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TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine
— 09-10/2013
—
69, 70, 71. In DVB-T/H mode,
the constellation diagram is
enormously useful in identify-
ing errors. The Deviser S7000
comes with all the expected
functions like Zoom, to magnify
quadrants. Also interesting is
the ability to focus the constel-
lation diagram onto single carri-
ers in the transponder. This lets
you analyze, for example, the
influence of a narrow bandwidth
interference signal on a single
carrier. Fantastic!
72, 73. And just like in Sat mode,
the spectrum in DVB-T/H mode
can also be displayed in various
forms, including a waterfall dia-
gram. The 50 MHz to 1000 MHz
spectrum is clearly refreshed at
a much slower rate than in Sat
mode and this is obviously due
to other signal modulations and
smaller spectrum frequency
bandwidths being used. How-
ever, Deviser gives the user
the ability to edit the spectrum
frequency bandwidth in incre-
ments. Depending on the set-
ting, the spectrum is displayed
in more detail or presented
much faster.
74. You don‘t want to analyze
the spectrum across the entire
frequency span here anyway -
it would be good enough if you
find out ahead of time on what
frequencies signals can be
found. It‘s critical here to enter
small enough ranges that would
allow a precise measurement of
the signal-to-noise ratio. There
are two markers available in this
mode for that purpose and the
S7000 automatically determines
this ratio in MHZ and dB.
75. An ECHO measurement
should be included in every
modern DVB-T/H signal analy-
zer. This type of measurement
is critical for Single Frequency
Networks (SFN). The idea here
is that various transmitters
in a region all broadcast on
the same frequency. This has
numerous advantages; for one
thing the rest of the frequency
band remains clear for other
uses and secondly, receivers
can be transported within a
region (such as in cars) without
having to reset the reception
frequency.
Unfortunately, SFN transmissi-
ons also come with a huge dis-
advantage: if the receiver finds
itself between two transmitters,
the signal will be received from
both transmitters. Since the dis-
tance of the receiver to both of
the transmitters is likely to be
different, the result is a shift in
the arrival times of both signals
known as ECHO.
The DVB-T/H standard provides
for a so-called Guard interval
that automatically compen-
sates for this time shift if it
is small enough, that is, lies
within the Guard interval. If the
ECHO is too large, it will ine-
vitably lead to reception pro-
blems even though a simple
signal analyzer will show a high
signal level!
The S7000 has mastered these
ECHO measurements and auto-
matically displays these mea-
sured ECHOs graphically and
in a table. For each ECHO the
corresponding level as well as
the delay in microseconds/kilo-
meters is shown. This will allow
the installer to determine which
transmitter is causing the inter-
fering ECHO and properly align
the antenna especially if he has
documentation showing the
regional distribution of these
transmitters.
A trick would be to use the slo-
ping roof of a house to block the
unwanted signal from a distant
transmitter.
76. Another relatively new mea-
surement that is found in very
few professional analyzers is
the MER vs. carrier measure-
ment. Here the Modulation Error
Rate (MER) is not shown as an
average value of all carriers
within a transponder, instead
it is shown for each individual
carrier.
These values are presented in
a graphic that resembles the
spectrum and consequently,
also in a waterfall diagram. The
marker allows the navigation
within the many carriers that
make up the transponder and
the analyzer will show the res-
pective matching MER value in
dB.
This measurement serves to
identify the smallest interfe-
rence such as that from audio/
video broadcasters in the VHF/
UHF range. But even LTE and
other services could interfere
with a DVB-T/H transponder.
With a basic MER measurement
this interference might not be
detected since the average
value is hardly affected. Never-
theless, it could be enough
to totally interfere with a few
carriers such that a considera-
ble portion of the transponder
stream can no longer be cor-
rectly received.
The simultaneous display as a
waterfall diagram is truly one-
of-a-kind; it allows you to catch
and document even intermittent
and momentary interference.
77. Just like in Sat mode, the
TS Analyzer can be accessed
at any time. In just seconds a
signal lock is achieved and the
reception parameters are read.
Thanks to the implementation
of many standard and also non-
standard CODECs, channels
such as the Portuguese TDT
(Televisão Digital Terestre -
digital terrestrial television)
can be displayed even though,
despite DVB-T modulation, the
video and audio are compressed
in MPEG H-264 and MPEG4 AAC,
respectively, as can be seen in
the PROG GRAPH screen.
