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Optimizing digital TV networks –
QoS maintained automatically
Network operators are naturally interested in offering as many programs as possible
in one transmission channel. But the limits are reached when high compression
starts producing visible artifacts that degrade picture quality. Digital Video
Quality Analyzer DVQ (FIG 1) automatically detects such defects and helps to
maintain quality of service (QoS).
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The future is digital
The creation and expansion of digital
TV networks is in progress worldwide.
Everything started with satellite-supported
systems because of the comparatively
small retrofi t effort. Meanwhile,
terrestrial operators have followed up
and are now establishing completely
new networks for digital TV. Cable
operators too are now beginning to
trim their infrastructure for the digital
age.
What are the driving forces behind
these activities? The most important
reason – besides the high quality of digitally
coded signals and the advantage
of simple insertion of additional information
accompanying the program
– is the possibility of simultaneously
transmitting several programs in one
channel. This is profi table for the network
operator since more programs
38 News from Rohde & Schwarz Number 166 (2000/I)
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usually mean more viewers and thus
bigger earnings through advertising.
Where to date only one program could
be transmitted at a bandwidth of 6, 7
or 8 MHz, it is now possible to combine
the picture, sound and additional
information of several programs using
time-division multiplexing provided that
the picture data (to MPEG2 standard)
are suitably compressed.
The limits of this compression process
are reached if the compression rate
is too high and picture quality suffers
from visible artifacts. And that is exactly
what has to be prevented: nobody
wants to lose customers because of
inferior QoS.
Ensuring service quality
To maintain constant quality of service,
an instrument is needed to measure,
FIG 1
Digital Video Quality Analyzer DVQ needs no
original picture as reference and offers extensive
possibilities of application not only in network
monitoring but also in development, production
and quality assurance
quantify and monitor QoS. Digital
Video Quality Analyzer DVQ from
Rohde & Schwarz was developed for
precisely this purpose. It can evaluate
picture quality in realtime according
to subjective criteria and monitor
and record vision and sound errors
in a digital TV program [1; 2]. If a
set picture-quality threshold is underranged,
it responds so that immediate
remedial actions can be taken.
While the picture quality of analog TV is
mainly determined by signal degradation
along the transmission path, encoding
quality and data rate are the decisive
criteria for digital TV. However, the
quality of the picture is not a constant
quantity, it can fl uctuate a lot depending
on the program material (newscasts,
documentaries, sporting events, etc).
So testing the transmission equipment
with still pictures or short test sequences
is not very informative.