Home Cinema Insight | Understanding HDR
The UHD specifications also call for other
improvements in bit-depth, colour gamut (both key
parameters for rendering HDR video) and refresh rates.
In recent years, technologies were developed for
increasing the dynamic range of pixels used in LCD video
displays (as well as the dynamic range of sensors in video
capture and post-production equipment) and last year the
CTA released HDR10, an open standard for high dynamic
range UHD video supported by major manufacturers.
Having all these pixels, whether 4K or UHD, is great
for rendering more detailed images, but moving the
digital data comprising them from source to display
requires more bandwidth than HD. Just how much more
is determined not only by the number of pixels, but also
by the image refresh rate and colour depth.
Refresh rates for cinema or consumer video typically
don’t require more than 60 frames per second and 24, 30
and 60fps is the common refresh rate for cinema or home
video. This parameter won’t change as video moves from
HD to 4K/UHD screens.
The HDTV specification for colour depth is eight bits.
Interestingly, a video signal of 4K or UHD resolution and
8-bits colour depth at 60 fps, fits within the 10.2 Gb/s
bandwidth of HDMI 1.4 (as well as HDBaseT) technology.
This allowed manufacturers to start selling 4K sets for
that generation of source and switching devices, but
their only advantage was higher resolution, not the
enhancements to dynamic range, which produce the
most visible improvements in quality. Even if the sets were
capable of HDR performance, they could not accept the
data needed. These sets also came to market prior to the
availability of high-speed, 18Gb/s HDMI 2.0, resulting
in a generation of 4K TVs that won’t work with new
generations of Ultra HD Blu-ray players, AVRs, switchers
and distribution amps.
While HDTV calls for 8-bit colour, the UHD
specifications call for 10-bit. This difference is more
significant than the small delta in digits suggests, because
each pixel is comprised of three ‘sub-pixels’ of red, green
and blue, with each element defined by its colour code.
For HDTV, this means each sub-pixel can render 256
colours (28), allowing each whole pixel to display up to
16.7 million colours (2563). However, with a display based
on 10-bit colour, each whole pixel becomes capable of
rendering over 1 billion, or (210)3, colours.
One might think this just allows 4K and UHD screens to
display more shades within the exiting colour range of HDTV,
but here is where Ultra HD with HDR makes the difference.
The dynamic range of a pixel refers to the range in
contrast between its darkest black and its brightest white.
Compared to standard LCD display technology, HDR
products can utilise pixel technology that expands the
range on both ends of the brightness spectrum, allowing
‘blacker’ blacks and ‘brighter’ whites.
HDR also expands the colour range that pixels are
capable of reproducing.
On the display side, this is manifested in pixels
enabling a wider range of contrast, brightness and colour.
On the signal side, this means employing 10-bit colour
encoding for accessing a wider colour gamut and adding
a layer of metadata to the signal that tells the TV how to
process the info.
For displays, the wide colour gamut covered by
HDR can approach a specification now used in cinema
exhibition, so that video will be closer than ever before to
the experience one enjoys in a good movie theatre.
Handling all this data requires bandwidth of more than
11 Gb/s, so only AV gear compatible with the HDMI 2.0
specification can provide the headroom. However, HDMI 2.0
doesn’t support HDR metadata. Only the latest equipment
meeting the recent HDMI 2.0a specification does.
This is important in choosing equipment because
it involves not only the source and the display, but any
components in between. In choosing HDR displays, look
for technical specifications conforming to the guidelines
for Ultra HD Premium certification by the UHD Alliance,
including a colour gamut of more than 90% of the DCI-P3
colour space. AV gear will also need to support HDCP 2.2,
the current protection scheme on most content.
Atlona, which distributes residential products in the
UK through RGB Communications, has announced its first
series of distribution components, including a switcher,
distribution amplifiers and an extender, to support HDR
video. All the new components are compatible with the
HDMI 2.0a spec and are HDCP 2.2 compliant.
Life would be simpler if HDR were a universal format,
but it’s not. In addition to HDR10, which is supported and
promoted by the UHD Alliance (as well as the CTA), Dolby
has created Dolby Vision, a version of HDR that includes
additional HDR-specific capability and can be optimised to
the profiles of specific Dolby Vision displays. Products that
meet different specifications will carry identifying logos: the
Alliance’s Ultra HD Premium logo and the Dolby Vision logo.
Since installation headaches often i