In Broadcast - June 2015 - 62


62

www.inbroadcast.com | Vol: 5 - Issue 6 | June 2015

InMedia

Comparing Codec Technologies
For Optimal 4K Content Delivery
Adam Schadle, Vice President of Video Clarity, evaluates the future of
4K systems and codec technology...

W

hile it is widely acknowledged
that 4K technology will play a
significant role in all forms of content
delivery, many in the television
industry still view it as an issue that
can be addressed in the future. In
reality, however, 4K technology is
becoming a major factor in content
creation today - one that is already
starting to affect other aspects of the
broadcast workflow and to influence
new equipment purchase decisions.
When considering adding 4K content
to the broadcast workflow, contentdelivery organisations are faced with
a number of technologies (such as
encoding, multiplexing, and HEVC
transcoding) designed to help them
create bandwidth. To assess the
impact of 4K technology, organisations
including programme originators,
broadcasters,
and
multichannel
video distributors have begun testing
programmes to evaluate video quality
and system performance.
Much of the ongoing testing can
be distilled to a few major areas
where 4K technology is expected to
have an impact, one of which is the
comparison of codec technologies. Like
the other major areas of testing, 4K
codec comparison requires both highperformance sources that are capable
of playing 4K video test sequences
in real-time, as well as solutions that
can capture and analyse video quality
using modern objective measurement
techniques.
Engineers from a variety of
organisations are currently analysing
how 4K codec technology will impact
different aspects of their workflows.
Doing so requires compiling a
significant amount of data to support
decisions to select the right codecs
(HEVC, H.264, J2K, or VP9).
Choosing
suitable
compression
formats that will meet a range of
financial and functional objectives
means making major business
decisions at many points in the 4K
broadcast chain. At each point from
source to viewer, tradeoffs must be
made between bandwidth, delay, cost,
and video quality that greatly influence

Fig 1: Codec Evaluation

the type of compression algorithms
selected. These decisions can have
significant
financial
implications,
particularly if a large population
of subscriber devices needs to be
upgraded to support a new codec such
as HEVC. To get a clear picture of the
technical benefits and limitations of
each candidate standard, functional
testing with real devices and software
is crucial.
To properly evaluate a candidate
codec, testing should be performed
with several devices across a range
of useful bit rates. The results can
help determine which compression
standard best fits into a given 4K
service rollout plan.
Workflow
Setting up the test properly is crucial
to ensuring consistent, measurable
results. The test setup shown in Figure
1 provides a straightforward platform
for comparing various compression
systems. A 4K video source supplies
uncompressed content to the input
of the encoder, using a video test
sequence that has been chosen to
stress the encoding process in some
manner. The encoder output or file
is then decoded, which recreates the
original 4K uncompressed source
sequence as closely as possible. This
output video sequence is recorded
and stored to form one element of
a full-reference test sequence set
that will be used for picture quality

analysis. The test is then repeated at
different encoded bit rates, and each
captured 4K uncompressed output
sequence is added to the full-reference
test sequence set. More full-reference
test sequence sets can be created as
desired using other source sequences,
which have different characteristics
from the initially chosen video source.
After the first codec (A) is evaluated, a
second codec (B) can be inserted into
the test workflow, and additional fullreference test sequence sets can be
produced.
The lower portion of Figure 1 shows
estimated trends of DMOS sequence
measurement averages as a function
of bit rate from two different codecs
that used the same source
sequence. Note that a
decrease in the DMOS score
indicates improved video
image quality.
Measurement
Image
quality
trends
affecting the selected source
content caused by each
compression system can be
measured in each sequence
set using objective perceptual
techniques such as MSSSIM (Multi-Scale Structural
Similarity), DMOS (Differential
Mean Opinion Score) or JND
(Just Noticeable Differences)
index as well as subjective
evaluation by human viewers.

Each of these objective full-reference
test measurements will provide a
quality value for each frame of video.
These frame-by-frame quality values
can be compared directly to the quality
values from other elements of the
same test sequence set (since they all
used the same source sequence), or
they can be compared to other sets
that have been generated using the
same source sequence using another
codec. Portions of output sequences
where objective measurements vary
greatly from one codec to another can
be more extensively examined using
subjective techniques if desired.
Although 4K broadcasting is not yet
widely deployed due to bandwidth
and technology limitations, forwardthinking content originators, television
networks, and multichannel video
delivery providers are already
deeply involved with evaluating new
equipment and processes that support
this new format. Choosing the right
codec technology is an important
part of that evaluation. Video Clarity's
ClearView video quality measurement
and analysis products support fullresolution 4K recording, quality
measurement, and playback today,
and are primed to help broadcasters
identify the codecs that will be best for
their operations when the time comes
to deploy 4K systems.

A side-by-side comparison using
ClearView video quality analyzer
http://www.inbroadcast.com http://videoclarity.com/ http://videoclarity.com/videoqualitymeasurement/
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