Promax TV Explorer HD Manual De Usuario página 177

USER'S MANUAL. TV EXPLORER HD / HD
Figure 40.- DVB-S2 (QPSK/8PSK) signals MER measurement screen.
In the case of a DVB-S2 signal (QPSK/8PSK) instead of the Noise Margin (NM)
appears the measure of the Link Margin (LM); in the previous figure with a value of
2.3 dB. The LM is equivalent to the NM and indicates the distance to the QEF (generally
defined as one lost packet per hour). The LM is measured in dB and its value is equal to
the safety margin that separates us from the QEF. As bigger LM better signal quality. An
LM with a negative value means that there is no signal reception or errors are beginning
to display clearly in the video or the audio. An LM equal to 0 (zero) displays a service
and occasionally some artefacts can be observed.
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.
MER represents the relation between the average power of DVB signal and the
average power of noise present in the constellation of the signals.
When measuring MER, it also shows the noise margin in DVB-T, C, S and the
Link margin in DVB-S2, which indicates the distance from the QEF point at the current
signal.
By example, QAM 64 demodulators require a MER greater than 23 dB to work.
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 a 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.
Finally it is shown a status line, which displays information about the detected
signal.
05/2011
LE
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