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SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
Electronic Broadcast Equipment
The transmitter’s internal limiter is peak-clipping type; this means that as soon at it cuts in, modulation
distortion increases sharply. For this reason, the modulation signal should be kept under control to prevent
intervention of the limiter. Anyway, do not over-estimate this problem: occasional action of the limiter is
mostly unperceivable.
The cut-in limiter threshold, when enabled, is factory pre-set to +2.5 dB (100 kHz peak value). It may
be set from 0 dB (75 kHz) up to +7.1 dB (170 kHz). This threshold value is mostly specified in the various
national standards, and tolerance to short over-modulating peaks varies from country to country. Some
countries do not permit the user to disable the limiter or change the level. Some countries do not allow the
user to disable the limiter or change its threshold. Note that the limiter action begins slightly after the pre-
set level, with no action at all till that. The difference between the threshold level and hard clipping is some
0.5dB.
In any case, the modulation peak value that is internationally admitted for FM is 75 kHz for peaks that
are not extremely short. For this reason, the limiter’s cut-in threshold should never be too high. It is highly
recommended to use an external multi-band limiter to optimise modulation, with higher tolerance for any
audio-signal peaks. Such devices momentarily reduce the amplifier circuits’ gain if the threshold is exceeded
and prevent severe, significant distortion.
Any external compressor, limiter or audio meter must be frequency compensated with the same time
constant of the pre-emphasis to modulate or monitor deviation properly.
Therefore, as much as is possible, the audio level shall be constantly and correctly monitored and
adjusted to prevent the internal limiter from cutting in. Done it, on the other hand the audio level should be
as high as it is possible, to achieve the best signal/noise ratio, and presence effect, on reception.
The tendency to over-process audio signals is common in a number of local broadcasting stations:
if an amount of processing is advisable, and we recommend using a top grade one, do not to compress the
signal too much, as this impairs the original dynamics.
The audio response of the SIRIO2000T transmitter is extremely flat, without any perceivable loss on
low and high audio frequency: for this reason large frequency alterations of the audio signal supplied by using
a so-called “frequency equaliser” are not advisable. An increase of the low and high frequency contents of
the audio signal by more than a few dB might cause general degradation of modulation dynamics and
improper functioning of the limiter.
