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Example set of Picture Quality Analysis Charts
Monitoring image & signal degradation
with Mike Hodson, OmniTek
OmniTek’s latest waveform analysis software focused on
assessing the degradation in video and audio quality and
timing suffered by images as they are transmitted or stored.
What approach has OmniTek taken to measuring these effects?
I n an ideal world, all broadcasters
would love to have a piece of
equipment that simply looks at a
video signal and tells the operator what
the quality of the signal is. Unfortunately
this is an almost impossible thing to do,
for many reasons. What is the meaning
of ‘quality’, and how do you measure
it? What happens if the video material
has deliberately been distorted or de-
focussed by the producer, for effect?
Will this confuse the measurement
system? In practice, the only reliable way to
assess the quality of a signal that has
been through a lossy transmission
path is to compare the received signal
with the original transmitted signal, and
look at the differences. This is called a
‘full-reference quality measurement’,
and it is what the OmniTek PQA
system does. With access to both
the source and received signals, it is
then possible to make a quantitative,
repeatable assessment of the amount
of degradation that has taken place.
Different full-reference comparisons can
be made, varying from comparisons of
two stored copies (used to assess the
quality of a copy stored in a particular
way), through comparisons of issued
and received transmissions (used to
determine the effect of the transmission
path on the quality of the signal), to the
comparison of the quality of the signal
at different points along a transmission
path (used to assess how the signal is
degrading). What metrics are used?
A range of different metrics are used for
making Picture Quality assessments.
The simplest and most ubiquitous is
the Peak Signal to Noise Ratio (PSNR),
which is a calculation of the actual
differences in the pixel values between
transmitted and received signals,
summed over the whole screen area
and expressed as a ratio in dB. This
gives a quick and easy comparison of
the two images, but is not very good at
determining subtle differences in images
that the human eye is more receptive to.
The OmniTek PQA also offers an
improved algorithm called the
Compensated Signal to Noise Ratio
(CSNR), where the pixel difference
values are modulated by an edge
map of the source image (to highlight
compression artefacts) and also a
luminance characteristic curve (to mask
errors where the image is very dark or
very bright). This technique gives results
which more accurately reflect a viewer’s
perception of the image quality.
Of course, Picture Quality isn’t the only
factor to be considered in relation to
transmission and storage of video.
There’s also Audio Quality which, in
accordance with ITU-R BS.1387-1 and
the PEAQ audio model, is assessed
in terms of an Objective Difference
Grade, a Distortion Index, and loss
of Amplitude. Also important are the
delays that become applied to the audio
and video streams, which when different
give rise to Lip-Sync problems.
How does OmniTek
measure Lip-Sync delay?
OmniTek offers two different methods
for measuring lip-sync delay. The first
technique involves the use of a special
video sequence, which consists of a
test pattern plus an audio channel ‘blip’.
This sequence is played out from a VTR
or file server, through a transmission
path: when received by any OmniTek
measurement system, this displays
the lip-sync delay. Calculation of lip-
sync using this technique is accurate
to one audio sample (typically 21
microseconds). The latest version of OmniTek software
now also includes an optional “in-
service” lip-sync delay measurement
system, which does not require any
special video sequence and is at least
as accurate if not slightly more accurate
than the test sequence technique.
As long as the OmniTek equipment is
provided with signals from both the
transmission and the reception paths,
the total loop delay of both video and
audio components (and hence the
relative A/V lip-sync delay) can be
calculated and displayed.
Where did OmniTek gain its
expertise in Waveform Analysis?
OmniTek has been creating high-
resolution waveform monitoring
systems for over 10 years. Our in-
house design team have a strong
background in audio and video signal
analysis and digital image manipulation,
and generating waveforms is a special
application of these signal processing
techniques. 54 | TV-BAY MAGAZINE: ISSUE 79 JULY 2013