Broadcast (BroadcastTech p47)
Ultra
High Definition (UHD) may be out of the bottle but discussions about
it are far from over. While studios, broadcasters, tech companies and
other stakeholders might agree on the ultimate goal of creating a set
of UHD standards that will drive public take-up of the technology,
there is much debate about what is needed to get there.
Let
us recap: already agreed is UHD-1 Phase 1, which, for all practical
purposes, is about a spatial resolution four times that of HD (3,840
x 2,160) and a frame rate of 50/ 60fps. Also agreed is UHD-2, which
at 7,680 x 4,320 pixels offers 16 times HD resolution.
UHD-2
was used for the joint NHK/BBC public demonstrations of Super
Hi-Vision during the London 2012 Olympics and is only being
considered in Japan at this time, with broadcasts expected to take
place by 2020.
UHD-1
Phase 2 is an attempt “to define an immersive viewing experience”,
explains Matthew Goldman, senior vice-president of technology, TV
compression, at Ericsson, and is earmarked for standardisation in
2017/18 – although this may slip.
Encouraging
adoption
The
reason for a second stab at UHD-1 is a realisation that resolution
alone does not produce a big enough uplift in picture quality and
other attributes are needed to encourage greater public adoption.
Some
of the main elements of UHD-1 Phase 2 include a wide colour gamut
(WCG) and high dynamic range (HDR), sampled from a bit depth of at
least 10 bits. Since all existing digital TV systems are only 8 bits,
they are not able to represent subtle shades of colour or details in
the shadows simultaneously with details in bright areas of the image.
“This
will have one of the biggest impacts on the viewing experience,”
says Goldman. “Colours will look more real, with subtle shading
possible without visible banding artefacts; and highlights – such
as the sun reflecting off surfaces or night-time stadium lighting –
will ‘pop’ with realism.”
Higher
than 50p frame rates are also on the table, with sports broadcasters
pushing for 100/120p to eradicate motion blur at high resolution.
Another
key element is dynamic range. The standard dynamic range (SDR) in use
today was defined in the 1950s based on cathode ray tube technology.
SDR is measured in candelas, for which the production standard
equates to 100 candelas per square metre (also known as ‘nits’).
This
can only deliver a limited contrast between the whitest whites and
the darkest areas of the picture, but a higher dynamic range
(starting at 5,000 candelas and rang- ing to 20,000) goes way beyond
the current TV production standard and will make the single biggest
difference to viewing.
There
are a number of proposals for implementing an HDR system – also
known as Extended Image Dynamic Range – from Dolby, Philips, the
BBC, NHK and Technicolor. Apart from Technicolor’s, each proposal
has been submitted to the ITU-R.
Dolby’s
approach, dubbed ‘perceptual quantisation’, is an absolute
measure of dynamic range that closely matches human vision but
requires complex computations. The BBC, by contrast, wants to adopt a
relative measure of
ynamic
range using log scales, which has the benefit of being simpler to
calculate.
Whichever
route is selected, when HDR, WCG, 10-bit sampling and higher frame
rates in UHD-1 Phase 2 are factored in, working with all the data in
production, let alone delivering it to the home, becomes fraught with
difficulty.
“It
is beyond the capability of current professional and consumer media
interfaces to handle that amount of data,” says Goldman.
HDMI
2.0, the latest consumer interface to transport signals between a
set-top box and a TV, arrives this year, but it won’t be able to
handle 2160p 100p, so a new version of HDMI must be developed.
With
the agendas of the various stake- holders at odds, they must trade
off what is desirable in UHD-1 Phase 2 with what can be realistically
delivered by 2017/18.
Since
the benefits of HDR are independ- ent of spatial resolution, the
debate has spawned interest in an enriched version of HD, known as
‘Enhanced HD’. “There’s a huge push to define a 1080p version
that includes 100/120p, HDR, WCG and deeper sampled bit depth that
will fit into HDMI 2.0,” says Goldman. “That type of signal will
fit into existing SDI cables as well. It takes up much less bandwidth
than UHD.”
Bandwidth
matters, especially to cable and telco service providers, so for them
it may become a question of what delivers the best ‘bang per buck’
in terms of changing the viewing experience. “Perhaps Enhanced HD
offers a more viable, compelling solution for service providers than
UHD,” says Goldman. “The argument is not whether to include HDR
but what form it should take.”
