There's More to HDMI Than Meets the Eye


In our first HDMI blog we covered standard HDMI features including subjects like High-Bandwidth Digital Content Protection (HDCP), High Dynamic Range (HDR), DVI and CEC. We also went through the various versions, resolutions and data rates of the HDMI signal transfer interface. Here in Part 2 we want to dig a little deeper into some of HDMI’s additional features that really distinguish one HDMI connection type from the next. 

Many of HDMI’s stand out high-end specs were introduced with the advent of HDMI 2.1, including the progression of some base features that change the way we look at HDMI as a A/V signal transfer medium. 

HDMI Expanded Features

From elevated high-definition resolution (HDR) graphics to auto low-latency, HDMI has been providing new and improved specifications to its ports and cables that solidify its place atop the consumer-centric video signal tech pyramid. Many of these new features are directly applicable to one of HDMI’s core use cases: gaming. 

Gaming HDMI Features

Variable Refresh Rate (VRR)

Depending on a visual scene’s complexity, selected resolutions, frame rates, and GPU horsepower, the time it takes a graphics processor to render the frame can vary. When a GPU can’t render the frame in time due to one of these factors, it must repeat the current frame or display a partially render frame of the next in sequence. 

A variable refresh rate enables a source such as a computer or gaming console to deliver video frames to match the frame rate output of the console or PC. Through VRR, an HDMI 2.1a connection will wait until the next frame is ready for transport, making the image smoother and eliminating judder, lag, and frame tearing.

Auto Low Latency Mode (ALLM)

Also contributing to lag-free viewing experiences, Auto Low Latency Mode (sometimes referred to as game mode) “enables the ideal latency setting to automatically be set” by allowing the game console, PC, or other device to send a signal to the attached display to automatically switch to ALLM. Now, instead of sorting through menu options and settings, then switching back when ready for normal viewing, a low-lag mode for gaming, conferencing, and more is available automatically. 

Since other viewing types like general TV watching may not benefit from low latency prioritization, ALLM will automatically disable the signal, reverting to the previous mode for the ideal picture. 

Quick Frame Transport (QFT) 

With applications in PC and console gaming, virtual reality, and other interactive digital media, QFT is yet another HDMI 2.1a specification upgrade that reduces latency to promote a smoother picture and decrease lag. 

When it comes to crisp visuals, latency is a hot button topic, that describes the full amount of time from the point a frame is ready for transport to being displayed in its entirety. To quickly break it down, latency describes:

- Transport time through a PC or console’s output circuits and across the interface
- Processing of the video frame data in the display
- Painting the screen with new frame data

Through Quick Frame Transport, each image is transported at a higher rate to reduce any latency in the display. This is done by reducing the time it takes to send only the active video across the cable. This means games using QFT will have better responsiveness reducing the time difference between a button being pressed on a source, and a resulting action occurring on screen. 

Quick Media Switching (QMS)

The ultimate goal of quick media switching boils down to the elimination of the untimely blank or black screens that sometimes occur when mode switching. The way this is accomplished is by allowing a switch from an original format to a format with a lower refresh rate (with the same resolution) without disrupting the video. This provides the monitor the ability to smoothly switch from the base refresh rate to the constant rate used for standard media. 

To boil it down further, QMS uses the variable refresh rate technology we covered earlier to change frame rates such as a change from 24Hz to 30Hz, 30Hz to 60Hz, 24Hz to 60Hz, etc. meaning the display never experiences a blank screen. 

Additional HDMI Features

Color Depth

In the context of video display, color spaces (such as RGB or YCbCr) can be described as the range of colors capable of being rendered in an image with their primary characteristics being split into two main categories: color depth and gamut. 

Color depth – The number of bits that are used to represent a single pixel’s color, which is also a determinant for its gradation.

Gamut – Simply refers to the number of colors available at a specific color depth.

At the start, HDMI specifications required support for 24-bit color depth, labeled simply as True Color.Now, with the introduction of newer editions of HDMI specs, billions (36-bit supports 68.71 billion) and trillions (48-bit supports 281.5 trillion) of colors are supported with Deep Color.

Chroma Subsampling

Luminance information and color data are the two primary aspects of a video signal. With luminance (often shortened to luma) being the driving force behind contrast, which defines the shapes we see on screen, it is ultimately more critical to a picture’s quality than chrominance (color information, sometimes shortened to chroma). 

HDMI uses chroma subsampling as a type of compression to reduce bandwidth without significantly affecting the picture’s quality. This is accomplished by reducing the amount of color information in an HDMI signal to allow for more luminance data.

HDMI uses chroma subsampling as a type of compression to reduce bandwidth without significantly affecting the picture’s quality. This is accomplished by reducing the amount of color information in an HDMI signal to allow for more luminance data.

HDMI Bandwidth

All HDMI products are limited by hardware to a maximum bandwidth, so understanding the bandwidth of source signals is critical to determine compatibility of downstream devices. It can be difficult to calculate from the beginning, but warning signs (sometimes called “artifacts”) such as odd coloration, flashes, or pixelation on the screen may indicate that the HDMI cable you are using isn’t negotiating an effective connection between source and display and may result in HDR data being grayed out.  

For more information on finding the A/V connector type that’s right for you, check out our blog A/V Signal Showdown: HDMI vs. DP here. 

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