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User Manual
Rev. 1.0 - 29/11/05
PTRL NV/3 & RXRL NV/3
Setting nominal input level for 0dB modulation in the transmitter is an easy task. The
field in the appropriate menu shows how modulation changes as you set input level
in 0.5dB increments. Modulation is expressed both in kHz as absolute deviation,
and in dB for 75kHz. In this measurement, displayed deviation includes all other
auxiliary signals, such as pilot tone, if stereo, and RDS / SCA signals if relevant. To
measure the input signal of audio channels alone, go to the LEFT/RIGHT channel
level menu.
Auxiliary channel level is also very easy to set, as it is also shown as deviation in kHz
or as sensitivity in dB. Please note that, in this case, 0dB equals 7.5kHz deviation,
i.e. 10% of total standard modulation. As a result, the level for the RDS coder will turn
out to be -11.5dB for the corresponding 2kHz deviation. The reading shown in this
menu field reflects deviation as determined by the auxiliary signal only. To observe
additional effects on total deviation, go to the MPX menu.
Because of the special characteristics of the RDS signal and the measurement
method, auxiliary channel modulation readout is slow to respond to changes. Allow
some time for the reading to become stable: correct reading will be the highest reading
shown among those fluctuating by a few tenths of dB.
The limiter incorporated in the transmitter is of the clipping type: as soon as it kicks
in, modulation distortion will increase significantly, so it is a good rule to keep the
signal under control to prevent the limiter from kicking in. However, limiter operation
is hardly appreciable when it kicks in rarely.
Normally, the limiter is disabled at the factory. However, recommended setting for the
limiter operation threshold is +2.5 dB (100kHz peak value). Threshold setting range
is from 0dB (75kHz) to +7.1 dB (170 kHz). This setting is often established
(mandatory) by national standards and acceptance of short overmodulation peaks
varies from country to country. In some countries, it is forbidden for users to disable
the limiter or modify its level. It should be noted that limiters will kick in shortly after the
calibration level, never before it. The difference between threshold level and operation
at full capacity (hard clipping) is about 0.5 dB, i.e. deviation 3-4kHz above the
threshold.
Under Italian law, the limiter must kick in at over 75kHz. This transmitter conforms to
Italian law requirements when the limiter is set to +0.5dB (80 kHz). Under these
conditions, the limiter produces a signal with 1 dB (85 kHz) maximum deviation
compared to nominal deviation, when a modulating audio signal with +6dB overload
is applied. Users who adopt less stringent limiter settings do so under their sole
responsibility.
Anyway, the internationally accepted peak modulation for FM transmission is 75kHz
for longer peaks. For this reason, limiter operation threshold should never be set too
high, and users are advised to use an external multi-band limiter to achieve optimal
modulation, as this will provide greater tolerance in the event of audio signal peaks.
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