The
difference is not insurmountable, given that several companies,
Ericsson included, are represented in two or more of the standards
organisations.
UHD-1
Phase 2 will also have a consider- able impact on production and
post. “Part of the process of deciding on commercial requirements
for UHD-1 Phase 2 is to con- sider the practicability of programme
produc- tion that could be used for the format,” says David Wood,
commercial module chairman,
DVB-UHDTV,
and SMPTE fellow. “The degree of compatibility between Phase 1 and
Phase 2 is one of the most important issues in its development.
Current thinking is that both backwards-compatible and
non-backwards-compatible versions of Phase 2 should be investigated,
so a decision on which to use could be taken on the basis of factors
such as the extra bandwidth the compatible version needs.”
Real-time
data handling
Even
if display technologies are capable of the frame rate, wider colour
and HDR of UHD-1 Phase 2, colour-grading systems need to be able to
handle the data in real-time.
“On
face value, the image for Phase 2 is still 3840 x 2160, but the data
differential between that and 10 bit UHD is 16 times,” warns SMPTE
EMEA governor and Sundog Media Toolkit co-founder Richard Welsh. “If
a facility thinks UHD/4K looks like a lot of data now, Phase 2 is an
eye-watering prospect.”
Another
challenge will be the number of deliverables. One suggestion is to
automate the process by using one master format to derive all the
deliverables downstream.
“This
is not a simple case of throwing data away – just reducing bit
depth from 16 to 12 to 10 requires careful management to avoid ugly
artefacts,” says Welsh. “Likewise, changing the frame rate and
colour space will present challenges. Will creatives want to see, and
tweak, all these configurations? And will productions be able to
afford it? That remains to be seen.”
Who’s
setting the standard?
What
are the differences between the SMPTE, ITU and DVB’s approach to
UHD? SMPTE’s focus is broadly on production and archiving
standards, the DVB’s interest is in content delivery to the home,
while the ITU acts as a bridge or programme exchange.
David
Wood, who advises the DVB and SMPTE, says: “SMPTE and ITU formats
for UHD-1 programme production and exchange are aligned. The DVB
broadcasting profiles will draw on the specifications, but not all of
the features in the ITU/SMPTE specifications may be included. For
example, UHD-1 programme production might be done at 12 bit, while
delivery might be done at 10 bit. Meanwhile, the MPEG committee is
exploring whether any changes should be made to the HEVC codec in
order to code HDR more efficiently.”
The
DVB hopes its specification for UHD-1 Phase 2 will be will be
finalised in the second quarter of 2016.
Other
bodies are keen to contribute to the formation of UHD standards.
Unveiled at CES (pictured) in January this year, the UHD Alliance is
mostly driven by film studios and dis- play manufacturers, and is
looking to agree a set of parameters for UHD viewing quality, and to
stamp a ‘kite mark’ onto certified UHD TVs. Its members include
Panasonic, LG, Netflix and The Walt Disney Studios.
The
Ultra HD Forum is a US-based organisation set up in 2014 by Harmonic
to cover all aspects of UHD production and delivery, including live,
OTT and VoD. It is not to be confused with the UHD Forum, another
group looking at the complete UHD ecosystem. The latter was launched
by the UK’s Digital TV Group (DTG) in 2013 and is chaired by the
BBC and BSkyB. The DTG is working with both US organisations, as well
as the Forum for Advanced Media in Europe (Fame).
The
UK’s Digital Production Partnership (DPP) has also committed to
drawing up a definition of a UK delivery standard for Ultra HD
programmes by the end of the year.
Q&A...David Wood on SMPTE
What
will the UHD-1 Phase 2 specifications include? The name ‘UHD-1
Phase 2’ is used by the DVB Project for a collection of features
that will be included in a future transmission format. It is pre-
paring the elements it believes Phase 2 will need for commercial
success. 2160p spatial resolution is included and it is likely to
include the BT2020 definition for wider colour gamut. HFR and HDR are
under discussion. Advanced audio may also be a part of Phase 2.
What
are the delivery requirements?
The
working assumption is that HEVC compression will be used for Phase 2.
Different combinations of features will affect the compressed bit
rate, and different compressed bit rates may be more or less
commercially viable for different delivery platforms.
When
will UHD-1 Phase 2 be adopted?
It
should be available in 2016, to allow services in 2017/18, but it is
still under discussion.
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