AP01 HD rev.09 UK
40
7.4.
QAM MEASUREMENTS
7.4.1.
CH BER in QAM
A QAM signal rarely has no errors; if its quality is excellent, it could have an error rate lower
than 1 out of 10 millions (1E-7), whereas if it has 1 out of 10000 (1E-4) its quality (
signal
quality, not picture quality
) will be bad. In this case, the converter is maybe going to
stop working, despite a perfect picture; a little worsening could stop reception completely,
without any premonitory sign. The BER is indicated in exponential format: for example, 1
out of 1000 becomes 1 E-3, 2 out of 10000 becomes 2 E-4, 7.5 out of 100000 becomes 7.5
E-5 and so on.
The instrument directly provides the measure of "Channel BER" (CH BER) or PRE REED
SOLOMON BER, which is the most significant parameter since:
•
it can be measured with all signals of the installation (from excellent to mediocre ones).
•
it indicates the margin of reception thresholds (if the installation can still receive a little,
of course).
7.4.2.
MER in QAM
The
MER
(Modulation Error Ratio) gives an idea of the demodulated signal in a range
between 17 and 34 dB.
MER can be compared to the Signal-Noise ratio (C/N) of an analogue signal, measured in
bandbase.
Do not confuse it with C/N measure, which is executed on carrier 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 may
get worse before the reception is compromised, according to the set reception parameters
and measured MER.
The higher the Noise Margin is, the lower the chance to have troubles is.
7.4.4.
RU
The BER is practically the number of errors detected and corrected through an algorithm
called Reed Solomon and it is a nearly instantaneous index of the signal quality.
The RU parameter gives an idea of the signal goodness during long periods. Some losses of
signal “blocks”, in particular, may occur over the time; they appears on the TV, for instance,
as mosaics of em-spaces that overlap the picture.
RU (Reed Solomon Uncorrected – detected errors not corrected) is a register which
increases just on these cases. The worse the BER will be, the faster the RU will increase, of
course.
This register value rises 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 corrections that the "Reed Solomon" stage carries out, but these
are already few when the "Channel BER" is about 1.10
-4
(an increase every 30 minutes).
