BSS Audio DPR 404 User Manual page 21

In the DPR-404, gain reduction is achieved by using a proprietary subtractive
circuit which minimises signal degradation and has a maximum attenuating
range of 30dB. The THRESHOLD control is used to position the input program
signal into the window of operating range for the circuit. Rotating the control
anticlockwise from the 'out' position will bring the signal onto the BELOW TH
meter. Gain reduction will commence once the dimly lit TH LED is
exceeded, and the actual amount of reduction in gain shows on the GAIN
REDUCTION METER, calibrated in dBs. The RATIO control is used to scale
the reduction effect once threshold is exceeded, and the scale numbers relate
to the amount of program signal, in excess of threshold, that will cause a 1dB
increase in output level. For general compression a ratio of between 2:1 and
4:1 is an optimum starting point. For limiting a ratio of between 10:1 and 20:1
is to be used.
The response of the DPR-404 to signals above threshold is further defined by
the ATTACK and RELEASE controls. These control how quickly gain reduction
occurs and, following signal level dropping below threshold, how slowly the
gain reduction is removed and unity gain restored. It is important that these
controls are set to suit the particular program envelope and the DPR-404 is
provided with two automatic ranges that are designed to carefully optimise
these times to the signal. With the switches in their released position, the
times are set to work within a range typical of vocals, mix program material
and other non transient sources. When compressing very fast percussive
sounds it is recommended that the fast auto modes are selected by depressing
the switches. Any combination can be used and it is best that the optimum
setting be found by listening for the desired effect. In general too fast an
attack will cause high frequency distortions and clicking sounds, whereas too
fast a release will give level pumping effects and low frequency distortions.
Because compression is a gain reducing effect, the output signal is often at a
lower level than normal.
The GAIN control, scaled in dBs, is provided to restore the output signal level.
It is positioned after the reduction process and therefore has no effect on the
threshold setting. The associated red LED will give an indication that output
level is approaching the maximum operating level for the channel. Should
you wish to bypass the compress ion effect, releasing the IN switch will
restore the output signal to that of the input. The green LED illuminates to
show when the channel is active. The LED metering remains active even in
the bypassed mode, so that some initial setting up can be done before
selecting the IN.
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