User manual

page 5 of 15
21016 SMU1-MPAB3 V1.3.doc
Product Description
The SMU1 is desk mounted microphone console for primarily designed for
use with Audix Systems Public Address and Voice Alarm controllers.
Led indicators provide comprehensive status display.
Red LED’s showing when zones are busy, green LED’s show when calls are
successful.
The SMU1 features a good quality dynamic microphone, chosen for its clear
and natural sound.
The MPAB3 is a low noise remote microphone amplifier board with balanced
input and built in AGC/compressor maintain maximum intelligibility. The
nominal compression ratio is 5 to 1, but increases towards the higher level
end of the range.
The underlying AGC operates slowly with an attack time of around 3s ( gain
control is mostly transferred from the compressor) and release time of around
10s, but has a fast recovery (approx. 1s rate ) which operates if the output
level falls below 10 to 15dB of its current nominal value for longer than 1.5s.
The AGC recovery rate of around 10s is re-established when the signal level
recovers to within the 15dB range of the nominal output level.
The compressor effectively operates "over the top" of the AGC to reduce the
dynamic range of the gain adjusted signal. The reason for this approach
(rather than just having a simple compressor is to reduce the amount of
crackle and distortion of the signal to improve overall intelligibility of the
speech signal.
The compressor operates normally with an attack time of around 10ms and a
release time of 200ms which gives good dynamic range control with minimal
attack distortion. To control the initial attack and any subsequent large
excursions of the signal, the attack time speeds to around 1ms for any
instantaneous input increase of greater than about 4dB above the current
output level.
To help prevent the compressor modulating the noise during short pauses in
the input speech signal (i.e. unwanted "breathing") the compressor release
time is increased greatly if the output level falls to below 10 to 15dB of the
current output level. Normal control is re-established when the output level
rises above this level.
The board also incorporates a 20kHz surveillance oscillator which injects tone
directly into the microphone input for use in the rest of the PA system, but
which will also detect (absence of tone) a short or open circuit fault at the
microphone. The 20kHz generator is arranged to inverse-track the gain