HDR on TV: What you need to know

LG has announced Flat-TVs with High-Dynamic-Range (HDR) for the IFA.


HDR anywhere


Thanks to our adaptive iris, the range of light intensity that the human eye can perceive is absolutely gigantic. Only a very small margin of this can be transferred from a TV device between black and white peak at all. What makes sense here was defined fifty years ago in the TV basic standards still valid today. For 1 to 100 candela per square meter (Nits) TV programs should be produced. A contrast ratio of 100: 1 was still considered a technical challenge. And the transition from analogue to digital TV to HDTV has not been shaken.


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The display technology, however, is significantly wider and can represent native contrasts of more than 1000: 1, dynamic in clusters even 10,000: 1. So far, however, there has been a standard for source material for this high dynamic range (HDR) as for the extended color space (WCG). However, the truly largest TV revolution for 50 years has now come into the new Ultra-HD specification.


Technologies hand in hand


This year, we were flooded with information about High Dynamic Range from everywhere, which, however, often confused and fueled false hopes. Since many LCD panels are HDR-capable or support HDR or streaming services call it already in one breath with richer colors and SD compatibility. There were hardly any concepts that could be conceived so far, because experts are just setting the stage for a new and surprisingly high quality of television.


The classical definition of HDR is an increased quantization (more bits) in order to keep brightness thresholds invisibly in the case of now significantly greater maximum light intensity. The HDR EOTF, the electro-optic transfer function, which replaces the well-known gamma curve, is now recognized as a future standard (ST-2084). It is significantly more curved in the diagram, thus providing considerably more values ​​of the source signal for dark content. Here, the human eye is extremely sensitive, and thus disturb brightness jumps more than in bright areas.


In order to be able to utilize the new bit depth properly, an LCD or OLED panel requires, in addition to at least 1,000 candela luminous intensity, more clearly than the 12 or 10 bits suggested for the source. In the hardware, the gradations are different from the source signal, so the conversion requires more depth.


In order to make a source device, which expects especially "normal" TV, to make HDR output, it needs a suitable communication of the new possibilities. Here signals have been standardized by the SMPTE, which are also used in the upcoming Ultra-HD Blu-ray and are transported via HDMI 2.0a.


Streaming services have made it easier here. For example, Amazon already offers HDR movies for Samsung TVs. In this case, no source signal has to be changed or consideration has to be given to world-wide standards. The app on Samsung TV directly fetches the right film in HDR from the server and prepares it specifically for its individual panel.


On the other hand, we will have to wait a long time for an HDR control operation for TV stations. The new signal is not compatible with classical broadcasts and should be transmitted by an additional broadcaster, which only pay TV providers afford. Here is the solution of Dolby, called Dolby Vision, forward-looking. It adds HDR as additional data, which need only 20-30% more capacity to the old HDTV data stream. For this, however, the Dolby decoder must be integrated into each HDR TV.


HDR brings sensational brilliance to the TV standard, but the technology has yet to be standardized and brought to the end user. A good way would be the combination with Ultra-HD resolution, extended color space and increased image rate. Then we would have complete the dazzling TV experience of the future.


In order to achieve perfect image reproduction, all technical parameters must be matched. So was the old technique, so 8-bit resolution and an exponential gamma curve correct for picture tubes with a maximum of 200 candela brightness and analog ground noise. Since modern screens work much more precisely and are much brighter, the smallest bit steps (quantization) are now often applied until the eye can differentiate them as streaks. This is particularly noticeable in dark scenes.


Even worse will be with extended color space. With 8 bit resolution, only 120 color saturation stages describe the path from the gray point to a corner of the color triangle. If the path increases from the current standard (BT.709, black) to that of Ultra-HD (BT.2020, white), the individual steps will become larger and easier to see.

In the Ultra-HD color space, 8 bits are therefore unacceptable, which is why 10 bits (four times as many values) are the minimum requirements. The new EOTF (electro-optic transfer function, image at the top in yellow) also makes critical dark areas considerably finer than the classic gamma.