AP301-FULL-HD_rev.09_UK
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7.
DEMODULATOR CARDS FOR DVB-S/S2, DVB-T/T2, DVB-C
7.1.
INTRODUCTION
The modulation systems adopted for digital signal transmissions are optimized for the
various types of transmission:
•
QPSK/8PSK: used for satellite transmissions due to their significant resistance to noise;
•
QAM: used for cable transmissions due to their particular efficiency in taking advantage
of the available band;
•
OFDM: for over the air signals, capable of ensuring reception even in the presence of
multiple pathways and signal echos. Used for reception from vehicles in motion.
A digital transmission appears as noise that occupies an entire channel, normally around 30
MHz in QPSK/8PSK and 7/8 MHz in QAM and OFDM.
⇒
In order to decode the signal, a number of transmission parameters must be known.
Many of these are standardized, but some may vary from one channel to the next and
must be set upon the instrument.
7.2.
MEASUREMENTS IN QPSK / 8PSK
When the QPSK/8PSK card is in function, the variable transmission parameters and the
measurements, as described below, are shown on the screen:
SYMBOL RATE:
indicates the set value.
Note:
This value must be set by the user.
MODE:
indicates the type of modulation (DVB-S or DVB-S2). This setting must be
performed by the user.
CODE:
indicates the modulation Code Rate and is followed by the indication of the value.
The recognition and the setting are both automatic.
SPECT:
indicates the modulation polarity and is followed by the indication of the setting.
Recognition is automatic.
LOCKED:
green when the signal is locked; or else
UNLOCKED
in red when the SYMBOL RATE and/or MODE values do not correspond to the
correct values and the signal is not locked, or else
NO CAR
in the absence of an input
signal.
CFO:
(Center Frequency Offset) indicates the difference between the set frequency (tuning)
and the true frequency of the carrier. This is an interesting parameter that provides
information regarding any frequency drifts in relation to the LNB's local oscillator.
PWR
index:
Followed by the message
OK
,
HIGH
or
LOW
indicating whether the input
signal
is respectively correct, too high or too low.
The PWR index indicates whether the signal at the tuner’s input is of a suitable level to be
decoded and measured digitally (this depends on both the attenuation configured within the
field strength meter as well as on the level of the signal available at the input).
MER:
(Modulation Error Ratio), a qualitative value for digital channel modulation expressed
in dB. The higher the value, the better the quality of the signal. This measurement is only
valid when the carried is locked.
NOTE:
For this measurement, all modulation theory causes for alteration, such as distortion
and interference, are taken into consideration.
