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In the figures 5.c-b and 5.d-b, as suggested by CEPT 54-01, the "Accumulated distribution plot of
deviation" have been reported on the graph, relating to the graphs of the left figures -a and -b ; in
this case all the samples from left to right have been added and the samples total value has been
normalized.
In other words, starting from left fig. 5.d-a (0KHz) and going towards right (150KHz) it' s
noticed that all the samples are towards right (100 %) till about 35KHz, to 50KHz over than the
80% of samples is on the right, at 70kHhz just the 5% of samples is on the right, as it has been
evidenced on the graphs –b ordinates.
The CEPT 54-01 rule and the equivalent REC. ITU-R SM.1268 and REC. ITU-R BS.412-7, at this
point stop and they do not give exact and rigorous information about the interpretation of the graphs
mentioned above.
On the opposite by connecting the different rules it' s possible to analyse the graphs of fig. 5.c-a and
5.c-b to draw some statistic parameters which, deriving from a reference system, can, as said
previously, be applied to a typical music broadcast.
So some quantities will be defined peculiar to the two graphs which will define just one over
modulation factor, whose value will be used as limit parameter.
Definitions:
M
:
OM
:
OM% :
K
:
The formula can be explained easily and intuitively, since the over modulation factor is directly
proportional to the peak number percentage detected over 75KHz (OM%), while the ones lower
than this threshold must not give any contribution to K, and it' s also directly proportional to the
peaks KHz value which have exceed 75KHz (OM–75).
If no maximum peaks measured through the 50msec samples has exceeded 75KHz, we are in a
favourable condition, OM = 0 and OM% = 0 and so K = 0
If all the peaks exceed 75KHz and their average is 78 then K = (78 –75) * 100 / 100 = 3
Now getting the example again of the transmitter modulated with coloured noise as to the CCIR-
559 and IEC-244 rules previously seen, which has originated the graphs of fig. 5c-a , 5c-b, 5d-a and
5d-b and we apply the above mentioned parameters and calculate them each minute. Thus it will be
obtained other graphs which can be added to the two previous couples, so originating a screen shot
full of all the parameters relating to the peak modulation measurement:
average of all the measured samples as peak maximum every 50msec
average of the samples which have exceeded the 75KHz threshold only
samples percentage which has exceeded 75KHz as to the total
over modulation factor, defined as follows:
K = (OM - 75)*OM% /100
Pag. 52
A07B500S manual
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