37
E
Fig. 4.2: The interactive relationship between the operating level and the headroom
It is therefore useful to keep the operating level as high as possible without risking signal distortion in order to
achieve optimum transmission quality.
It is possible to further improve the transmission quality by constantly monitoring the program 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; signal peaks are hard to predict and impossible to compensate manually in the available time.
Manual control is simply not fast enough to be satisfactory.
The need therefore arises for a fast acting automatic gain control system that can constantly monitor the
signals and which will always adjust the gain to maximize the signal-to-noise ratio without incurring signal
distortion. This device is called a Compressor or Limiter, and is a component of the BEHRINGER
ULTRA-DYNE PRO.
4.1.3 Compressors/limiters
With broadcasting and recording, signal peaks can easily lead to distortion due to the high dynamic range of
microphones and other musical equipment. Compressors and limiters reduce the dynamics by means of an
automatic gain control. This reduces the amplitude of loud passages and therefore, restricts the dynamics to
a desired range. The dynamic range of a microphone channel can thus be compressed from 90 dB to 50 dB
or less, ensuring trouble-free processing in broadcasting, stage or recording environments.
Although compressors and limiters perform similar tasks, one essential point makes them different:
limiters abruptly limit the signal above a certain level, while compressors control the signal "gently" over a
wider range. Both continuously monitor the signal and intervene as soon as the level exceeds a user-adjust-
able threshold. Any signal exceeding this threshold will be immediately reduced in level.
Limiters reduce the output level to the adjusted threshold whenever the input signal exceeds this point. With
compression, in contrast to the action of a limiter, the signal is reduced in gain relative to the amount the
signal exceeds the threshold. The output of a compressor will still rise if the input level is increased, while the
maximum output of a limiter will always be equal to the threshold level.
Generally, threshold levels for compressors are set below the normal operating level to allow for the upper
dynamics to be musically compressed. For limiters, the threshold point is set above the normal operating level
so that it only intervenes to protect subsequent equipment from signal overload. The ULTRA-DYNE PRO is
equipped with such an output limiter.
There are generally also significant differences in the control times. Although both limiters and compressors
use very short attack times, the release time of a compressor are in the 100 ms region whereas a limiter uses
release times of seconds. To be exact: The release time is a time constant of an exponential function. It is the
time it takes the gain reduction to decrease by 36.8% (= 8.7 dB).
Because fast level changes are more noticeable than slow changes, long release time are used where unobtru-
4. TECHNICAL BACKGROUND
