AP301-FULL-HD_rev.09_UK
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7.4.
MEASUREMENTS IN QAM
7.4.1.
CH BER in QAM
It is quite rare for a QAM signal to be entirely free of errors; a signal of optimal quality will
have an error rate of less than 1 per 10 million (1E-7), while a poor signal will have an error
rate of more than 1 per 10,000 (1E-4)
(This refers to the signal and not an eventual
image)
. In this case, the converter may be about to stop functioning, although a perfect
image may be present; even slight additional deterioration could entirely block the
reception, with no warning signs. The BER is indicated by the instrument in exponential
form. For example, 1 in 1000 becomes 1 E-3, 2 in 10000 becomes 2 E-4, 7.5 in 100000
becomes 7.5 E-5, and so on.
The instrument directly provides the “Channel BER” (CH BER) or PRE REED SOLOMON BER
measurement. This is the most important parameter because:
•
It can be measured with all of the signals present within the system (from optimal to
mediocre).
•
It provides an indication of the reception threshold margin (obviously if the system is
such to allow for even minimal reception.)
7.4.2.
MER in QAM
The
MER
(Modulation Error Ratio) index gives the user an idea of the quality of the
demodulated signal within the range of 17 to 34 dB.
In fact, the MER is similar to the Carrier-Noise (C/N) ratio of an analogue signal, measured
in base band.
It must therefore not be confused with the C/N measurement, which is carried out in Radio
Frequency.
7.4.3.
NM (Noise Margin)
The Noise Margin is a calculated estimate (in dB) of how much the carrier/noise ratio can
deteriorate before reception will be compromised, based on the set reception parameters
and the measured MER.
The higher the noise margin, the lower the possibility of encountering any problems.
7.4.4.
RU
The BER is effectively the number of errors identified and corrected by an algorithm, known
as the Reed Solomon code, and is a nearly instantaneous indicator of signal quality.
The RU parameter gives the user an idea of the quality of the signal over medium to long
periods of time. In particular, over time the loss of signal “blocks” can be observed and
seen on the television, for example, as a mosaics of squares that are superimposed over the
image.
RU (Reed Solomon Uncorrected) is a register that increases when events of this type take
place.
Obviously, the worse the BER, the more quickly the RU will increase.
The RU register value increases from 0 to 65535 and stops at the maximum value. It is
reset whenever there is a loss of signal lock.
Its increase depends on the corrections that the “Reed Solomon” stage makes, but these
are already very few when the “Channel BER” is in the order of 1.10
-4
(one increase every
30 minutes).
