PreSonus CL44 - Manual (Page 7)

2.1 FRONT PANEL BASIC LAYOUT
Notice that the front panel is divided into four identical sections. These are the four signal processing channels of the CL44.
Each channel contains:
Compressor/Limiter
Lo Filter
Compander
Gain (Output Control)
Link & Bypass Control
Optical and Auto Mode
2.2 COMPRESSOR/LIMITER CONTROLS
Lo Filter
The setting determined by the lo filter control allows signals passing through/above the selected frequency to be acted upon
by the compressor while the frequencies below the selected frequency are rolled off and ignored by the processor. The Lo
frequency control is variable from 20 Hz to 12 kHz with the twelve o’clock position being 2 kHz. An example of the Lo filter
in use would be to set the cut-off frequency at 4 kHz to reduce annoying sibilance (SSss’s) in a vocal performance. The
compressor will de-accentuate the frequencies above the selected 4 kHz while the frequencies below 4 kHz are allowed to
pass. Remember, the Lo filter effects the processing, but the audio path remains full range.
Threshold
The Compressor threshold sets the level at which the onset of compression begins. When the signal is above the threshold
setting, it becomes 'eligible' for compression. Turning the threshold knob counter-clockwise causes more of the input signal
to become compressed if you have a ratio setting of greater than 1:1.
Ratio
Ratio sets the compression slope. This can be 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 setting will be compressed at a compression ratio of 2:1. This
means that for every l dB 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 setting of the threshold. For example, if you have the threshold
knob set at 0dB, and the ratio turned fully clockwise, the compressor becomes a limiter at 0dB. This means that the signal
will be limited to an output of 0dB regardless of the level of the input signal.
Compander
The Compander works like an upward expander. Essentially, it increases the level of the signal that is below the threshold
according to the percentage specified on the knob. The range is from 0% to 100%.
Compander is derived from a combination of the word “compressor” and “expander”. The effect a compander circuit
achieves is that of restricting the dynamic range of the signal. The compressor prevents the transient of the signal from
going above a setting while the expander ensures that the signal is boosted below a predetermined point. This results in the
signal being maintained within a very restricted range without the loss of any information. The signal “seems” louder
without actually going above the level determined by the compressor. Apparent loudness is the result.