The last few blog entries I’ve written have made the case for HDR assuming that viewer accessible screens will be launched soon. Begs the question, how safe an assumption is this and what luminance performance can we expect?
The current crop of screens is restricted to 100 nits (a nit being the unit of measurement for luminance) and 120 nits if you really remove all of the stops! Compared to the cinema this is great, where luma is restricted to 48 nits typically within the confines of specifications that allow it to reach 55 nits.
But cinema regains all this lost luma by having you watch in darkness and films have vastly improved chroma performance compared to TV. So, how come TV screens have been available with extended dynamic range and wider color range for some time even though the specs have been revised since the 1930s when phosphors in cathode ray tubes were state-of-the-art?
The answer is screens with improved brightness have been with us for some time, after all the original Trinitron screens were developed so Americans could watch TV poolside in the glare of the sun. Herein lies where most of the effort on improved luminance and chrominance handling was featured to counteract the effects of high ambient light levels in the case of luminance and make certain content really zing with color. The problem with this approach is that what looks good on certain content (like that used in TV showrooms) looks gaudy and oddly saturated on more representative content.
This brings us back to specifications and restricting performance to achieve wider dynamic range and gamut in a defined and repeatable manner, so all content looks impressive. Whether you buy a UHD or HD screen you will still be restricted to HD color space, Rec.709 to the techies. UHD color space is on the way in the form of BT 2020 but only currently available in high-end production monitors used for grading. Consumer screens are on their way later in 2015, but I only know of 4 models that will be very high end and expensive. Ok, so let’s tackle this another way then! Even within the confines of the costs that can be borne in screens destined for the consumer market extending the dynamic range is possible.
The extended Rec.709 proposals (there are three to my knowledge: BBC, Philips and Technicolor) all rely on being able to extend the Dynamic Range within what can be sustained in a backwards compatible manner with Rec.709. Without going into too much detail in a blog, luminance can be extended to nearly 3,000 nits. This well exceeds what’s practical on screens destined for the consumer at CES 1,000 to 1,200 nits matched the top-show flow HDR capability with it slightly extended to 1,500 nits for private demo room showings. Sure there are very impressive screens that can achieve truly impressive HDR performance, up to 4,000 to 6,000 nits but they’re professional monitors, both expensive and resembling power sub-stations rather than the latest slick screen viewed as an asset to the living room!
So, in conclusion I see Rec.2020 making its way into the market for professional mastering and production purposes, with the latest HEVC VUI and SEI signaling to down-sample both luma and chroma to extended Rec. 709 constraints being signaled to screens via HDMI2.0a! Simple, so now all that remains is to see this working in practice….roll on IBC!
Stay tuned for my new Harmonic Whitepaper on HDR due for completion this summer!
– Ian Trow, Sr. Director, Emerging Technology & Strategy, Harmonic