BSS Audio OPAL Series DPR-422 Owner's Manual - Manual (Page 14)

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The effect of the ratio control can be shown on a graph which plots input level
against output level (see figure 6.1). This clearly shows that below threshold the
DPR-422 acts purely as a linear amplifier.
In applications where gentle compression is required, it is beneficial to change
from the linear to compression region in a very gradual manner, rather than the
more conventional abrupt manner as shown above. The DPR-422 has been
configured so that for low settings of the ratio control and low levels of
compression, the transfer is soft, and for increasing ratio settings and high
levels of compression the transfer becomes harder. This ‘progressive knee’
gives inaudible compression for low levels of ratio and gain reduction, whilst
allowing harder compression for effective limiting and extreme control when
required. This feature is exclusive to BSS equipement, and is why the DPR-422
doesn't require a 'soft knee' control as all other compressors do.
When the THRESHOLD control is set fully clockwise to the position marked
OUT the threshold can not be reached, regardless of the input level, and the
signal will always reach the output uncompressed.
How the compressor performs on signals above the threshold is controlled by
the RATIO, ATTACK, and RELEASE controls (see the following sections for
more info.)
Control Operations
6.3 Ratio
Fig 6.1 Effect of ratio