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TUBE COMPOSER T1952
Fig. 4.3: The interactive relationship between the operating level and the headroom
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 maximize the signal-to-noise ratio without incurring signal
distortion. This device is called a compressor or limiter.
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. This application is particularly useful with microphones, to compensate for level changes.
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-adjustable 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 speed, or rather time settings used can differ greatly depending on use. Although both limiter and
compressor use very short attack times, the release time of a compressor is 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 63.2% (= 8.7dB).
Because fast level changes are more noticeable than slow changes, long release times are used where
unobtrusive signal processing is required. In some cases however, the principal goal is to protect devices as
loudspeakers and power amplifiers. In those cases a short release time is more appropriate to ensure that the
limiter only intervenes when it is needed and the level returns to normal as soon as possible.
Long release times are better suited when the limiter should remain inaudible for instance with broadcasting
or club applications or when a signal is transferred to (analog) tape. Please note that when using slow release
times you should switch to the level meter menu where the functioning of the limiter can be monitored.
4.1.4 Expanders/noise gates
Audio, in general, is only as good as the source from which it is derived. The dynamic range of signals will often
be restricted by noise. Synthesizers, effects devices, guitar pickups, amplifiers etc. generally produce a high
4. TECHNICAL BACKGROUND