—This month, I thought we’d tackle one of the less controversial subjects in the media
business today, high dynamic range. Who could argue from a creative perspective
or from the viewer’s perspective, the ability to capture and delivery stunning
video imagery with a dynamic range far in excess of the current HDTV system.
Sorry, the world is never that simple.
While there is agreement that HDR is one of the key components that should be
included in UHD TV, the details of what HDR consists of, what the
specifications will be or which encoding and compression methods will be
standardized, has not been settled. In an attempt to help frame this situation,
I’ll start at the beginning….
In the beginning of television, the
one and only display technology was the cathode ray tube. An incredible
technology for its time, it did have certain limits, one of which was the peak
brightness possible, limited by the maximum spot size (resolution) as well as
the power supply. CRTs have a characteristic electronic to optical transfer
function (EOTC) implicit in the physics of the CRT, which we know as gamma(γ). The
gamma transfer curve has been documented officially in ITU Rec. BT 1886:
The dynamic range of this curve is
typically from 0.1 cd/m2 to 100 cd/m2 , which can
represent three orders of magnitude or about 10 f-stops. Those of us who have
had to shade cameras know that there are limits to where to set iris
(exposure), pedestal (minimum black) and the knee point (peak white). Beyond
the limits of minimum black and peak white, the video signal clips and all
detail is lost.
The CRT limitations constrained the
design of the original analog TV systems (NTSC, PAL and SECAM) as well as early
digital video systems (ITU Rec. BT 601) and HDTV systems (ITU Rec. BT 709). These
systems implicitly limited the range of display luminance values to match the
CRT capabilities and using the inverse gamma (0.45) to convert the linear light
values from the electronic sensor (CCD/CMOS or tube) to electronic values
(voltage or digital code numbers).
With the development of flat panel
technologies (LCD, plasma, OLED) along with improvements in display brightness
including LED backlighting, display performance is no longer a limitation to
reproduce the brightest scene elements while still handling the dark shadow
details providing a dramatic increase in overall picture contrast ratio.
High dynamic range tonal reproduction
goes beyond the brightness of standard dynamic range, both in the toe (minimum
black) and shoulder (peak white/highlights). SMPTE has standardized a new
transfer curve defining the conversation to/from linear light values and video
levels (code values) in ST 2084. Known as PQ (perceptual quantizer), this curve
is based on the perceptual quality of the human visual system and was defined
so as to minimize detectable errors (banding and contouring) over a large range
of light values from .0005 cd/m2 to 10,000 cd/m2.This
range covers most of the range of human vision as well as natural scene
lighting and represents eight orders of magnitude or 28 f-stops! PQ preserves
this extreme range of light values while fitting them within of the 10- or
12-bit values of digital video. Below is a comparison of different electrical-to-optical
transfer function (EOTF) curves:
However PQ is not strictly a display
transfer curve. For each display, there has to be a conversion from the PQ
values back to linear light and then another conversion from linear light to
display light values. This is sometimes referred to the electronic-to-electronic
transfer function (EETF).
There are two meta-data items that are
needed by the display to correctly convert the PQ values to display light
values, MaxCLL (peak white) and MaxFALL (frame averaged light level). These two
metadata items anchor the scene tonal range so that the mid-tones are reproduced
correctly and the overall scene light balance is maintained.
So far so good.But what about displays fixed to the old
gamma curve? And will a program produced for HDR look right if converted to the
more limited CRT display tonal range?
The BBC and NHK have proposed a
compromise curve between PQ and gamma, which uses the lower part of the gamma
curve and then mapped to a logarithmic curve to extend the upper range of peak
white. Called HLG (hybrid log gamma), the definition of the curve can be found
in the ARIB B67 standard, “Essential
Parameter Values for Extended Image Dynamic Range Television.” While
Rec. 709 is anchored in a definition of the peak white value (100 nits) and PQ
is a mapping of absolute light values, HLG defines the code value of 0.5 to
represent a reference white level (relative to the SDR code value of 1 = 100
nits). Typically the range of HLG encoding is from .001 nits to 1000 nits for a
dynamic range of 20 f-stops.
Similar to Rec. BT 709 (SDR), HLG is a
“relative light” transfer curve, so the display device tonal range
characteristic has to be known at the time of mastering. While this works for a
HDR display, what about SDR displays?How will HLG look on these legacy (and some of the first generation 4k
displays)? Tests by the BBC and NHK report that HLG, when displayed on a SDR
monitor, produce reasonable results. Can HLG might be a good candidate-mastering
curve for live television production as a common format for both HDR and
So far, we’ve gone over the concepts
in terms of capture and display of HDR, but what about delivery? How can HDR
programs be delivered to the viewer?s
it possible to simultaneously deliver a signal that can be faithfully reproduced
on any display, HDR or SDR? What about all the different types of content
distribution such as Blu Ray, OTT, VOD, cable, satellite and of course over the
air?I’ll address HDR distribution and
delivery in my next article. Stay tuned.