May 26, 2022 - As a manufacturer of video distribution products sold internationally, at AVPro Edge we are hyper-confident our affiliations with HDMI, HDCP, HDBaseT and the Imaging Science Foundation enable our engineers to design best-in-class products containing HDMI inputs, outputs and without-a-doubt-puts. Yet at times in our Sioux Falls headquarters, walking past the tech assistance offices that monitor our phone support lines, and pausing to hear integrators calling from project sites with a myriad of aural puzzles, I become re-amazed (sure, feel free to use that one...). Has our industry become so devoid of training it is taking for granted all products with HDMI ins / outs / ups and downs now simply work, regardless of their born-on date? In one sense that is true, as HDMI was designed for licensed products to produce an image when connected together.

As a major update, HDMI 2.1, was announced on January 4th, 2017, and while for some that may have passed unheralded, the industry is now on the cusp for 8K resolution and other significant enhancements to become mainstream. 

Inattention to HDMI designations with legacy devices, particularly cabling, produces unanticipated compatibility issues for consumers and integrators alike, as next generation products attempt to mesh within existing systems. And adding to that, HDMI LA (HDMI Licensing Administrator, Inc.) announced prior to the opening of CES 2022 a minor revision, but a change in designation nonetheless, to HDMI 2.1a.

AVPro Edge sponsors AVPro Academy regional trainings and presentations at CEDIA Tech Summits, affording the integration community opportunities for keeping pace with new technology convergence. HDMI 2.1a crosses the HDMI 2.0 series Rubicon, ushering forward a profound collection of advancements to enhance the viewing experience; it is still not, however, sans pitfalls. 

Below I detail these features and the effect they impart on the future of video distribution and playback. 

HDMI 2.0/a/b

The previous major leap forward with HDMI was the step up from the 1.3/1.4 versions maximum data rate of 8.16 Gbit/s and maximum transmission bit rate of 10.2 Gbit/s to version 2.0 with a data rate of 14.4 Gbit/s and a maximum transmission bit rate of 18.0 Gbit/s, suitably positioning HDMI 2.0 into the UHD era. While teasing BT. 2020 color space, no consumer products emerged for that advantage. The audio sample frequency doubled to 1536kHz, and the format made provisions for 32 audio channels. 

HDMI 2.0 retained TMDS encoding (Transition-Minimized Differential Signaling) for video signal transmission as in previous versions, relying on powerful clock recovery in the receiver for high skew tolerance to accommodate longer cable lengths.

Version 2.0a, released in April of 2015, added HDR10 support, while version 2.0b in December 2016 added capabilities for hybrid log-gamma (HLG). Worthy of mentioning is the establishment of the HDMI Forum, by HDMI founding members, in October of 2011. From October 25, 2011, forward, additions and changes to the HDMI specification are the responsibility of the HDMI Forum, with more on that to follow.

HDMI 2.1/HDMI 2.1a

By the numbers, data capacity for HDMI2.1 catapults to a maximum data rate of 42.6Gbit/s and maximum transmission bit rate of 48.0 Gbit/s, and how this is accomplished within the constraint of using the same 19 pin form factor is remarkable.  

As with previous version updates initiating substantial changes, implementation often trails press day fanfare by years. HDMI 2.1's entry delay was hampered by chip availability prior, and unrelated to, the pandemic. 

A quantitatively significant leap in bandwidth is made possible in HDMI 2.1, in part attributable by shifting from three data channels of 6 Gbps (the 18Gbps total for HDMI 2.0) into three data channels each doubling the signal rate to 12 Gbps. How the data is structured also changed, using a packet-based model embedding the TMDS clock signal into the data 3 lane, and converting the TMDS clocking channel (which in previous HDMI versions never transmitted audio, video, or data), into a fourth data channel of 12Gbps. Encoding efficiency gained in the packet-based format allotted more bandwidth to data transfer compared to TMDS, enabling the four channels an aggregate maximum data rate of 42.6 Gbps, nearly three times that of HDMI 2.0 with a maximum transmission bit rate of 48.0Gbps.

Fixed Rate Link

This rearrangement to the physical layer architecture, increasing HDMI data transfer, is referred to as Fixed Rate Link. Ranked from FRL1 through FRL6, it is broken out to define the number of "lanes" (using paired wires contained in the HDMI cable) and the lane rate bandwidth of each, expressed in Gbps. Combinations range from a minimum of three channels of 3Gbps with FRL1, to a maximum of four channels of 12Gbps for FRL6. 

Embedding the TMDS clock signal within FRL signal packets enables the bandwidth density necessary for HDMI 2.1 to host features like Dynamic HDR, Variable Refresh Rate, Link Training, and more. Despite the transmission mode of AC-coupled FRL replacing DC-coupled TMDS, HDMI 2.1 is not exclusively free of using TMDS. FRL maintains backward compatibility were data rates to fall under FRL minimum support range. TMDS fallback encompasses FRL1 and FRL2, although a particular device may not support all possible signals in those rates.

AVPro Edge introduced the world's first 8K matrix switcher, the AC-MX-42X, which operates within Fixed Rate Link 5 parameters. 

Link Training

Link Training, enmeshed in FRL for HDMI, is a protocol for switching between TMDS and FRL and is intended to create reliable and stabilized communication between a Source and a Sink. Six Link Training states exist in HDMI ultimately governing the quality level of the image to be displayed. Link Training plays no direct part in image fidelity parameters rather, it parses EDID information from the sink on behalf of the source, verifying FRL support and confirming the maximum compatible rate of data that can be exchanged (and other display criteria such as timing, color depth, etc.). Link Training can assess HDCP status but that is optional. The source initiates Link Training, with the Sink negotiating for a specific FRL rate. If the Sink requests a new Link Rate, LTS: 4 (Link Training State) can be used to change FRL rates. Upon agreement, FRL transmission commences with LTS: P (Passed). If Link Training fails (LTS: L) TMDS is initiated, with an image of some construct designed to appear. 

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By Matt Murray, Chief Technology Officer

AVPro Edge