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162

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.

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.