Pagina 226 - TELE-satellite-1205

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TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 04-05/2012
How a tuner for VCM operates
- an indepth look into today‘s most
advanced transmission technology
Jacek Pawlowski
The DVB-S2 standard, published in
2003, is a significant step forward from
DVB-S. Its most obvious advantage is a
better spectral efficiency as compared
to DVB-S. Within the same transmis-
sion channel you can send up to 30%
more data. Its performance is very
close to the Shannon limit – only 1 dB
below, so in fact one should not expect
any further improvement in data rate
achievable within a given transmission
channel. However, there is another as-
pect of digital data transmissions – for-
ward error correction or FEC.
Although the actual implementa-
tion is very complex, the essence of
FEC is simple. Having digital informa-
tion (e.g. video, audio, Internet data)
the encoder divides it into packets or
frames and adds extra bits calculated
in a special way from the useful digital
content. Those extra bits are transmit-
ted together with the “useful” bits. If
some of the bits, either the useful ones
or the additional ones, are distorted (1
becomes 0 or vice versa), the recep-
tion system which “knows” what the
relationship between the normal and
additional bits should be, can do so-
phisticated calculations and restore the
original form of useful bits.
DVB-S2 introduced new algorithms
for FEC: BCH and LDPC. BCH (Bose-
Reed Solomon coding used in DVB-S.
LDPC (Low Density Parity Check) re-
placed Viterbi inner coding. LDPC, a
method invented in 1960’s by Gallag-
er, became feasible only recently after
advances in IC technology had been
made. LDPC uses the following inner
code rates: 1/4, 1/3, 2/5, 1/2, 3/5, 2/3,
3/4, 4/5, 5/6, 8/9, 9/10.
These numbers are quite familiar for
satellite fans. The meaning of them is
simple – this is the ratio of the useful
bits to the sum of useful bits and ad-
ditional bits added for error correction.
As you can deduce, the most effective
inner coding rate is 9/10 but at the
same time is most vulnerable to inter-
ference. Inner coding rate 1/4 is very
robust as for one useful bit there are
three additional bits for error correc-
tion but, of course, the efficiency of the
transmission is very poor.
In the old days of DVB-S we had to
use one fixed inner coding rate for one
transponder. One transponder trans-
mitted only one transport stream.
Every TV channel or the other service
included in the transport stream was
transmitted with the same spectral effi-
ciency and interference robustness be-
cause the same inner coding was used
for the whole transponder.
DVB-S2 offers more flexibility in this
aspect. Except for CCM (Constant Cod-
ing and Modulation), we have two more
options: VCM (Variable Coding and
Modulation) and ACM (Adaptive Coding
and Modulation). VCM and ACM are the
methods to send more than one trans-
port stream within the same carrier
(same transponder). In VCM, we can
set different coding rates for different
transport streams and in ACM we can
even dynamically change the coding
rates depending on the feedback infor-
mation from the reception system.
While ACM is very useful in point to
point transmissions where there is only
one recipient of the information, VCM
can be quite useful for satellite TV
transmission for a large audience. The
content provider may choose to trans-
mit the very basic package of the SD
TV channels with a strong error cor-
rection – thus making it receivable also
at the edges of the satellite footprint,
while the premium HD channels may
be transmitted with weaker correction,
so that their reception is guaranteed
only in the central part of the footprint
where most of the subscribers live. To
achieve stronger or weaker error cor-
rection, the provider chooses both
modulation type (e.g.: QPSK or 8PSK)
and inner coding rate.
As already described in TELE-satel-
lite (see issue 12-01/2012), there are
already transponders transmitting in
VCM and ACM format. However, there
are still quite few receivers or receiver
cards available today that can receive
VCM signal. How does the tuner of such
a receiver differ from its standard CCM
brother? Before we discuss the receiver
front end suitable for VCM transmis-
sions, let us focus first on the transmit-
ter part – DVB-S2 modulator. Figure 1
explains the principle of operation of a
DVB-S2 modulator with MIS (Multi In-
put Streams) feature.
VCM Tuner
Figure 1. DVB-S2
Modulator with MIS