What's new with HDMI 2.1?

The Playstation PS5 and Microsoft Xbox Series-X were the first HDMI 2.1 sources to be released from consumer electronics in 2020. PC graphics cards can also produce HDMI 2.1. Since then, there have also been TVs and AV receivers for the new standard. But what are the advantages and what is new?

Higher video resolution up to 10k

The HDMI specification 2.1 supports resolutions up to 10k (10240 x 4320) and higher frame rates. The most common are still 4K and the new 8K. At 4K (3840 x 2160), the maximum frame rate increases to 120Hz. This enables razor-sharp UHD images even with very fast movements. Sports broadcasts and computer games benefit most from this - provided they were produced this way. 8K (7680 x 4320 pixels) delivers twice the horizontal and vertical resolution of 4K. Four times the number of pixels combined with a refresh rate of 60 Hz enables smooth and sharp display of content on large screens. In addition to 4K and 8K, a range of other resolutions are supported, including 5K and 10K for commercial AV, industrial and specialised applications. It will be many years before media such as films and series are produced in 8K. This is because cameras, the entire post-production process and transmission paths will have to be retooled for this.

HDR formats

The HDMI 2.1 specification supports "multiple static and dynamic HDR solutions" - as did HDMI 2.0b. Not much changes here. HDR enhances video images with an expanded dark to light contrast range for deeper blacks and brighter whites. This allows more detail to be seen in both the dark and light parts of the same image. The colour space is also expanded. With Static HDR (e.g. HDR10), the values for depth, detail, brightness, contrast and colour space are set for the entire film. With Dynamic HDR (e.g. Dolby Vision), the ideal values are displayed frame by frame. HDR-supported content is available for movies, videos, TV shows, console and PC games and VR.

VRR - Variable Refresh Rate for Gaming

One of the highlights HDMI 2.1 specification is the variable refresh rate Variable Refresh Rate (VRR). PC gamers have long known this function under the names FreeSync and G-Sync. VRR reduces or eliminates lag, judder and frame tearing for smoother gameplay. VRR allows a game source (console or computer) to deliver video images as fast as possible, which in many cases is slower than the normal static refresh rate.

Because graphics processors take different amounts of time to render an image. This time depends on the complexity of the scene, the power of the GPU, the resolution chosen and the frame rate. A GPU that is too heavily loaded will not be able to render the next frame in time. Therefore, the display must either repeat the current frame or show the only partially rendered next frame. This results in judder or cracked graphics. VRR waits until the next frame is ready to be transported. The frame rate is therefore not constant, but variable depending on the graphics performance. If the graphics card is heavily loaded, the frame rate drops slightly so that complete frames are displayed smoothly.

ALLM - Automatic Low Latency Mode

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Many TVs already offer a low-latency gaming mode. If activated, the TV switches off picture enhancers and the like to display the video signal as quickly as possible. This benefits action games and time-critical applications such as video telephony. HDMI 2.1's Auto Low Latency Mode (ALLM) enables this mode to be switched on automatically. The user therefore no longer has to activate the gaming mode manually and saves the settings in the TV menu.

When switching to a film stream, the source deactivates the ALLM signal. The screen reverts to the previous mode for optimal viewing.

QFT - Fast Frame Transport

The Quick Frame Transport (QFT) reduces latency for games and real-time applications. This allows frames to be transported at a higher rate. This speeds up the responsiveness of games, e.g. the time between pressing a button and the resulting action on the screen. QFT only refers to signal transport. The bottleneck in practice is still the performance of the graphics unit GPU.

QMS - Fast Media Switching

HDMI 2.1 offers Quick Media Switching (QMS) for movies and videos. This eliminates the delay or short black image that occurs when the frame rate changes (as long as the resolution remains the same).

For example, if you are watching trailers from a streaming service, they may be in 24Hz, 50Hz or 60Hz. Each time a trailer with a different frame rate is selected, the entire system has to change its clock and resynchronise, resulting in a momentary black image. Streaming services can now compensate for this and add a built-in delay so that users do not miss the first part of the trailer. So if the service and devices support QMS, the viewer never sees a blank screen.

eARC - the enhanced audio return channel

HDMI 2.1 features the Enhanced Audio Return Channel (eARC), which is the successor to the earlier Audio Return Channel (ARC). Via the audio return channel, the TV at an HDMI input sends the sound to the output of the sound system - in principle, backwards. This means that no additional cable is needed for audio playback. The sound from Smart TV apps, from the internal tuner or connected devices is transmitted via eARC/ARC via HDMI.

With eARC, this audio connection should become more reliable and support more formats. eARC is fully backwards compatible with ARC. So you can also operate an ARC sound system on a new television with eARC. However, then only with the maximum audio formats of ARC, i.e. Dolby Digital Plus (incl. Dolby Atmos). eARC, on the other hand, supports the latest audio formats with high bit rates up to 192 kHz, 24 bits and uncompressed 5.1 and 7.1 as well as uncompressed 32-channel audio. It also supports DTS-HD Master Audio, DTS:X, Dolby TrueHD, Dolby MAT / Atmos.

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The new Apple TV 4K 2nd generation uses eARC to play back TV audio played back through Homepod speakers.

Cables for HDMI 2.1

In principle, any existing HDMI cable can also be used for HDMI 2.1. This is because the plugs and cable assignments are identical. As long as no higher data rates are used than for 4K Ultra-HD, no problems are to be expected. At resolutions of 4K 120Hz, previous cables reach physical limits. Then specially tested cables should be used. For the 48 Gbps data rate at full resolution, however, an HDMI cable with copper conductors may only be 3 m long. In view of the large screens for 8K, this is very tight. Longer lengths require cables with optical data transmission.

Other innovations

HDMI 2.1 uses a new transmission method. Instead of TMDS (Transition-Minimised Differential Signaling) with 3 data channels plus clock signal, FRL (Fixed Rate Link) with 4 data channels is used. Advantage: The omission of the clock signal alone increases the possible data rate by 33 %. In order to achieve the highest resolutions and colour depths, additional signal compression was agreed.

The following resolutions were introduced with HDMI 2.1:

  • 4K 50/60Hz (including 4K RGB 12-bit / 24 Gbps)
  • 4K 100/120Hz
  • 5K 50/60Hz
  • 8K 50/60Hz
  • 10K 50/60Hz
  • 10K 100/120Hz


For all resolutions, colour scans of 4:2:0 for films and 4:4:4/RGB for PC applications are regulated. The colour depth can be 8 bit, 10 bit, 12 bit or 16 bit. HDMI 2.1 supports various static and dynamic HDR solutions.

So HDMI 2.1's FRL process is completely different from TMDS. Therefore, it seems unlikely that simultaneous mixed operation will be possible. If, for example, the source and sink transmit via HDMI 2.1 or the FRL method, an AV receiver connected in between must also do so. Alternatively, sound output via HDMI-ARC is possible via the TV's return channel.

Incoming HDMI 2.1 devices such as splitters and matrix switches will thus only enable the new functions and data rates if all connected devices support HDMI 2.1.

Our conclusion: HDMI 2.1 is especially interesting for computer games on the TV. Console gamers benefit the most from this. For PC gamers, special monitors and the DisplayPort connection offer more advantages than HDMI 2.1. For home cinema, the extended audio return channel enables better sound formats and more stable operation. Higher picture quality is not on the cards for now, because Ultra-HD 4K has not yet been fully exploited. 8K transmissions will probably only be available for special live events such as the Olympic Games or concerts in the foreseeable future.