T.E.M. A07B500S, Manual

Page: 55 / 131

A07B500S manual
Pag. 55
In the case of a sinusoidal quantity, which may be voltage or deviation, the power is calculated for a
time equal or multiple the semi period of the wave form, while in the case of a music signal the
calculation is to be made by the integral which defines the power. Besides the modulation power
value in absolute form would be of a difficult understanding, for this reason any sinusoidal signal is
taken as reference whose power, for long observation periods, doesn’t depend on the sinusoid
frequency but only on its peak value square.
So the rule provides to measure the modulation power, which is as previously seen equal to
modulating signal electric power, for one minute time period and to compare it to a sinusoidal
modulating signal one which deviates ±19KHz. The result, expressed in dB, must be lower or equal
to zero to comply the rule.
The modulation power integral calculation is made inside the transmitter by integrating, between 0
and 1 minute, the modulating signal square. The integration is made in a discreet manner by
calculating the function area in the integration time; the signal sampling is made at a double speed
respecting its bandwidth, so microprocessor is practically locked for a minute to follow instant by
instant MPX signal value. After this period it performs the set values square, add them up, which is
equal to the integral, then it calculates the logarithm respecting the reference sinusoidal value.
The value is displayed in a numerical form or on a graph (on the PC) which has in the abscissas the
time (discreet with 1min steps) and in the ordinates the value in dB of the music signal power and
the reference sinusoidal one ratio.
Even in this case it’s possible to refer to a modulating signal made of the usual coloured noise in
accordance to CCIR-559 particularly Rec.ITU-R BS.412-7 mentions at pag.5-note 4:
The power of a sinusoidal tone causing a peak deviation of 19KHz is equal to the
coloured noise modulation signal according to Recommendation ITU-R BS.641 , i.e. a coloured
noise signal causing a quasi-peak deviation of 32KHz
So, for the modulation power measurement instrument alignment, it’s possible to refer either to a
500Hz sinusoidal signal (the frequency is not important) which makes the carrier to deviate of
19KHz or to the coloured noise which deviates 32KHz. Both signals give the listening the same
sensation of “ volume intensity “, told in non technical words and not considering physiological
effects of the ear sensibility at the different frequencies.
7.4 Considerations on the real measurements performed
Performing modulation measurements with the methods described so far on broadcast networks
which have been modulating for years without over modulation problems, one realizes how the
limits imposed by the rules mentioned so far are particularly restrictive and maybe not in
compliance with current reality.
There are some contradictions and gaps the rules self sometimes point out. We report two examples
which give the idea of the real difficulty about the strict application of them:
REC. 641 , Rep.1064 5. FREQUENCY DEVIATION OF THE SIGNAL GEN.