In Broadcast - February 2016 - 60


60

www.inbroadcast.com | Vol: 6 - Issue 2 | February 2016

InForm

A Methodology For
Identifying Target
Bit-Rates For 4K Services
Adam Schadle, Vice President of Video Clarity, examines the importance of quality testing
and evaluation on business decisions, regarding the adoption of 4K technology
infrastructure and services...

T

echnology leaders are rapidly
responding to the looming
challenge of 4K video both by
embracing new production methods
and by validating compatibility with
existing workflows.
This validation needs to cover all
aspects of the production process,
and any compression and processing
applied for storage and delivery of
content. Although most broadcasters
today view 4K as an issue to be
addressed in the future, some
forward-looking groups recognise
that the technology is starting to have
an impact on current engineering
practices. At the same time,
engineers from many organisations
are examining how 4K technology
will impact different aspects of their
workflows.
A significant amount of data needs
to be compiled to support decisions to
select the right codecs (HEVC, H.264,
J2K, or VP9 to name a few examples),
the optimal bit-rates, the appropriate
file/container
formats
and the best suppliers for
contribution, distribution,
and archiving functions.
Some
organisations
are also grappling with
the trade-offs between
delivering content to
consumers as 1080p60
or 2160p60, and defining
whether better quality
can be achieved for
very low bit-rates by
relying on upconversion
at the display device.
In the sections below,
we describe a testing
protocol,
workflow,
and measurements for
identifying target bitrates for delivery of 4K
signals.

Selecting an appropriate bit-rate for
delivering 4K signals is a billion-dollar
question for broadcasters around the
globe. Delivery networks will only be
cost-effective if the bit-rates chosen
for 4K delivery allow enough channels
to be offered to consumers, while still
remaining within the capacity limits
of the network. For example, a cable
provider might require at least two
4K signals within each 6 MHz channel
slot. Using 256 QAM that generates
a usable bandwidth of 38 Mbps in a
channel, each 4K channel would need
to consume less than 19 Mbps. Similar
decisions are faced by satellite, overthe-air, and IPTV providers.

input. The compressed output of the
encoder can be fed directly to the
decoder's input. The 4K output of the
decoder is captured in real-time to
form an element of a full-reference
test sequence set. Additional outputs
are then captured and added to the
sequence set by making small step
changes in the encoder bit-rate. More
full reference test sequence sets can
be made by using different source
content, and repeating the process
of capturing and storing the decoder
output sequences. This workflow can
also be performed as a file process
in place of real-time play/record
functions.

Workflow
Testing can easily be performed to
determine an optimal transmission bitrate using the test configuration shown
in Figure 1.
A 4K video source playing a
video source sequence feeds an
uncompressed signal to the encoder's

Measurements
Several types of measurements
can be made using a complete fullreference test sequence set, that
leverages objective measurement tools
such as MS-SSIM metric on a DMOS
scale, or PQR metric on the JND scale.
At times and for certain high bit-rate

codecs such as JPEG2000, a PSNR
measurement will be more appropriate
to better define performance between
bit-rates. Figure 1, Diagram A shows
an estimated trend of sequence score
averages that would be expected if
quality is measured by using Peak
Signal-to-Noise Ratio (PSNR), where
better
comparative
performance
through less image degradation is
indicated by a higher value. Figure
1, Diagram B shows sequence score
averages and trends that would be
observed if quality is measured using
MS-SSIM/DMOS; again, in these indices
a lower score indicates better picture
quality. For any given source content
type, some points in the curve of the
encode/decode process may perform
differently at different bit-rates. This
same test configuration can be used
to evaluate video quality performance
for codecs used for different functions
(say contribution vs. distribution) at
different points along the broadcast
chain.

Figure 1. Target 4K bit-rate designation

Conclusion
Much of today's 4K
testing is being done on
prototype or 'first-article'
system elements, but
the results are already
providing valuable input
to help guide important
business decisions for
future generations of
services.
As new 4K systems
and products reach the
market, video quality
testing and evaluation
will continue to be
important,
both
for
processing
technology
performance benchmarks
as well as vendor
selection.
http://www.inbroadcast.com https://youtu.be/9-i_XHOUURw
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