Promax PROLINK-4C, Manual

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USER’S MANUAL. PROLINK-4C
02/2003 Page 41
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In a digital reception system for cable signals, after the QAM demodulator an
error correction method called Reed-Solomon is applied (see figure 13). 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) and the number of non-correctable errors (W.P., Wrong Packets)
received after Reed-Solomon in the measuring time.
Figure 13.- 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).
In the next line it is shown the number of non-correctable packets received
'wrong packets' (up to a maximum of 126) in the time detailed on its right. A packet is
considered wrong when one, at least, non-correctable bit is detected. To reset this
measurement just modify the measurement conditions: for example change the tuned
frequency.
Finally it is shown a status line which displays information about the detected
signal. The possible messages that can appear and its meaning are shown in the
following list. The messages are exposed from less to more fulfilment of the MPEG-2
standard:
No signal received
Any signal has been detected.
Signal received
A signal is detected but it can not be decoded.
Carrier recovered
A digital carrier has been detected but it can not be decoded.