3 Key Take-aways from CEDIA & CTA’s New Recommended Practices for HDMI Systems

Walt Zerbe, Senior Director of Technology and Standards, CEDIA | Dec 10, 2021
Thanks to the tireless efforts of volunteers and staff, CEDIA and the Consumer Technology Association (CTA) recently released the joint best practices document CEDIA/CTA-RP28 HDMI System Design and Verification Recommended Practice. This document is free to CEDIA members, and should be considered an essential job aid for anyone installing HDMI systems. Within, you can find a deep dive into HDMI devices, connectivity, and functions, including accessible explainers on all the latest features supported by HDMI 2.1. I strongly recommend downloading it now and reading it cover to cover—but seeing as it weighs in at nearly 70 pages of directly actionable design best practices, testing procedures, references, and tools, you’d be forgiven for wanting some highlights.  

Consider this an appetizer, then, of the many take-aways within RP28. 

Cooler Features, Trickier Design 

HDMI 2.1 has ushered in support for a wide range of features that end customers are really excited about. For instance, High Dynamic Range (HDR) dynamic metadata can adjust color and contrast display settings on a scene-by-scene or even frame-by-frame basis. With this feature implemented end-to-end in a properly designed and calibrated HDMI system, the infamous season 8 Game of Thrones episode “The Long Night” might even be good, or at least legible. Variable Refresh Rate (VRR) syncs the refresh rate of a sink with the refresh rate of a source. Real time interactive content – such as a video game – doesn’t render at a fixed frame rate, so VRR prevents screen tearing in the middle of a high-stakes Call of Duty mission.  And of course, HDMI 2.1 is capable of ultra-high-definition video formats up to 10K resolution 60Hz with HDR and 4:4:4 sampling, or 120Hz with HDR and 4:2:2 sampling.  

These are specs that directly, substantively impact the customer’s experience, and that they are willing to upgrade media systems to get. They’re also fragile: any weak link in the signal chain can cause these advanced features to falter or fail, resulting in fallback to legacy video modes or even signal loss. The more complex a system is – the more links there are – the greater the likelihood of disruption. Thanks to new features and passionate consumer interest, HDMI system design requires more expertise than ever. 
Protecting Advanced Features from Source to Sink 

HDMI has always been famous for its plug-and-play functionality, but not all advanced features are automatically enabled. Customers looking to experience ultra-smooth gameplay due to VRR may not realize that this feature needs to be enabled in the settings of both source and sink. Similarly, many displays don’t support all features on all input ports – integrators will need to pay close attention to which features, such as HDR, are supported on which ports. And of course, all required features must be supported by every link in the signal chain, with full metadata pass-through. Any manipulation of the signal data, including scaling and processing, can result in loss of HDR if the repeater devices don’t support all the desired formats. If there are multiple displays with varying capabilities, design to the highest resolution and feature set needed and downscale as required at the legacy sinks.  

More complex HMDI systems generally require longer cable runs, including repeaters, alternative transport technologies (extenders), and/or active cabling. When using category cable to extend the HDMI signal, including for HDBaseT extension, be aware that HDMI is very sensitive to crosstalk. Unshielded Twisted Pair (UTP) cables carrying HDMI signals shouldn’t be bundled or pinstriped, especially in the first 20 m (65 ft) of the run, and the conduit fill rate for such cables shouldn’t exceed 40%. RP28 recommends instead using shielded Cat 6A cable or better, which enables bundling and up to 60% conduit fill.  Integrators will also need to pay careful attention to the native bandwidth capabilities of whatever extension technologies they’re using – and, if that bandwidth is lower than the requirements for full resolution and sampling, they’ll need to examine what data rate reduction strategies the technology deploys. Some extenders use pixel bit-depth down-sampling or video compression to offer higher format support at lower bandwidth. These strategies can impact the image quality and HDR functionality – integrators will have to discuss the trade-offs with their clients.  
True Lossless Audio Arrives 

As more customers are upgrading their media rooms, home cinema, and gaming environments to support lossless Hi-Fi audio, multi-channel surround sound, and object-based audio, the HDMI audio return channel has gotten a much-needed upgrade as well. The former HDMI Ethernet Channel (HEC) can now be repurposed to support Enhanced Audio Return Channel (eARC). eARC is capable of transmitting audio at a frame rate of 768KHz, meaning it’s fully compatible with lossless audio formats, expanded surround sound formats like Dolby Digital Plus 7.1, and object-based formats like Atmos. Integrators don’t necessarily need all-new Ultra High Speed HDMI 2.1 cables to take advantage of this new feature – though all Ultra High Speed cables will support eARC, any legacy cable labelled “with Ethernet” is also compatible.   
Bonus Insight: Planning makes Perfect 

The many advances that came with HDMI 2.1 prove that the standard will continue to evolve in ways that are compelling to end users, growing more bandwidth-hungry all the while. Keep the likely need for future upgrades in mind when planning infrastructure and cable runs. No matter what source resolutions and video/audio features are in use at the outset, design upgrade pathways into the infrastructure design, including overspec’ing the bandwidth throughput of your cables and components whenever possible. There is bound to be a shiny new set of features coming online in a year or two, and the system should be ready to support them.  

RP28 includes detailed testing procedures for verifying the functionality of HDMI features; bandwidth charts to help you assess system throughput needs; and a design checklist to help ensure you’ve carefully planned every necessary element of the video system. Free access to recommended practices like this one is one of the most tangible, immediately applicable benefits of CEDIA membership. Now that you’ve had a taste, go get the full meal at cedia.net/standards.