2017-01-26 | Technical changes, design modifications, errors and misprints are subject to change without prior notice. 23
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.
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.
3.5.2.
Fill level
The data rate of the sender on the DVB-S/S2 transponders may vary depending on the image contents and on
the transmission quality. To ensure an undisturbed reception, a reserve must absolutely be observed.
We recommend you to set the maximal fill level to 90%.
If the current fill level exceeds the maximal fill level, it may cause image disturbances, e.g. mosaic images.
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.
