The HDMI protocol has had a minor upgrade, with some big implications. In this concluding article, Matt Murray continues his exploration of the changes and how integrators will be affected.

In the last issue of Connected magazine, I started to look at terminology and features affected by changing aspects of the HDMI protocol.

Let’s continue to explore this area:

HDMI forum variable refresh rate

It will probably enter the lexicon as VRR, but the HDMI Forum prefix is key for HDMI 2.1 products. Variable Refresh Rate (VRR) is nothing new to the PC-based gaming community, having begun life – and eternal conquest – as graphics card maker AMD with FreeSync battles Nvidia and G-Sync for format domination.

PC users found the VRR format available to them was dependent on who manufactured the graphics card inside their PC. As such, a wide belief extends this to TV support for either FreeSync or G-Sync to properly support VRR game consoles. Just throwing this out as a free life lesson, never make something thought to be proprietary into a tattoo. HDMI Forum VRR (or if you prefer, HDMI VRR) is the consequential standardisation of this process, with the HDMI group establishing its own version of the VESA Adaptive Sync protocol.

Next Gen consoles Xbox Series X supports HDMI VRR in conjunction with FreeSync VRR, and in April 2022 Sony announced VRR support for HDMI 2.1 VRR compatible TVs for PS5, a presumed alignment with HDMI Forum VRR.

For ardent gamers, VRR is a critical feature to image clarity. Next Gen consoles can send 120fps information to a TV that is capable of displaying that frame rate (see Refresh Rate previously…trust, but verify!), so crucially for gaming, the match-up seems idyllic: the gaming device will send video frames to the display at a rate it can efficiently process them.

In reality, the in-game frame rate fluctuates perpetually as plot scenarios evolve. In an F1 driving simulation, a sequence with the car being prepared in the garage parses out frames less frantically than a 320kph (biggest numberitis shamefully in use) pass in the Belgian countryside.

An HDMI VRR-featured television allows the frames to be displayed at the rate they are sent, not at a fixed rate, adjusting the display’s refresh rate in real-time. Sudden changes in frame rate cause image tearing on displays with static refresh rates. HDMI VRR dynamically adjusts refresh rates to match rapidly changing frame rates for a consistent, smoothly transitioning image.

A VRR-capable display will vary refresh rates anywhere from 40-120Hz.

Xbox Series X and PS5 both are HDMI VRR capable, so the display will require this feature designation as will any device the signal traffics through, such as an AVR. Ultra High Speed HDMI cables are necessary at all device connection points.

While an exciting feature aimed at gamers, be aware that VRR technologies are yet to be deemed perfected. In some instances, they can affect input lag and may not completely eliminate all screen tearing artifacts.

Auto low latency mode (ALLM)

With some display manufacturers, getting into game mode was like trying to get into Studio 54 back in the day…you had to know somebody. Deep dives into normally bypassed parts of the TV menu were necessary to access and activate Game mode, often temporarily disabling settings directly relative to image fidelity on non-gaming sources. For HDMI 2.1 feature set televisions, ALLM signals the display to switch into Game mode as well as activate all the necessary settings to reduce delay and lag.

It essentially optimises pixel processing for best latency or best pixel processing.

It won’t contribute to your gaming experience, only properly prepare for it. ALLM isn’t activated every time a game console is powered up, as it is capable of detecting game play, media streaming, or disc use. Not all televisions with HDMI 2.1 ports necessarily have the feature, as some models launched required updates to enable ALLM.

Quick frame transport

Quick Frame Transport (QFT) is designed to improve the transfer rate of video game data from console output to display input.

Display latency is measured as the time necessary for a frame of data at the source GPU to be displayed on the television screen. The data path is longer than simply the HDMI cable, as it includes output circuitry in the source device, TV processing, and screen refresh. QFT accelerates the transport time between the source HDMI 2.1 output port to the display’s HDMI 2.1 input port at a higher rate than normal.

In gaming terms, the delay between pressing a controller button and discernible movement on the screen is reduced, with any incremental reduction in latency considered quantitative in the gaming world.

HDMI quick media switching (QMS)

Using HDMI VRR, HDMI 2.1a Quick Media Switching eliminates the momentary screen blackout when frame rate changes with a source device. With QMS, employ of the HDMI VRR feature supplies an elegant solution. One caveat, however, is the black screen will still occur if resolutions differ between content.

An example might be Apple TV (set to match content and frame rate). A switch between from 4K30fps on Disney+ to 4K24fps for a film on Netflix will occur seamlessly, however switching from 4K24fps Hulu at 1080p will incur a black screen until sync and HDCP handshake are established via hot plug detect.

Display stream compression 1.2

Companions to higher bandwidth requirements at the as-yet, fringe side of HDMI 2.1, specifically 8K and beyond, are new compression methods for data transport.

Most common is Display Stream Compression (DSC). AVPro Edge carefully examined DSC when designing our first products and determined the criteria for DSC relied too heavily upon declaration that tangible differences to static images were negligible, heavily skewing the conclusion that it was an artifact free platform. As a result, AVPro Edge’s in-house engineering team designed ICT, our Invisible Compression Technology algorithm.

