Promax MO-180 Manual De Usuario página 73

USER'S MANUAL MO-180
In master MFN mode, the modulator is locked to either the internal 10 MHz
TCXO clock reference or to the external 10 MHz GPS reference. In this mode, the
MO-180 is able to work with any incoming bit rate as long as the net bit rate resulting
from dropping all NULL packets present in the stream is strictly lower than the value
given in the DVB-T/H specification for the modulation parameters in use (see
Section 4.14). The input TS bit rate is adapted (bit rate adaptation) to the useful bit rate
required by the DVB-T/H signal by stuffing the TS with NULL packets (packet stuffing).
This stuffing process alters the sequence of PCR values embedded in the TS. These
values have to be re-stamped for the resultant PCR jitter to remain within the limits
specified by the DVB. In hierarchical modes, operating the MO-180 as master has the
added advantage over the slave mode of being able to use any of the three TS inputs
as the HP input, LP input or both.
Whenever possible, in master MFN mode it is advised to use an input bit rate
considerably lower than the nominal value given in the DVB-T/H specification.
Otherwise, an input rate too close to the required value might eventually lead to
overflow of the TS packet buffer implemented in the modulator.
In SFN mode, the modulator can be locked to either the external 10 MHz GPS
reference, or to the internal 10 MHz TCXO, or to the input HP TS data rate. A loss of
sync with the external 10 MHz reference can be used to trigger a swap of the
synchronisation over to the input TS rate. This reduces the number of disruptions to the
output IF/RF COFDM signals. Periodic or a periodic MIP packets are constantly
monitored in the HP TS input so as to dynamically adjust the delay of the modulator for
accurate SFN synchronisation. If required, a positive or negative delay offset with
100-ns resolution can be added locally.
In non-hierarchical transmissions the modulator can be instructed to seamlessly
switch between ASI inputs when it detects a loss of sync in the currently selected TS
input. Additionally, in SFN operation the IF/RF outputs of the modulator can be
optionally muted in the presence of processing errors.
The modulator can be configured to generate any of the transmission modes
listed in the DVB-T/H specification. In hierarchical modes, the HP and LP streams can
be encoded with different convolution code rates. The channel bandwidth can be set by
the user to 5, 6, 7 or 8 MHz as required by the application. Several test modes are
available in the MO-180 (blanking of carriers, single tone output, test TS generation,
CBER and VBER injection).
Many of the configuration parameters can be optionally obtained from the MIP
packet embedded in the input transport stream. This can be done in both MFN and
SFN operation. By setting the Transmitter ID of the modulator we can extract from the
MIP the configuration parameters which are addressed to a particular transmitter and
which may differ from those of the rest of the network. The ID and centre frequency of
the network can be updated in the corresponding entries of the Network Information
Table (NIT).
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