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The analogue TV switch off is fi nish in Europe. Some countries, such as Spain and Italy, have decided to install digital TV “SFN”
(Single Frequency Networks), in other words a national television broadcaster has the same frequency/channel all over the
country. This provides a fantastic opportunity, but also means that in areas between two cells, it is possible to receive the same
signals from more than one transmitter.
If the “SFN” network has been designed well, the SFN signals’ slight propagation delay (which we will call “echoes”), coming from
the different distances in which the transmitters are situated, becomes absorbed in the invaluable GUARD INTERVAL function,
present in the DVB-T & T2 (COFDM) modulation and consequently there will not be any reception problems. In any case,
experience over the last few years has shown us that reality is different to theory, especially when there are many local television
networks that could generate many interferences.
You could therefore encounter the unpleasant experience of receiving a signal with good power, but that cannot show any
pictures and not be able to establish the cause of the fault. In this case it is indispensible to measure the IMPULSE RESPONSE
in real time, to measure the echo’s delay or advance compared to the main signal.
When changing direction and position of the
antenna it is possible to optimize reception intuitively, by maximising the power of the main signal and minimize the power of
interference echoes, also at the expense of the channel power.
Once again Rover Instruments is the fi rst company to supply meters for TV installers, that can measure up to 16 ECHOES and
PRE-ECHOES in real time. ROVER meters allow you to see ECHOES, measure the power and the delay in uS and the distance
of the interfering broadcaster in Km. There are currently very few meters that allow you to measure ECHOES and PRE-ECHOES,
in real time and at a distance of up to 75 Km, higher than the maximum amplitude possible with the GUARD INTERVAL and above
all that can highlight, using the green mask, the useful reception area, in other words within the guard interval.
The width of the GUARD INTERVAL varies according to the modulation parameters: consult the table below to fi nd the width of
the GUARD INTERVAL and all the possible DVB-T confi gurations.
Fig. 4:
DVB-T-64Q CONSTELLATION:
The table to the right shows all the
received modulation parameters
N.B.* Valid examples for a DVB-T
OFDM 8k signal with an 8 MHz
Bandwidth and a 1/8 Guard Interval,
this data is shown on ROVER meters
to the right of the Constellation, see
below Fig. 4.
ECHO & MICROECHO MEASUREMENT in “SFN” TV NETWORKS
HOW TO REDUCE INTERFERENCES IN “SFN” NETWORKS
DVB-T 2.000 carriers (2K DVB-T)
GUARD INTERVAL
1/4
1/8
1/16
1/32
max time (microsecondi)
56
28
14
7
max distance (Km)
16.8
8.4
4.2
2.1
DVB-T 8.000 carriers (8K DVB-T)
GUARD INTERVAL
1/4
1/8
1/16
1/32
max time (microsecondi)
224
112
56
28
max distance (Km)
67.2
33.6
16.8
8.4
TEMPORAL GUARD INTERVAL WIDTH
(already automatically highlighted by the GREEN mask)
Fig. 1:
OPTIMUM RECEPTION:*
no ECHO present either outside or inside
the guard interval mask (green area)
Fig. 2:
GOOD RECEPTION:* 2 ECHOES present,
but within the guard interval mask (green
area) coming from a distance of:
1st echo: 15 Km, the same as a 50 µs delay
2nd echo: 25 Km, the same as a 83 µs delay
Fig. 3:
MARGINAL RECEPTION (or IMPOSSIBLE):*
2 ECHOES present outside the guard interval
mask (green area), coming from a distance of:
1st echo: 40 Km, the same as a 133 µs delay
2nd echo: 45 Km, the same as a 150 µs delay
Summary of Contents for HD TAB 900
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