PAG E 6
A M I S C L 2 I N S TA L L AT I O N & O P E R AT I O N M A N UA L
S E T U P A N D O P E R AT I O N
Fig 1 The interactive relationship between the operating level and the
headroom
It is possible to further improve the audio quality by constantly
monitoring the programme material with the aid of a volume fader,
which manually levels the material. During low passages the gain is
increased, during loud passages the gain is reduced. Of course it is
fairly obvious that this kind of manual control is rather restrictive; it is
difficult to detect signal peaks and it is almost impossible to level them
out. Manual control is simply not fast enough to be satisfactory.
The need therefore arises for a fast acting automatic gain control
system which will constantly monitor the signals and which will always
adjust the gain to maximise the signal-to-noise ratio without incurring
signal distortion. This device is called a compressor or limiter.
C O M P R E S S O R / L I M I T E R S
By measuring the dynamic range of musical instruments and micro-
phones, you will find that extreme amplitudes will occur which will
often lead to overload in subsequent signal processing equipment.
These signal peaks can lead to heavy distortion. To avoid this kind of
distortion or, for example, to avoid loudspeakers being damaged by
overload,
Compressors
or
Limiters
are used.
The principal function used in these devices is dependent on an
automatic gain control as mentioned in the previous section, which
reduces the amplitude of loud passages and therefore restricts the
original dynamics to a desired range. This application is particularly
useful in microphone recording, to compensate for level changes
which are caused by varying microphone distances.
Although compressors and limiters perform similar tasks, one essential
point makes them different:
A
Limiter
abruptly limits the signal above a certain level, while a
compressor controls the signal “gently” over a wider range.
A limiter continuously monitors the signal and intervenes as soon as
an adjustable level is exceeded. This level is called the threshold.
Any signal exceeding this threshold will be immediately reduced
back to the adjusted threshold level.
A
compressor
also monitors the program material continuously and
has a certain threshold level. However, in contrast to the limiter, signals
exceeding the threshold are not reduced abruptly but gradually. Above
the threshold, the signal is reduced in level, relative to the amount the
signal exceeds this point. This gain reduction is set by the ratio control.
Generally, threshold levels for compressors are set below the normal
operating level to allow for the upper dynamics to be musically com-
pressed. For limiters, the threshold point is set above the normal oper-
ating level ini order to provide reliable signal limiting and thus protect
subsequent equipment.
E X PA N D E R S / N O I S E G AT E S
Audio, in general, is only as good as the source from which it was
derived. The dynamic range of signals will often be restricted by noise.
Synthesisers, effects devices, guitar pickups, amplifiers etc generally
produce a high level of noise, hum or other ambient background hiss,
which can disturb the quality of the program material.
Normally these noises are inaudible if the level of the desired signal
lies significantly above the level of the noise. This perception by the
ear is based on the “masking” effect: noise will be masked and thus
becomes inaudible as soon as considerably louder sound signals in the
same frequency band are added. Nevertheless, the more the level that
the desired signal decreases, the more the noise floor becomes a
disturbing factor.
Expanders or noisegates offer a solution for this problem: these
devices attenuate signals when their amplitudes drop, thereby fading
out the background noise. Reliant on this method, gain controlling
amplifiers, like expanders, can extend the dynamic range of a signal
and are therefore the opposite of a compressor.
In practice, it is shown that an expansion over the entire dynamic range
is not desired. With an expansion ratio of 5:1 and a processed dynamic
range of 30dB, an output dynamic range of 150dB will be the result,
exceeding all subsequent signal processors, as well as human hearing.
Therefore, the amplitude control is restricted to signals whose levels are
below a certain threshold. Signals above this threshold pass through the
unit unchanged. Due to continuous attenuation of the signals below this
threshold, this kind of expansion is termed “downward” expansion.
The noisegate is the simplest form of an expander: in contrast to the
expander, which continuously attenuates a signal below the threshold,
the noise-gate cuts off the signal abruptly. In most applications this
method is not very useful, since the on/off transition is too drastic.
The onset of a simple gate function appears very obvious and
unnatural. To achieve an inaudible processing of the program material,
it is necessary to be able to control the signal’s envelope parameters.
