USER’S MANUAL. TV EXPLORER II/II+
Page 54
12/2013
The
BER
measurement before error correction is shown:
BER before FEC
(Forward Error Correction).
In a digital reception system for cable signals, after the
QAM
demodulator an error
correction method called
Reed-Solomon
is applied (see following Figure). Obviously,
the error rate after the corrector is lower to the error rate at the
QAM
decoder output.
This is the reason because this screen provides the
BER
measurement before FEC
(Forward Error Correction).
Figure 30.-
Digital reception system via cable.
The
BER
measurement is provided in scientific notation (i.e. 1.0 E-5 means
1.0x10
-5
that is to say one wrong bit of every 100,000) and through an analogue bar (as
its length is smaller the signal quality will be better). The analogue representation is done
on a logarithmic scale (not linear).
With the aim to have a reference about the signal quality, it is considered that a
system has a good quality when it decodes less than one non-correctable error for every
transmission hour. This border is known as
QEF
(
Quasi-Error-Free
) and it corresponds
approximately to a BER before FEC of
2.0E-4 BER
(2.0x10
-4
, that is to say two incorrect
bits of every 10,000). This value is marked on the measurement bar of the
BER
and
therefore,
BER
for acceptable signals must be at the
left
side of this mark.
Below the
BER
analogue bar it is shown the tuned frequency (or channel) and
the
frequency deviation in kHz between the tuned frequency and the one, which optimizes
the BER
(i.e.
800.00 MHz + 1.2 kHz
). This deviation must be adjusted specially from the
C/N
measurement in satellite band, by tuning again the channel in frequency mode
[24], to the lower reachable value.
5.14.7.2
DVB-T/H signals
Once determined the parameters of
COFDM
signal, it will be possible to measure
BER
.
Two types of measurements appear:
Following is shown the
BER measurement
before the error corrections
:
BER
before the FEC
:
CBER
.
