HDMI glitch: Fast media switching does not work on some HDMI devices

The sentence, which can be read in LG’s recently published press release about the technical capabilities of the company’s new OLED televisions, does not stand out very much at first: “The 2023 LG OLED TV models are (…) the first televisions certified by the HDMI organization for the recently announced Quick Media Switching Variable Refresh Rate (QMS-VRR),” it reads, followed by a brief explanation of what the feature is all about.

To put it simply, QMS prevents the TV screen from briefly going black when the player changes the refresh rate – for example, because it switches from the 60 Hertz surface to a 24 Hertz film. So far, this triggers a so-called “handshake”, in which the television and player agree on the new frequency. Users of an Apple TV 4K know this when the streaming box adapts the HDMI output to the respective content. With QMS, on the other hand, playback continues seamlessly as long as the resolution does not change. The prerequisite is that both the TV and the player are capable of QMS – as well as any audio/video receiver that may be connected in between.

However, those who have been dealing with the digital audio/video interface HDMI for a long time will start pondering. Because Quick Media Switching was by no means “recently announced”, but was already a feature of the HDMI specification 2.1 when it was officially published more than five years ago. Audio/video receivers with HDMI 2.1 connections where QMS is listed as a feature have been on the market since 2020. The statement in the LG announcement that QMS has been “certified” by the HDMI organization for the TVs also causes astonishment. If you look at previous announcements, there was usually talk of certification for the entire AV interface, not for individual parts.

How does this all come together? The solution to the riddle: QMS was specified twice – most recently in version 2.1a published at CES 2022, but before that in version 2.1. The latter can also be read on the HDMI website from 2019, which can be found in the Internet archive (see screenshot). When asked, Jeff Park, CTO at HDMI Licensing, also confirmed this duplication to c’t – and added that HDMI 2.1a was ultimately just a clarification. The reason he gave was that the 2.1 specification left too much room for manufacturer interpretations.

According to Park, the technical implementation of QMS has not changed with HDMI 2.1a compared to HDMI 2.1; Then as now, VRR is the basis, i.e. the function known from the gaming sector to support variable refresh rates. So VRR is used here to increase or decrease the frame rate when the content changes.

Repeated unnecessarily

However, this “clarification” sounds more harmless than it actually is: Based on the unclear wording in Specification 2.1, chip manufacturers approached the implementation of QMS differently – sometimes without VRR as a basis, as industry representatives reported to c’t. That is probably also the reason why the HDMI consortium now likes to call QMS “VRR-QMS”.

The correction by 2.1a only came when AV receivers with QMS according to HDMI 2.1 were already on sale. Jeff Park then admitted to c’t that the HDMI consortium made a mistake in specification 2.1, which could have negative consequences for customers.

This evokes memories of the HDMI 2.1 bug uncovered by c’t in 2020, which prevented UHD content from the Xbox Series from being routed correctly through AV receivers. At the time, the problems were also triggered by misunderstandings on the part of the chip manufacturer Nuvoton Technology when implementing the specifications of the HDMI consortium.

limbo

What’s next? The HDMI Consortium’s official position is that devices that strictly adhere to the 2.1a specification have correctly implemented QMS – which explains LG’s wording regarding certification to the “recently announced” specification. This could also be the reason why QMS was already being discussed when the new Apple TV 4K was released last year, but is not yet available.

But what about the devices equipped with QMS to specification 2.1? This is currently practically a black hole, as Jeff Park admitted to c’t. Because up to now there has been no official test procedure that reliably determines whether the respective implementation fits the current specification or not. That means: customers with HDMI 2.1 players become involuntary beta testers.

Apparently, Denon itself does not assume that the HDMI chip from Nuvoton used in its receivers implements the QMS function in accordance with specification 2.1a: The manufacturer only supports the – unclearly defined – QMS in its models specified according to HDMI 2.1 , but denies it for the devices specified according to HDMI 2.1a. As a result, none of these receivers can use QMS together with the new LG TVs.

And this problem not only affects Denon and its sister brand Marantz, but also receivers from other manufacturers such as Yamaha. After all, since the misery surrounding the HDMI 2.1 bug, we have known that the Nuvoton chips are in almost all receivers on the market.

So now, for the second time, customers are allowed to settle for misunderstandings between the HDMI consortium and the industry. However, there is probably no solution to this problem – in contrast to the HDMI 2.1 bug: According to the current state of knowledge, the QMS implementation of the current chips cannot be “bent” to fit with a firmware update. According to c’t information, Nuvoton’s roadmap also does not envisage the next revision of the HDMI chip until 2026. Until then, no processor with QMS according to HDMI 2.1a will come from the manufacturer. With receivers, you will probably have to live with the black intermediate images for years to come.

(nij)