BSS Audio OPAL Series DPR-422 Owner's Manual - Manual (Page 23)

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8.2 Compressors
and Limiters
Compressors and limiters have closely related effects, and in general a limiter
will reduce gain very strongly once a certain level has been reached, whereas
a compressor will act gently, but over a much wider range of volume levels. A
limiter will continuously monitor program levels, but only commence to
reduce gain once the level has exceeded a preset amount. This point is called
the threshold level. Any program level in excess of the threshold will
immediately be reduced to this threshold level. A compressor will also
continuously monitor the program and has a threshold level. However,
program signals in excess of this threshold will be progressively reduced by an
amount (ratio) depending on the degree to which it initially exceeds the
threshold. Generally, threshold levels for compressors are set below the normal
operating level to allow them to reduce the dynamic range of the signal
gradually, so that they are acceptable to following equipment. For limiters, the
threshold point will be set above the operating level in order to provide a
maximum level for signals to following equipment.
Fig 8.2 Operating level
and headroom
One solution to this problem is for the operator of the equipment to be
continuously monitoring the program, and manually adjusting the gain to suit
the moment. When the program is quiet, the gain can be increased, and when
the program is loud the gain can be reduced. However, in most types of
program there are instantaneous short duration level peaks or transients, which
would be difficult to anticipate and impossible to respond to in the required
time. Even a sound engineer with the quickest reflexes could not bring gain
knob or fader down quickly enough. The need therefore arises for a fast acting
automatic gain controlling device which will track the program material
constantly, and which will always adjust the gain to maximise the signal to
noise performance without incurring distortion. This device is called a
compressor or limiter, and is one part of the DPR-422