At first glance, DSC 1.2 appears to be very similar in concept to Software Defined Video-over-Ethernet (SDVoE) especially in using the approach of splitting a video frame into equally sized slices along horizontal and vertical, and then processing these slices in parallel.

Theoretically, it can be considered similar to interlaced analogue video and scanning lines. In another facet, DSC 1.2 is said to be able to increase maximum color bit depth to 16-bit with YCbCr 4:2:0 and YCbCr 4:2:2 without the need for conversion to RGB first, taking on the persona of Colour Space Conversion.

With bandwidths and resolutions to be addressed by this compression technology at 64Gbps and 10K respectively, much can change prior to implementation and many years may pass before it is needed.

HDMI enhanced audio return channel (eARC)

HDMI 2.1a features Enhanced Audio Return Channel (eARC) as the follow-up generation to ARC. The primary benefit with eARC is a robust boost in audio bandwidth and speed, from approximately 1Mbps to 38Mbps, supporting up to 32 audio channels including eight-channel, 24-bit / 192kHz uncompressed data streams.

High bitrate audio formats from Blu-ray, 4K Blu-ray and streaming providers including Dolby TrueHD, DTS-HD Master Audio plus object-based formats Dolby Atmos and DTS:X are compatible with eARC. Simplified connectivity with reliable HDCP negotiation were design goals with the actual audio path clear of CEC pathways, avoiding unwanted control protocols. Audio to a TV originating from DBS, CATV, connected source devices, or internal streaming apps can be routed back via eARC to a surround system or to a sound bar through a single HDMI cable.

While connected devices are not specifically required to be HDMI 2.1a certified, certification practically assures eARC support. Manufacturers can produce devices compatible with both ARC and eARC however, eARC is not defined as backwards compatible with ARC.

Dynamic HDR

While HDMI 2.1 standardised the transport of dynamic HDR metadata over HDMI, it only formalised dynamic metadata interfaces already in use with HDMI 2.0 by Dolby Vision and HDR10+. Both formats function properly with HDMI 2.0 and HDMI 2.1a does not provide any additional capabilities. Standardisation incorporates compliance testing to ensure Static and Dynamic HDR metadata can be exchanged through the HDMI interface.

All the benefits of static HDR10, HDR10+, and Dolby Vision remain unchanged in the HDMI 2.1 ecosystem.

Source based tone mapping

The “a” in HDMI 2.1a refers to the inclusion of Source Based Tone Mapping to the HDMI 2.1 specification, introduced on January 5, 2022, prior to the start of CES.

The HDMI Forum members recognised that some streaming services were combining SDR, HDR, Dynamic HDR and graphics overlays simultaneously on one screen. In that mixed-bag environment, metadata is absent as it cannot be generated for a composite image. The source device optimises the image suspending metadata, preventing the display device from attempting to tone map.

SBTM is not a new HDR standard, rather, it is designed to partner with HDR10 and HLG and adjust luminance and colour range per display capabilities when presented with this scenario.

Frame by frame, the source signals the display it is tone mapping the composite content to characteristics outlined by the display’s EDID. Signalling stops when the composite image ceases.

Another instance where SBTM is unique and beneficial is the enabling of ‘plug and play’ HDR gaming without the need for pre-play calibration. From reading the EDID, the gaming device knows the display’s preferred colour volume, and the gaming engine can appropriately tone map the content. No dynamic range in the display needs to be reserved for pixel values outside the specified colour volume, applying full capability of the display to the content being played,

So, there can’t be a downside, correct? As with other unique HDMI 2.1 features, SBTM is a manufacturer-optional feature with no requirement to provide support. The same options apply to HDMI VRR, ALLM, and extended resolutions, bandwidths or 120Hz refresh support. Perhaps this is fodder for a future article, but rescinding listing of the HDMI version numerically and resorting to a provided feature list is a tedious imposition upon the consumer, and perhaps that might be the HDMI Forum’s ultimate intent.

Every square inch of carton real estate will be required to list ALL supported features. The HDMI Forum’s return argument, weak as it appears, is that this is always how its standards have worked and features that are optional give manufacturers flexibility in the functionality they offer.

Summary

Without question, the features scheduled to become active with HDMI 2.1 provide beneficial, ease-of-use image enhancements in lockstep with a massive boost to the format’s bandwidth signal handling capabilities, when all devices and cabling are within the 2.1 ecosystem.

HDMI 2.1a is considered a minor upgrade, adding source-based tone mapping to the HDMI 2.1 specification, yet significant of itself to warrant a designation change.

In the Link Training section (last issue), I commented upon the value of test gear and procedures designed to verify if specified products deliver their expected performance when installed into a new or existing system.

That sentiment interleaves with the notion expressed at the end of Source-Based Tone Mapping that deciphering what features are on-board particular products by list only, sans any inclusive, governing numerical designation, will at times seem and ultimately prove futile, contributing to a collision of confusion we haven’t seen as an industry in quite some time.

As end-users yourselves, enjoy the new features of HDMI 2.1a, but as integrators, buckle up!