Metz Slim TV manual Addition Cancellation

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 improve 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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