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WTF is HDMI 2.1 and 2.1a?

HDMI 2.1 enables a lot of capabilities for TVs and other devices, but the product manufacturer decides which features to implement. This means confusion for many consumers.

HDMI 2.1 Use Cases

Did you know that you’ve been using HDMI for almost 20 years? Most consumers probably don’t even know how many times it has changed and how it even works with the technology in their homes. HDMI 2.1 is a rather significant upgrade and we think it requires a more detailed explanation. There is also a tweak referred to as HDMI 2.1a that we will also explain.

HDMI Overview

HDMI Logo Cable and Connector

HDMI stands for High Definition Multimedia Interface and was developed as a secure method of transferring both digital video and digital audio through a single cable. It was also designed to be upgradeable to meet changing needs. HDMI standards are developed and enforced by the HDMI Licensing organization and HDMI Forum

For security, HDMI incorporates HDCP (High Bandwidth Digital Content Protection) which prevents unauthorized copying of video content. This results in a feature of HDMI that is referred to as “The Handshake”. Both an HDMI source and connected display need to recognize via the HDCP signal or nothing will be displayed on the screen. However, even if both the source and display incorporate HDCP, sometimes there can be glitches that may block communication. Check out some troubleshooting tips

Here is a brief rundown of HDMI version history from 2002 to 2015.

  • Version 1.3: Expanded Color Gamut, Faster transfer rate (10.2 Gbps), compatibility with Dolby Digital Plus/True-HD, and DTS-HD/HD Master Audio. Auto Lipsync was implemented. In addition, a mini-HDMI connector was introduced for mobile applications.
  • Version 1.4: 4K/30hz, 3D, Audio Return Channel, HDMI Ethernet Channel. A Micro Connector was introduced for broader mobile applications.
  • HDMI 2.0: 4K resolution at 50/60Hz, 18 Gbps transfer rate, up to 32 channels of audio for Dolby Atmos, DTS:X, and Auro 3D Audio support, Dual video streams for viewing on the same screen, up to four audio streams (taking into account the total channel number support). HDR (High Dynamic Range) capability added in HDMI versions HDMI 2.0a and 2.0b. 

What HDMI ver 2.1 Adds

Introduced in 2017, HDMI 2.1 builds on the previous versions by adding the following capabilities:

  • Resolution/Frame Rate: Up to 4K 50/60 (fps), 4K 100/120, 5K 50/60, 5K 100/120, 8K 50/60, 8K 100/120, 10K 50/60, and 10K 100/120.
HD to 10K Resolution Comparison
Rec 2020 Color Gamut
Dynamic HDR Scene
VRR Example
  • Quick Media Switching (QMS): This feature borrows from the VRR feature to eliminate the blackout period when switching HDMI sources on a TV or monitor. Many times, when switching between sources, the TV or monitor may display a black frame. What QMS does is eliminate that black frame so that you see one image from the new source without a delay even if the frame rate of each source is different. However, it must be noted that the black frame elimination only works if both sources are providing the same input resolution, if not, you may still see the black frame. 
HDMI 2.1 Quick Media Switching (QMS)
  • Quick Frame Transport (QFT): This feature reduces latency for gaming, real-time interactive virtual reality sources. This shortens the time when you press a game controller button and when that function is displayed onscreen. 
  • Display Stream Compression (DSC): The feature allows the transfer of resolutions higher than 8K with refresh rates of 60 or 120Hz using VESA DSC. VESA DSC also supports higher resolution and frame rate compatibility for HDMI cables that are limited to pre-HDMI 2.1 resolution and transfer rates. 
Toslink vs. HDMI ARC vs. HDMI eARC Comparison
  • Bandwidth/Transfer Speed:: Bandwidth and signal transfer speed capability increased to 48 Gbps.
HDMI Bandwidth Evolution
  • Cable Support: To access the larger bandwidth and transfer speed you also need to connect your devices via Ultra High-Speed HDMI cables.
How to Identify and Ultra High Speed HDMI Cable Certification

HDMI 2.1a 

In 2021, HDMI ver 2.1a was announced that adds Source-Based Tone Mapping (SBTM)

This HDR feature allows a portion of the HDR mapping to be performed by the Source device in addition to tone mapping done by a compatible TV or monitor.  Just as HDMI allows sources devices can detect the resolution of a TV SBTN enables the source device to detect the TV or display device’s HDR capabilities

For example, an HDR TV or monitor might be able to display as much as 1,000 nits of light, but others may have a maximum of 600 or 700 nits light output. 

