Promax PROLINK-3 Premium Manual De Instrucciones página 157

USER'S MANUAL. PROLINK-3/3C
Figure 11.- BER measuring screen for QAM-modulated signals.
First of all, you will see the modulation error ratio measurement: MER.
Analogue and digital carriers are very different in terms of signal contents and
power distribution over the channel. They, therefore, need to be measured differently.
The modulation error ratio (MER), used in digital systems is similar to the Signal/Noise
(S/N) ratio in analogue systems. The MER represents the proportion of power lost
through wrong data with respect to the mean power of an ideal QAM signal.
To operate, QAM 64 demodulators require an MER greater than 23 dB. Though it
is preferable to have at least a 3 or 4 dB margin to compensate for any possible
degradation of the system. While QAM 256 demodulators require an MER greater than
28 dB with margins of al least 3 dB. Normally, the maximum MER value seen in
portable analysers is of approximately 34 dB.
Secondly, it is shown the 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 figure 12). 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 12.- Digital reception system via cable.
The BER measurement is provided in scientific notation (i.e. 1.0 E-5 means
-5
1.0x10 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).
06/2005
Premium
MER
BER BEFORE FEC
FREQ: 800.00 MHz + 1.2 kHz
W.P. 0 IN 00:22:59
MPEG-2
32 dB
Page 39