Rohde & Schwarz SR8000 Series, System Manual

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Chapter 5 Operation
5300.9677.71 - 5.57 - EN-4
2.7.4.1 Deviation and MPX Limiters
The limiter is able to monitor both peak deviation and average modulation power (to ITU-R
SM 1268-1). The modulation power is defined in ITU-R SM 1268-1 as follows:
The modulation power is a floating average value for the last 60 s. It therefore takes 60 s
from powering up before the correct modulation power is measured.
If one or both limiters are activated, the limiter monitors the signal and readjusts the audio
signal as necessary. Other components in the MPX signal, such as the pilot and RDS, are
not affected.
Deviation limiter
In order to keep to total harmonic distortion low and enable the simultaneous use of external
sound processors, the limiter operates in two stages:
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Stage 1: Audio level controller
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Stage 2: MPX peak clipper
The audio level controller calculates the theoretical deviation when the MPX peak clipper is
inactive. If this value exceeds the deviation limiter threshold for longer than 5 seconds, the
audio signal is gradually attenuated until the average value is more than 5 second below
the threshold.
Short peaks are clipped by the MPX peak clipper. Compared to a pure clipper, this mecha-
nism significantly reduces the total harmonic distortion if the input signal is permanently
over large.
It is also possible to use external sound processors, since the audio signal is not affected
in the event of transient out-of-limit conditions, and the clipper ensures compliance with lo-
cal regulations. The external processor then ensures that the MPX signal is always below
the threshold.
Note In MPX mode (input 2 or input 3) only the MPX peak clipper operates. In all other operating
modes the audio level controller can be switched off by means of the "Clipper Only" position
of the
Dev. Limiter Active switch, so that only the MPX peak clipper is then working.
Modulation power limiter (MPX power limiter)
According to the ITU recommendation, the average modulation power is measured contin-
uously for a period of 60 seconds. During that time it must not exceed a certain value, usu-
ally 0 dBr. This means that the modulation power would always exceed this value over a
shorter period than 60 seconds.
The MPX power limiter is therefore in fact a controller rather than a limiter. It estimates the
volume trend in the audio signal and readjusts the gain on the audio inputs accordingly. The
algorithm has also been designed to distinguish between sudden peaks or notches and
longer changes in volume (e.g. a change from music to speech). This prevents the "pump
effect" that would otherwise result in the case of rhythmical changes in volume, such as