To account for this variance, tone mapping is used. Most HDR TVs have tone-mapping capability, but it may not be that precise. 

What SBTM does is assist further by enabling the source device (such as UHD Blu-ray Player or Media Streamer) to detect what the TV or monitor HDR capabilities are and making automatic adjustments that map the HDR range of the content source to that of the TV or monitor. SBTM also takes into account the differences between SDR and HDR source content so that a consistent look is provided. 

SBTM benefits the viewer by limiting the need to make additional manual HDR adjustments. This is especially convenient for Gaming needs. 

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Source Used Tone Mapping Example

HDMI Feature Chart

HDMI Version Feature Comparison

Fake HDMI 2.1 – Confusion for Consumers

Since the introduction of HDMI 2.1, implementation issues have resulted in some components being referred to as having “Fake HDMI 2.1”.

What this means is that a monitor, TV, or another component that is labeled as being HDMI 2.1 enabled, doesn’t necessarily have to include all possible HDMI 2.1 features. This also applies to the SBTM feature added in HDMI 2.1a – implementation is optional.

The problem is that although HDMI 2.1 supports all the capabilities discussed in the previous sections, how it is implemented is at the discretion of the product manufacturer. To look at this another way, HDMI 2.1 provides a “menu” of options, but not all options have to be made available to the consumer for the product to be labeled as being HDMI 2.1 enabled. This can make things very confusing when using HDMI.

For example, a TV that only implements eARC can be labeled as supporting HDMI 2.1 even though it doesn’t support any other HDMI 2.1 capabilities. The reason for this is that HDMI 2.1 is required to access eARC.

Also, TVs and monitors don’t have to include all of the gaming features offered by HDMI 2.1 if the TV is not promoted as a gaming TV. 

In addition, if the TV is a 4K UHD TV, it doesn’t have to include HDMI 2.1 capabilities required for 8K TVs.

This situation has resulted in additional confusion as to when a device is HDMI 2.0 or HDMI 2.1 compliant

Unfortunately, although the HDMI Forum encourages TV and other device makers to list what HDMI 2.1 features are enabled, it is not required, leaving it up to the consumer to figure it out.

This means that when shopping for TV, monitor, or other HDMI-enabled if you are looking for specific HDMI features check any available product labeling or specifications that are available online or at your local dealer.  

In addition, make sure you purchase the HDMI cable that will meet your needs – HDMI Licensing provides a guide. However, when in doubt, buying an Ultra High-Speed cable is the best insurance policy.  


HDMI Backwards Compatibility

HDMI is backward compatible. This means that if your TV is only compliant with an older version of HDMI (for example you have an HDMI 1.3 compatible TV, but your source device includes HDMI 2.1 features), you can still use your TV with that source device, you just won’t be able to access to any of the additional HDMI 2.1 features that may be available.  

Also, make sure you check all the TV’s features and capabilities beyond HDMI 2.1 before reaching into your wallet!

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HDMI vs DisplayPort

Displayport vs. HDMI Connector

The HDMI and DisplayPort connection options have some overlapping features. However, HDMI is designed to be more applicable to home theater and home entertainment needs, while DisplayPort is primarily used in the computer environment. On the other hand, while HDMI is used in conjunction with PCs and Laptops, there are currently no TVs that have a direct DisplayPort connection option.

Related reading: Buying a New TV: Everything You Need Know

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