BSS Audio OPAL Series DPR-944 Owner's Manual - Manual (Page 29)

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noise performance without incurring distortion. This device is called a
compressor or limiter, two such devices are provided in the DPR 944.
10.2 Compressors &
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 10.2 Operating level
and headroom