32-channel sound, and much more are added to the mix. If a new cable is required for all of this, fine by me.S|A