The latest glitch in the matrix has come from the new AV receivers of usually rock-solid brands Denon and Marantz. Both brands recently launched, to great fanfare, their first AV amps and receivers equipped with 'full bore' HDMI 2.1 ports capable of, so we were told, handling everything the latest HDMI standard could throw at them. 

Now that sources capable of outputting such cutting-edge video formats have appeared, it's turned out that they come a serious cropper with 4K/120Hz sources.

If you try to feed a 4K/120Hz signal through the HDMI 2.1 ports on these new receivers, you get a blank screen on your display. According to German magazine c't, the source of the upset is the Panasonic Solutions HDMI 2.1 chipset that all the receivers use that renders them unable to handle HDMI 2.1's Fixed Rate Link (FRL) feature correctly - a feature that the Xbox Series X console and new Nvidia RTX 30 graphics cards use for their 4K/120Hz delivery. 

It's possible that the problem may not happen with 4K/120Hz feeds from the PS5, as it handles its high frame rates in a different way. But at the time of writing this hasn't been confirmed. 

The models affected are the Marantz SR5015, SR6015, SR7015 and SR8015, and the Denon AVR-X2700H, AVC-X3700H, AVC-X4700H, and AVC-X6700H.

As if the HDMI 2.1 issues weren't bad enough already, it's currently looking like they can't be fixed by a mere firmware update. In other words, a hardware replacement will be required—either of the entire AVR or the affected HDMI input board.

It seems possible, too, that the issues are going to spread to other brands. The same faulty HDMI 2.1 chipsets may have been supplied to the likes Yamaha, Onkyo and Sony, meaning that the same 'game-breaking' problem could also plague their next generation of AVRs.

Sound United, which owns the Denon and Marantz brands, has confirmed that the bug exists, and issued the following lengthy formal statement:

Some new gaming source devices that support 4K/120Hz output may not work fully with Denon (or Marantz) 8K AVRs. You may discover this incompatibility issue due to a HDMI chipset mismatch between the devices. When the affected system is connected to the AVR via 8K HDMI input and set to output at 4K/120Hz, and the AVR's 4K Signal Format option is set to “8K Enhanced,” you may not see the system's source video on their display, and may not hear the system's source audio processed through the AVR. This problem is only present when a display that supports 4K/120Hz is used. We are currently investigating the issue further and will offer a permanent solution at a later date.

Sound United has offered a couple of workarounds to prevent the issue in its current state suggesting the following:

• You can connect the system to the display directly via HDMI and use the display's ARC/eARC functionality to feed the native audio back to the AVR using the connected HDMI cable between the AVR and display. This will allow users to decode the native audio format sent from the source. With this method, the display's CEC/ARC option must be enabled as well as the AVR's HDMI Control and/or the AVR's ARC option. In the AVR, this option is located within the GUI under “Video – HDMI Setup.”
• Another workaround is to leave or change the source's video output to 4K/60Hz instead of 4K/120Hz until a permanent solution is available. This will ensure reliable communication between the source, the AVR and the display. The source's default is set to output at 4K/60Hz, so if no change was initiated out of the box, then nothing further needs to be done.

Sound United continued:

We apologize for this inconvenience and we are currently working tirelessly to release a permanent solution so you can enjoy the 4K/120Hz experience using the latest sources with your AVR. We will have an update soon regarding the timeline of a permanent solution. We appreciate your patience.

The eARC suggestion in Sound United's statement is certainly the most useful, as it should mean you suffer no degradation with your picture or sound quality. And pretty much any TV that has HDMI 2.1 ports capable of handling 4K at 120Hz will also support eARC. It's worth noting, though, that eARC can cause lag issues, and many eARC TVs only pass Dolby sound formats, not DTS.

We'll share future updates on this issue from all potentially impacted brands as and when we get them.