Promax Promax TV EXPLORER HD ISDB-Tb, Руководство пользователя

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USER’S MANUAL. TV EXPLORER HD ISDB-T/T
B
Page 64 11/2010
Following, it appears the Noise Margin ( NM ) measurement (in the figure value
8.4 dB). It indicates a safety available margin according to the MER level measured that
allows signal degradation until arriving to the QEF ( Quasi - Error-Free ) limit value.
Figure 43.- DVB-S2 ( QPSK/8PSK ) signals MER measurement screen.
In the case of a DVB-S2 or DSS 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 ISDB-T/T
B
, 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.