HDMI Hikes Rates—Boosts Bandwidth for UHD

A quick look at the lens to display pipeline highlights some trivial-seeming bottlenecks that place important constraints on motion imaging systems. One of these is the connection to the consumer display. The announcement of HDMI 2.1 is key to advancing to 4K and beyond, with additional support for HDR, and increased frame rates for UHD video.

HDMI 2.1

The HDMI Forum has announced the release of the HDMI 2.1 specification. Supporting a bandwidth up to 48Gb/s, it is a big increase over the 18Gb/s of the previous HDMI 2.0 and over 4x the bandwidth of HDMI 1.4, which is still commonplace in the home.

The wider bandwidth now supports 8K to 60fps and 4K to 120fps. Resolutions up to 10K are also supported for other applications.

As developers of HDR transmission soon found, without the release of HDMI 2.0, it was difficult to display UHD video without multiple connectors, a no-no for consumer applications. The first UHD content I saw was on a monitor with four DVI connectors, carrying four HD quadrants.

It wasn’t just the video bandwidth either, support for HDR static metadata (added in 2.0a) and HLG signalling (2.0b) also awaited updates to the interface specification. HDMI 2.1 now adds support for dynamic HDR metadata.

Dynamic HDR support ensures every moment of a video is displayed at its ideal values for depth, detail, brightness, contrast and wider color gamuts—on a scene-by-scene or even a frame-by-frame basis.

Dynamic HDR support ensures every moment of a video is displayed at its ideal values for depth, detail, brightness, contrast and wider color gamuts—on a scene-by-scene or even a frame-by-frame basis.

The step from 1.4 to 2.0 was key for UHD displays, increasing the frame rate for 4K images from 24fps (OK for movies) to 60 fps for the new higher frame rate UHD sports programming. 2.1 now means that all the planned HDR features can be utilized in the next generation of displays.

A new cable specification was required for such a high speed interconnection. It should exhibit exceptionally low EMI (electro-magnetic interference) which reduces interference with nearby wireless devices. The cable is backwards compatible and can be used with the existing installed base of HDMI devices. The Forum designates the maximum speed of HDMI cables as follows:

DesignationData RateTypical application
Standard2.23 Gb/s1080P30 and 1080i60
High Speed8.9 Gb/s1080P60 and 2160P24
Premium17.8 Gb/s2160P60, 8, 10 &12 bit 4:2:2
Ultra48Gb/s2160P120 12 bit 4:4:4
To support the higher data rates a new 'Ultra' cable is specified.

To support the higher data rates a new 'Ultra' cable is specified.

Since the connector form-factor is basically the same, the label—Ultra or whatever—indicates whether that cable will support the necessary video data rate. Consumer beware! Many of the early adopter of UHD display riushed out and purchased receivers with 1.4 connectors, and soon found that they are not much use for watching 50 or 60 fps sportscasts.

Display Stream Compression (DSC)

Resolutions up to 8K (4320P120) are supported with DSC, a Video Electronics Standards Association (VESA) standard. DSC is visually lossless compression that is also used with DisplayPort connections.


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