On the Bright Side: How HDR Enriches Our Viewing Experience from Lattice Semiconductor
When Hollywood content providers contemplated the upgrade from 1080p to 4K video content, they carefully studied whether consumers would see a visible difference in 4K motion video. Even though a 4K television displays four times more pixels than 1080p, and fluid video at far viewing distances is one of the inherent benefits, this is not always an advantage with big screen displays in typical living rooms where viewing distances are shorter. It was obvious that 4K content would need to offer more than just more pixels… but what?
Filmmakers knew that a great deal of color and contrast information which was captured on film could not be seen using 1080p HD technology. The color/brightness standard of HDTV, known as rec.709, is based on the video display technology of 1950s television sets. Rec.709 is based on a reference display with a brightness range of only 0 to 100 nits. However, many newer LCD and OLED televisions deliver a brightness range of 0 to 700 nits, or even more.
When a television’s brightness range increases by 700 percent, the range of both contrast and color increases dramatically. When mastering for rec.709, the brightness and color ranges needed to be “crushed” into the available window of 0-100 nits. And if a display’s brightness was turned up higher than 100 nits, there was no way to map the narrow rec. 709 colors and brightness ranges to the wider brightness range.
What’s more, a television panel’s brightness range can vary – some panels might achieve 1,000 nits or more, and at the same time, the viewer may have reduced the maximum brightness of the display. Movie directors needed a way to control the display of their films, so they could decide how blue the sky should be, whether there should be contrast in the cloud areas, how much details to display in darker areas, and so forth.
Enter HDR, or High Dynamic Range. With HDR technology, content which is mastered for a very wide brightness range and very wide color “gamut” can be accurately and effectively displayed on televisions. The HDMI Forum has enabled this by adding support for a new “colorspace” called bt.2020, which replaces the narrow colorspace of rec.709. This allows the content and the display to map colors over a much wider color and brightness range. This supports the wider brightness ranges of today’s televisions, but it requires upgrading the bit depth of content and television sets from 8-bits per pixel (16.7 million colors) to 10-bits per pixel (over four billion colors).
With the release of HDMI Specification 2.0b, HDMI now enables the transport of High Dynamic Range Metadata, which transmits descriptors supporting two different High Dynamic Range modes: The display-based Perceptual Quantizer, also called HDR-PQ and HDR-10, and the scene-based Hybrid Log Gamma, or HDR-HLG. In HDMI 2.1, this EOTF Metadata can be transmitted on a frame-by-frame basis, enabling even more tightly coupled, synchronized presentation of high dynamic range video, which can reflect the differences between indoor and outdoor scenes, and even differences from frame-to-frame.
What does all of this mean? It means that new 4K Blu-rays and 4K televisions with HDR support can deliver a much brighter, enriched, colorful and life-like experience than ever before.
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