Promax Promax TV EXPLORER HD ISDB-Tb, Manual

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USER’S MANUAL. TV EXPLORER HD ISDB-T/T
B
Page 62 11/2010
In a digital reception system for satellite signals ( DVB-S2 ), after the QPSK
decoder other two different correction methods are applied (see following Figure). In this
case, as the previous one, each time we apply an error corrector to a digital signal, the
error rate changes, therefore if we measure in a digital satellite television system, for
example, the error rate at the output of the
QPSK/8PSK demodulator, at the output of
the Low Density Parity Check ( LDPC ) decoder, and at the output of the BCH decoder,
we obtain nothing more than different error rates. This is the reason because the BER
measurement is provided after LDPC (LBER ). It also shows the amount of packet errors
( PER ), that is, the amount of packets receiving during the measurement time, which are
not correctable by the demodulator (WP).
Figure 40.- Digital reception system via satellite. (DVB-S2).
Figure 41.- DVB-S2 ( QPSK/8PSK ) signals LBER measurement screen.
The BER measurement is provided in scientific notation (i.e. 2.7 E-7 means
2.7x10
-7
, that is to say incorrect bits of every 10,000,000) 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 after Viterbi of 2.0E-4 BER (2.0x10
-4
). This value is marked on
the measurement bar of the BER after Viterbi and therefore, BER for acceptable signals
must be at the left side of this mark.
Next it is shown the tuning frequency and the frequency deviation in MHz between
the tuned frequency and the one which optimizes the BER.