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User manual
4. - ELEMENT DESCRIPTION
4.1.- INTRODUCTION
Once the digital terrestrial TV (DTT) is being
broadcasted
in
a
modulation
COFDM, different from the one employed
for satellite (QPSK) and cable (QAM), the
user should use a special receiver for vie-
wing these services (COFDM IRD) that basi-
cally consists of a COFDM demodulator and
an MPEG decoder.
This inconvenience could be solved distribu-
ting the COFDM signal by means of a
COFDM to PAL converter. In this way the
same TV set could be employed to receive
the analogue and the new digital programs,
and the user wouldn't have to buy a new
IRD. But with this method, transmodulating
to an analogue format (PAL), the quality and
the added value services that are offered by
the DTT are lost.
On the other hand the DVB project appro-
ved the ETS300473 norm, which establishes
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, QPSK, to QAM format.
By unifying the modulation format to QAM in
format,
the distribution, a single receiver may be
used for all types of signals, regardless of
their original modulation format (QPSK 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
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
speed, 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. The COFDM-QAM
transmodulator makes it possible to select
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COFDM QAM - TS PROC
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