2016-01-11 | © AXING AG Schweiz | Subject to change in design and type; errors and misprints reserved
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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 ½ - 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.
DVB-T transmission modes (2 k and 8 k):
Using the 8 k mode is recommended for reflections with really long durations. Due to the long symbol duration,
long guard intervals are possible.
Output level:
The
Output Level
options include attenuation and deactivation of the modulator. The settings are separate for
each of the four modulators. The max. output level of approx. 105 dBµV is reached with an
Output Level
setting of 20 dB.
Vernier adjustment:
The vernier adjustment of the output channel is performed in 1 MHz steps.
FILL LEVEL:
If the net data rate of the signal exceeds the net data rate of the output channel, the modulator overflows. This
overflow leads to disturbances. If the modulator overflows, the status LED on the front side of the device lights
in red.
Filtering the programs (see chapter 3, section 3.1) reduces the net data rate of the input signal. Subsequently,
the net data rate of the output signal is also reduced.
The data rates of the programs are not constant. They are dynamically changed by the sender. These
fluctuations can be noticed in the fill level.
To avoid an overflow, we recommend that you don't let the maximal fill level exceed 90%.
Chosen programs:
The program table
Selected Programs
shows the programs that were activated in phase 2 and that have been
selected in the Cross Multiplex.
