Display Interfaces: HDMI Marches On

Ultra High Speed Cable for Display Interfaces

An update on display interfaces following CES 2021.

You might have noticed that, in my coverage of CES 2021, I didn’t spend much time discussing display interfaces. In typical years, both the Video Electronics Standards Association (VESA) and the HDMI Forum have multiple, significant announcements at the show pertaining to speed upgrades, new protocols and adoption rates.

This year was different, though. The folks at VESA were conspicuous by their silence, promising that more news about DisplayPort (DP) would be forthcoming later this year. You might recall that, at last year’s show, DP 2.0 was announced, offering a combined maximum data rate in excess of 77Gb/s across all four lanes as well as support for Alternate Mode through USB version 4.0. That speed limit was achieved by faster lane speeds in transmitters and receivers and by more efficient signal coding, reducing the usual 20-percent overhead found in today’s interfaces to about three percent.

And that’s where things stand right now. No one is manufacturing DP 2.0 chipsets yet, and it’s hard to find products that support the older 1.3/1.4 versions (32.4Gb/s). You won’t see this connector on televisions—those are firmly in the HDMI camp—and even some laptop brands have added HDMI ports to DP connections, with the latter typically running through a USB 3.1 port in Alternate Mode.

Things are much different in the HDMI camp, where the HDMI Forum and HDMI LA had a joint press briefing prior to CES. The latest iteration of the interface, HDMI 2.1, was announced more than three years ago (has it really been that long?), and it’s a major departure from earlier versions in that the signaling method has moved to a 100-percent packet-based architecture. Whereas earlier HDMI versions used three lanes for red-green-blue (RGB) signals, plus a fourth for clock/sync signals, HDMI 2.1 embeds the clock information into packets, making all four lanes available for video signals at speeds up to 12Gb/s per lane, or 48Gb/s total.

When HDMI 2.1 was announced, it was just a concept on paper (much like DP 2.0 last year). Over time, though, more chipsets became available and, now, HDMI LA states that several TV brands—among them, Samsung, LG, TCL, Hisense, Sony, Panasonic and Sharp—are shipping 4K and 8K TV models with more than one HDMI 2.1 input. Both Q4/20 PlayStation and Xbox gaming consoles support the format, as do select models of AV receivers and computer graphics cards designed for gaming.

Most of the CES press briefing, however, focused on other matters—for example, the latest enhancements to HDMI 2.1. Those include an Enhanced Audio Return Channel (eARC) that can handle 1,500Kb/s, sufficient for Dolby Atmos spatial sound, and some “smart” functions that appear to be targeted at the rapidly growing gaming market.

The first of those is Auto Low Latency Mode (ALLM), which lets a game console, PC or other device send a signal to the display, which will cause it automatically to switch to a low-latency, low-lag mode for gaming. Variable Refresh Rate (VRR) tells gaming consoles and computers to deliver video frames as fast as they can, free of judder. That’s another beneficial enhancement for gamers.

HDMI 2.1’s Quick Frame Transport—another feature aimed at the gaming marketplace—reduces latency for smoother, no-lag gaming and real-time, interactive virtual reality (VR) by transporting each frame at a higher rate to decrease “display latency.” Finally, Quick Media Switching uses VRR to eliminate the blackout period when all devices in the HDMI connection chain change video modes. As long as the resolution remains the same, and only the frame rate changes, QMS will smoothly switch between frame rates.

The HDMI press briefing also focused extensively on the new Ultra-High-Speed HDMI Certification Program. This is the first HDMI-cable specification to be defined by the HDMI Forum, and its goal is to ensure cables will support high-resolution video modes with fast refresh rates (4Kp50/60/100/120 and 8Kp50/60). As is usually the case, higher clock speeds in central processing units (CPUs) and associated cabling can generate strong electromagnetic interference (EMI); thus, good shielding is also important in these cables.

There are certification programs for ultra-high-speed HDMI cables that are supposed to entail testing at HDMI Forum Authorized Test Centers. These cables will come in packaging that displays an UltraHigh-Speed HDMI Certification label, complete with a hologram and quickresponse (QR) code that can be scanned to verify that the cable is legit. According to the HDMI Forum, “The combination of holographic technology, QR code and label ID number are unique to each individual cable package and enable compliance tracking and anti-counterfeit confirmation.” (Counterfeit HDMI cables are apparently a big problem!)

Although copper cables will still play a part in HDMI 2.1 interfacing, the clock speeds and data rates associated with higher image resolutions, greater color bit depths and faster frames rates almost beg for optical fiber cables. Those are already in widespread use for connecting 10Gb/s IT networks, running a much lower speed than a gaming console spitting out 4K video @ 120Hz with 10-bit RGB color (43Gb/s) or 8K video @ 60Hz with 10-bit 4:2:2 color (47.7Gb/s).

Active optical HDMI cables that use LED light modulators are already widely available. Unlike copper cables, which become increasingly lossy at shorter lengths as clock rates increase, optical cables can easily transport that same 8K/60 10-bit 4:2:2 signal over several hundred feet with minimal attenuation. For that matter, one could simply buy several 100-foot active optical HDMI cables, make all the necessary interconnections—even short ones—and leave the excess coiled up in a neat loop.

So far, I’m not aware of any TV, monitor, projector or interface manufacturer that is building optical HDMI interfaces directly into its products, but it’s bound to happen eventually. Right now, there’s really no call for ultra-high-speed HDMI signal management. The commercial AV products that are out there are still employing HDMI 2.0 interfaces, capped at 18Gb/s but fast enough for 4K/60 eight-bit RGB or 10-bit 4:2:2/4:2:0. How much influence the gaming market will have on AV-signal-interfacing products remains to be seen.

Interestingly enough, although HDMI 2.1 supports Display Stream Compression (DSC) along with DP—almost no one uses DSC, aside from the SDVoE platform— HDMI 2.1 isn’t onboard yet with USB 3.0/4.0 Alternate Mode. For those unfamiliar with it, Alternate Mode allows the trans port of DP signals over a USB connection.

This feature is attractive to laptop manufacturers. A USB Type-C connector takes up less room than a DP jack does, and it’s about the same size as a Mini DisplayPort receptacle (another interface type that seems to be falling by the wayside). Given that consumers generally want more high-speed USB ports on laptops, this double-duty approach makes a lot of sense. And HDMI 2.1, being a packet-based digital interface not all that different from DP, should be a cinch to ride on the USB highway.

It appears that HDMI is not ceding any ground to DP, but, rather, is gaining more of it. Some laptop manufacturers that eschewed the format at first—for example, Apple—have now come around and added it. If HDMI gets onboard with Alternate Mode, that action could really hamper DP-utilization rates in the future, even though there are still no royalties or licensing fees for using DP.

As to the need for higher data rates that would approach the 48Gb/s speed limit…well…we’re a few years away from that obstacle. Check back the middle of this decade!

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