Functional Description - Promax MO-170 Manual De Instrucciones

1.2 Functional description

The MO-170 is a general purpose DVB-T 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 signals even in the absence of a valid TS input.
In slave mode, the useful bit rate at the TS input to the COFDM modulator has to
be the one defined in ETSI EN 300 744 for each choice of DVB-T 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 0.1l of the values specified in
the DVB-T standard (See section "4.10 DVB-T useful bit rates") and approximately
constant. This operating mode is useful when re-modulating an off-air DVB-T 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-170 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 receiver is unable to acquire sync.
In master mode, the MO-170 is able to work with any incoming bit rate as long as
this is strictly lower than the value given in the DVB-T specification for the modulation
parameters in use (See section '4.10 DVB-T useful code rates'). The input TS bit rate is
adapted (bit rate adaptation) to the useful bit rate required by the DVB-T 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-170 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, it is advised to use an input bit rate considerably lower than
the nominal value given in the DVB-T 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.
The modulator can be configured to generate any of the transmission modes
listed in the DVB-T 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 6, 7 or 8 MHz as required by the application. Several test modes are available in
the MO-170 (blanking of carriers, single tone output, test TS generation, CBER and
VBER injection).
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USER'S MANUAL. MO-170
12/2006