Functional Description - Promax MO-180 Manual De Usuario

1.2 Functional description

The MO-180 is a professional SFN/MFN DVB-T/H modulator contained in a 19"
1U chassis. The unit has three selectable MPEG-2 TS inputs (two serial ASI inputs and
one parallel SPI input). Either of these inputs can be used to modulate the COFDM
signal in both hierarchical (one TS input) and non-hierarchical (two TS inputs) modes.
An additional test TS can be generated internally in the modulator. This allows to
generate compliant DVB-T/H signals even in the absence of a valid TS input.
The modulator has a one pulse per second GPS input (1pps) and a 10 MHz
GPS input. These are used in combination with the Megaframe Initialization Packet
(MIP) carried in the transport streams to achieve synchronisation inside an SFN
network. The 10 MHz input can also be used in MFN networks to frequency lock the IF
and RF outputs to a common reference. A loop-through 10 MHz output is also
available. The input impedance of both the 1pps and 10 MHz inputs can be set to 50 Ω
or high impedance.
In MFN networks we can operate the modulator in master and slave modes. In
slave MFN mode, the useful bit rate at the TS input to the COFDM modulator has to be
the one defined in document ETSI EN 300 744 for each choice of DVB-T/H
transmission parameters. The modulator automatically synchronises its internal clock
to the incoming TS packet rate. The slave mode allows to use one TS input with
constant bit rate in non-hierarchical modes. When using hierarchy, the user has to
choose which TS (HP or LP) the selected TS input is mapped to. This is the stream the
modulator actually synchronises to. The other hierarchical TS is generated internally as
a PRBS test sequence.
The input bit rate in slave mode should be within 0.1‰ of the values specified in
the DVB-T/H standard (see Section 4.14 for actual figures) and approximately
constant. This operating mode is useful when re-modulating an off-air DVB-T/H signal
with the same parameters without the need to demultiplex and re-multiplex the
transport stream (as it would be the case in master mode).
The lock-in range of the MO-180 with respect to the TS rate is typically greater
than that of a COFDM demodulator. It's thus possible that the modulator is perfectly
synchronised in slave mode and, however, a DVB-T/H receiver is unable to acquire
sync.
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USER'S MANUAL MO-180
04/2008