Promax Promax TV EXPLORER HD ISDB-Tb, Руководство пользователя

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USER’S MANUAL. TV EXPLORER HD ISDB-T/T
B
Page 58 11/2010
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 33.- Digital reception system via cable.
The BER measurement is provided in scientific notation (i.e. 1.0 E-8 means
1.0 x 10
-8
that is to say one wrong bit of every 100000000) 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.
5.14.7.3 ISDB-T/T
B
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 .