Presonus Audio electronic Smart Compressor - Manual (Page 10)

CONTROLS & CONNECTIONS
9
The Input control adjusts the gain on the input amplifiers for both
channels. Amplification gain varies widely from -20dB to +40dB.
The large range is necessary to amplify instruments such as guitar.
Thus the input stage to the Blue Max is actually a pre-amp.
Note: The input control is always active, even in manual mode.
Ratio
Ratio sets the compression slope. This is defined as the output
level versus the input level. For example, if you have the ratio set
to 2:1, any signal levels above the threshold (note: threshold is a
preset parameter in the Blue Max) will be compressed at a
compression ration of 2:1. This simply means that for every 1dB
of level increase into the compressor, the output will only increase
1/2dB, thus producing a compression gain reduction of 0.5dB. As
you increase the ratio, the compressor gradually becomes a limiter.
A limiter is defined as a processor that limits the level of a signal
to the compression threshold. For example, if you have the ratio
set to 20:1, input set at 0dB, the output of the Blue Max will be
limited to the internal threshold of the compressor, which is set to -
10dB in manual mode.
Note: The ratio control is only active in manual mode.