Televes DVB S2-QAM Manual De Instrucciones página 31

User manual
4. - ELEMENT DESCRIPTION
4.1.- INTRODUCTION
The DVB Project has developed the stan-
dard for DVB-S satellite transmission impro-
ving its features, with a view to increasing
the capacity of satellite transponders and its
flexibility to transport different kinds of data
beyond the MPEG transport stream.
For the purpose, newer and more complex
coding and modulating setups are used
(8PSK). As an error protection, we have
abandoned the Viterbi and Reed Solomon
DVB-S setups by more complex algorithms
(LDPC and BCH).
On the other hand, the DVB project establis-
hes the QAM modulation format as one of
the formats to distribute digital signals in
SMATV networks, due to the good beha-
viour against the noise and mismatches and
its very high spectral efficiency. In this way,
the digital satellite TV signals are being dis-
tributed in SMATV networks using QPSK-
QAM transmodulators, converting the sate-
llite modulation to QAM format.
Besides, it is possible transmodulate
COFDM terrestrial TV signals to QAM.
Unification of the modulation format to QAM
in distribution enables the user to use a sin-
gle receiver for all types of signals, regard-
less of their original modulation format
(DVB-S, DVB-S2 or COFDM).
Also, terrestrial COFDM signals vary their
binary rates from country to country depen-
ding on the transmission parameters (in
Spain, a bandwidth of 8 MHz, 64QAM, FEC
2/3 and a guard interval of 1/4). The same
occurs with satellite signals. For this reason,
when transmodulating COFDM or QPSK
signals to QAM, different binary output rates
are obtained, for example:
QPSK satellite (27.5 Mbaud FEC 3/4 )
-> 64QAM 6.875 Mbaud
COFDM terrestrial (64QAM FEC 2/3 IG 1/4 )
-> 64QAM 3.600 Mbaud
This complicates the scanning process. The
receiver has to detect all of the QAM signals
of the distribution by scanning the whole
band, using both the frequency and the
binary rate as variables. This process can be
very slow. The scanning process is made
easier if all of the QAM signals are identical
(with the same modulation format and sym-
bol speed). To do so, stuffing techniques are
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DVB S2-QAM TS-PROC
applied to the demodulated data, before
they are modulated to QAM.
Stuffing
In many cases the QAM receivers have soft-
ware that scans for the frequency band,
using one single symbol speed as the varia-
ble, for example 6.875 or 6.9 Mbaud, which
is the maximum binary rate that can be used
in 8 MHz channels for QAM modulation. To
do so, the transmodulator devices installed
in the head-end must provide this symbol
rate, independently from the binary rate of
the input signal.
To obtain these characteristics of the QAM
signals, a "stuffing" of the MPEG2 transpor-
tation packet must be carried out in the
head-end.
PID filtering
At the same time, in specific distributions
such as small CATV, the network operator
may decide to eliminate certain services
(PID filtering) to avoid having to pay rights to
the service provider. DVB-S2 transmodula-
tor allows to select services availables at the
input that will not be able to be decoded at
the output