DVB-T modulation:
The modulation can be set on QPSK, on QAM 16 or on QAM 64.
The QPSK-setting provides the smallest data rate to the output channel.
The QPSK-modulation process is used in bad distribution networks
because of its robustness against disturbances and of its safe
transmission.
The QAM-modulation process allow reaching higher data rates, so that
more programmes can be transmitted on a channel. The QAM 64-
modulation gives the highest data rate.
•
QPSK (2 bit)
– small data rate – very robust signal.
•
QAM 16 (4 bit) - middle data rate.
QAM 64 (6 bit) - high data rate.
DVB-T FEC (forward
error correction):
Thanks to the error correction, errors resulting from high-disturbed
transmission routes can be balanced by restoring data.
The data required to restore the signal are included in the transmitted
FEC bits.
Changing the FEC factor modifies the part of the FEC data in relation to the
application data.
A higher part of FEC data means an higher transmission redundancy. But
this reduces the bandwidth for the useful data too.
A FEC of 7/8 means the highest rate for the useful data and the smallest
transmission redundancy.
•
FEC 1/2 - small data rate - strong protection against errors.
FEC 7/8 - high data rate - weak protection against errors.
DVB-T bandwidth:
The DVB-T standard plans a broadcast on 6, 7 or 8 MHz channels. A
bigger bandwidth means that more data can be transmitted on a single
channel.
In the CCIR channel spacing, the lower channels: C5…S20 have a
provided bandwidth of 7 MHz. The other channels have a bandwidth of
8 MHz.
If the bandwidth is changed, the channel does not correspond any longer to
the set channel spacing. Therefore, the output frequency for all
4 modulators also changes.
DVB-T guard interval:
A guard interval is transmitted between the symbols of the useful signal.
This guard interval avoids the intersymbol interference during the DVB-T
transmission.
The delayed signals of other synchronized DVB-T senders or reflections
have no effects on the decoding of the useful signal if they arrive during the
guard interval. The period of the echoes must be shorter than the duration
of the guard interval.
Changing the guard interval adjusts the ratio between the transmission
duration of the useful symbols and the duration of the guard interval.
A great guard interval (e.g. 1/4) leads to a really small data rate.
When transmitting on a perfect coaxial distribution network, a really small
guard interval (1/32) is enough.
