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TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 09-10/2012
— www.TELE-satellite.com
Now you might wonder what it is that
so many receivers encounter troubles
with low symbol rates, while a few brave
boxes seem to be unaffected at all. The
root problem lies in the quality of the
tuner as well as in software fine-tuning.
If a transponder with extremely low
symbol rates comes in, the bandwidth
has to be reduced exactly in line with
the symbol rate in order to arrive at a
low noise level and achieve an optimum
carrier-to-noise (C/N) ratio.
In turn, the frequency has to be main-
tained even more precisely for a stable
and reliable signal. Many manufactur-
ers are trying to cut costs both in the
hardware and the software fine-tun-
ing fields, which consequently creates
problems with ultra low symbol rates.
Yet, manufacturers are hardly to blame
as their specifications in most cases
explicitly state that only symbol rates of
2 MS/s and above are supported.
Low FEC
If a high-capacity transponder is
required for a feed and the symbol rate
cannot be reduced TV stations have the
additional option of reducing the For-
ward Error Correction (FEC). FEC is a
mathematic correction process to set
off signal errors that occur during satel-
lite transmission. If it weren’t for this
process, most of us would never be able
to enjoy reliable satellite reception with
reasonably small antennas.
The FEC N/M value specifies how
many M (gross) bits need to be trans-
mitted for each N (net) bit. An FEC of
1/2 therefore means that two gross bits
are required for each net bit, with 3/4
it is four gross bits for 3 net bits and so
on. The higher the FEC value, the lower
the possibilities of mathematically cor-
recting a faulty signal. If, for example,
an FEC of 7/8 is in place you will need
a very large antenna for reliable recep-
tion.
So basically, the FEC defines the size
of the required antenna and this way the
FEC of a transponder can be modified
to allow for either easy reception with
an FEC of 1/2 or very difficult recep-
tion with an FEC of 7/8. As a matter of
fact, the new DVB-S2 standard brought
about even more FEC modes such as
2/3, 3/5, 4/5, 8/9 or 9/10.
Juan Carlos Duarte from Santiago, Chile
Vincent Witjhun from Pontianak, Indonesia
Ingo Salomon from Johannesburg, South Africa