Versión de HTML Básico
■
1
228
TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 04-05/2012
— www.TELE-satellite.com
Three exemplary transport streams
(yellow, green and blue) at the input of
the modulator consist of rather short
transport packets. The TS packets are
converted to longer BB (Base Band)
frames. They are either 16 or 64 kilo-
bytes long. In fact, there are more op-
erations done in this module (like mode
adaptation, stream adaptation) but as
they are not so relevant for understand-
ing the principle of operation, there is
no need to describe them here. Then,
BB frames are time-multiplexed on the
same carrier. In the process of coding,
BB frames are appended with error cor-
rection bits (see Figure 2). Depending
on the user settings, LDPC coding rates
can be different for different streams.
When no data is present, the modulator
pads incomplete BB frames or inserts
dummy BB frames if needed. Multi-
plexed BB frames are then interleaved
and modulated.
As mentioned earlier, each transport
stream (converted to BB frames) can
have its own modulation: QPSK, 8PSK,
16APSK or 32APSK and inner coding
ratio.
Except for the BB header, DVB-S2
also uses PL (Physical Layer) headers.
The PL header is always transmitted
using a robust modulation and strong
FEC rate. This helps the receiver lock
on the signal, as well as it provides the
information necessary to appropriately
demodulate the remainder of the PL
frame. The first thing the receiver does
when tuned to a given frequency with
a given symbol rate is looking for a PL
header.
A block diagram of the reception sys-
tem is shown in Figure 3. This is in fact
a partial and simplified block diagram
of a VCM compatible DVB-S2 receiver.
Signal from an LNB is converted in the
L-Band tuner to a pair of quadrature
signals: I and Q, which are then con-
verted to the digital domain in the dual
analog-to-digital converter. Such archi-
tecture is typical for most of the con-
temporary RF devices – not only satel-
lite TV receivers.
The DVB-S2 demodulator, after lock-
ing and processing the PL header,
learns what is inside the signal: how
many different transport streams, what
modulation they use and what their
FEC settings are. At the output of the
DVB-S2 demodulator we get BB frames
along with additional error correction
Figure 2. In the process of coding, BB frames
are appended with error correction bits.