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MIRAGE MK III - Broadcast Audio Processor
Introduction - General Description
The MIRAGE AM Audio Processor is a complete audio signal processing system for AM
transmission.
The image and professionalism of the broadcaster is defined by its sound quality; hence the
obvious importance of the AUDIO PROCESSOR.
Multi-band processing is the best method of audio signal processing; MIRAGE AM is based on
the processing of the audio signal split into THREE BANDS.
MIRAGE AM allows the integrated control of processing parameters in such a way as to provide
your broadcaster with a competitive sound with more punch, continuity, presence and clarity,
without pumping or other undesirable effects.
Total control of peak modulation
MIRAGE offers total control of audio signal levels for FM transmission and can be interfaced to
any commonly used transmitter, anywhere in the world.
The MIRAGE audio processor gives dynamic control over the level of pre-emphasis;
intervention is inaudible and produces a clear, open and brilliant sound, better than the original
program.
MIRAGE exerts a extremely precise control over peaks, limiting overmodulation levels to just
1.5 dB!
The stereo encoder of the MIRAGE uses a digital control circuit to contribute to the excellent
performance of the system, featuring high stability and total control of the signal. Both
amplitude and phase are adjustable to ensure the best adaptation to any transmission system
and the highest level of stereo separation.
MIRAGE helps prevent distortion arising in successive encoders and other transmission
components, through the use of bandwidth limiting filters (10 KHz low-pass filters), following
the overmodulation compensation circuits.
Concepts of audio processing
It is possible to define average and peak values for any sound source.
The average value contains a large part of the signal power and is therefore the most
important part of the signal, as opposed to the peak value whose contribution is very low in
terms of signal power.
Unfortunately, however, the maximum deviation of 75 KHz for FM transmission is determined
by the peak value; this constrains the average value to a low level which produces a dull
sound.
The sound level can be raised by reducing the peak/average ratio; if the peaks can be
reduced, the average level can be increased within the modulation constraints.
This can be effected by limiting and expanding the signal.
Clipping is performed on that part of the signal which cannot be compressed; this operation is
always necessary to guarantee that the maximum deviation never exceeds 75 KHz.
Clipping does not produce audible effects providing it is done with moderation. If excessive
clipping is applied, it can give rise to unpleasant distortion effects.
Initial power-up
Before switching on the MIRAGE, check that the line voltage corresponds to the voltage
selected on the rear of the power supply unit. Check also that the supply to which the MIRAGE
FM is connected has a good ground connection.
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