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41. Basic differences between analog terrestrial
reception and DVB-T
Addition
Cancellation
In most cases, only a few of the 8,192
carrier frequencies will be fully cancelled
out and the information which is lost can
be reconstructed by the error-correction
function of the receiver. Reception beco-
mes impossible only if very strong reflec-
tions cause the loss of too many carrier
frequencies.
The phase relationship between two sig-
nals depends on the path taken by these
signals on their way to the receiver.
If a rooftop antenna is used, almost all of
the signals reach the receiver by the same
path.
If an indoor antenna is used, the signals
reaching the receiver may have been
reflected from
- objects outside the building,
- objects such as thermally insulated
windows (coated with metal films),
- reinforced-concrete parts of the
building,
- persons in the vicinity or
- the furnishings of the room.
The signals may also have come from a
different transmitter.
In such cases, the reception quality
depends not only on the actual signal
strength, but also on how many carrier
frequencies are cancelled out.
Such problems can often be corrected by
adjusting the position and the alignment
of the antenna. In general, moving the
antenna a few centimetres to one side, or
tilting or turning the antenna, can impro-
ve reception far more than the use of an
additional antenna amplifier.
Simply moving the antenna a few
centimetres to one side can often
cause the signals to be added
together instead of cancelling each
other out.
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