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Idiomas disponibles
Idiomas disponibles
Two measures are shown :
1)
CSI: Channel status information
2)
BER after Viterbi
The CSI measure (Channel Status Information) is a qualitative measure about
channel state, between 0 and 100%. The optimum value corresponds to 0%. This
measure permits to look for the best situation even in those measuring conditions where
the measured BER is best than the minimum readout (in this way, in the example of the
previous figure, the measured BER is lower that the minimum readout, 1.0x10
CSI measurement, 27%, still can be improved).
Next it is shown the BER after Viterbi measurement both in numeric and
graphic bar format.
In a reception system of terrestrial digital signal, after the COFDM decoder two
error correction methods are applied. Obviously, each time we apply an error corrector
to the digital signal, the error rate changes, therefore if we measure the error rate at the
output of the COFDM demodulator, at the output of the Viterbi decoder, and at the
output of the Reed-Solomon decoder, we obtain nothing more than different error rates.
The PROLINK-4 provides the BER after Viterbi and the number of Wrong packets
received after Reed- Solomon.
The BER measurement is provided in scientific notation (i.e. 3.1 E-7 means
-7
3.1x10
, that is to say 3.1 wrong bits of each 10000000) and through a graphic bar (as
its length is smaller the signal quality will be better). The analogue representation is
done on a logarithmic scale (not linear), that is to say, the bar divisions correspond to
the exponent of the measurement.
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
of each 10000). 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.
In the lower line of the screen it appears the W. P. counter (Wrong Packets
counter). This counter shows the number of non-correctable packets received after
Reed-Solomon during the measuring time. This counter is automatically activated when
the unit detects an MPEG-2 signal.
Page 44
Figure 15.- COFDM reception system.
USER'S MANUAL. PROLINK-4
-7
, but the
-4
, that is to say 2 wrong bits
02/2003
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