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With the advent of richer television and gaming content, consumers¡¯ expectations have gone from ultra-high-resolution 4K displays to 10K with finer image details, more color gamut, and higher bandwidth. To deliver premium content to digital televisions and trending HDMI-based source devices, the HDMI Forum recently announced the HDMI specification, version 2.1. The re-architected HDMI 2.1 offers new features and capabilities including higher video resolutions (7680x4320 or 8K and 10240x4320 or 10K), more vivid colors and brightness, higher-quality and more sophisticated audio format, and a fluid gaming experience. This article describes HDMI 2.1¡¯s key enhancements such as: uncompressed 8K resolution, Dynamic HDR, enhanced Audio Return Channel (eARC), and Enhanced Refresh Rate.

Higher Bandwidth

An uncompressed 8K resolution, at 60 frames-per-second, requires transmission of a vast amount of data at up to 48 gigabits per second (Gbps) from a source device to a sink or display device via a single HDMI link or cable. This is a big jump in the amount of required bandwidth considering that a typical 4K video at 60 frames-per-second demands up to 18 Gbps bandwidth. To transmit the additional pixels, the HDMI 2.1 specification provides:

• Packetized low-layer transport with fixed rate link which greatly improves the efficiency of data transmission
• Embedded clock scheme that frees up one more differential signal pairs for data transmission

HDMI 2.1 supports a variety of resolutions and frames-per-second to meet the need for higher data rates at higher resolutions as shown in Figure 1.

Dynamic HDR

High Dynamic Range (HDR), allows more accurate color depth, and transfers more luminous and detailed images/videos to the display. It greatly improves the quality and viewing experience for consumers regardless of content resolution and display size.

The HDMI 2.0 specification supported HDR; however, it was static meaning the image/video data packet was transmitted with a constant value stream for color depth, detail, brightness, etc. With HDR, the viewing experience vastly improves from the quality provided by the Standard Dynamic Range (SDR). But SDR is not fully optimized for upcoming HDMI devices.

To overcome this limitation, the HDMI 2.1 specification supports Dynamic HDR by encompassing a more capable metadata in the HDMI link called Extended Metadata Packet (EMP). EMP transmits multiple sets of HDR data frame by frame or scene by scene. For example, when capturing video at night or indoors with low ambient lighting, optimized (for dark) HDR settings must be applied and transmitted with those sets of video data. Similarly, when capturing video outdoors or in a very bright setting, an optimized (for bright) HDR setting must be applied and transmitted. EMP carries the required HDR settings for specific scenes in the HDMI link, delivering the best video quality in terms of accuracy, color depth and gamut, and brightness regardless of video resolutions and screen size. Figure 2 shows Dynamic HDR with better detail, brightness, and wider color gamut than static HDR and SDR. 

Enhanced Audio Return Channel (eARC)

Because of modern digital televisions¡¯ slim chassis and thin bezel design, not much physical space is allocated to speakers which may force consumers to purchase a separate speaker system. In the past, consumers connected these speakers with a pair of analog signals (a cable with red and white banana connectors) or a TOSLINK (a fiber optic cable). These configurations, did not provide enough bandwidth for the best sound quality. To overcome this limitation, HDMI introduced Audio Return Channel (ARC), which allowed transmission of the highest quality audio format, via two signals, in the HDMI interface.  

Today, with eARC, the HDMI 2.1 specification offers higher-quality digital audio formats with higher audio sample rates and more surround sound channels than ARC. eARC supports the maximum 32 audio channels, and improves the interoperability between HDMI display devices and speaker systems.

Enhanced Refresh Rate

Gaming content is generated and displayed differently than video content for traditional consumer electronic devices. Some gaming content contains static images that don¡¯t have many moving objects (simple frames), and some may contain many moving parts and dynamic objects (complicated frames) where the GPU needs more time to generate and process. To provide a smoother gaming experience, HDMI 2.1 supports Variable Refresh Rate (VRR) and Quick Frame Transport (QFT) features which allow transmission of simpler frames as soon as they are generated so more time is allocated to the complicated frames that are within the same bandwidth.

In addition, gaming content requires a more fluid viewing experience at 50 to 60 frames-per-second versus a ¡°movie-like¡± video experience that requires only 24 frames-per-second. The difference between video and gaming content creates display challenges when switching from one type of content to another on the same device, such as a TV. For example, one can switch from watching a movie to playing a game using the same device. For a smoother transition, HDMI 2.1¡¯s enhanced refresh rate feature offers Auto-Low-Latency Mode (ALLM) and Quick Media Switching (QMS).