What is a Compressor-Limiter?
A compressor-limiter
is a device for controlling the “dynamic range” of an audio signal. The dynamic range is how we describe the variance
between quiet and loud passages of audio. For example, a snare drum has a very dramatic, loud attack which dies out to quiet very quickly,
whereas a note played on a violin has a fairly constant moderate sound level
– these have very different “dynamics”. Sometimes we would like to
be able to make quiet passages less quiet and loud passages less loud
– this is called “compressing” the dynamic range.
The following graph shows how audio level over time is affected by compression of a ratio 2:1
It may be preferred to maintain the quieter passages of audio unaffected and only apply the compression at the highest volume levels, set by
a “Threshold” parameter. This is known as “Limiting” and can be useful to control maximum sound levels for safety of listeners and to protect
audio equipment.
Since compression or limiting can allow the lower frequencies to be emphasised, an ENHANCER processor is added, which when turned up,
helps to restore the balance of higher frequencies.
In addition to these functions, the Citronic CL22 has an EXPANDER/GATE for each channel. When there is a period of silence in audio, often
there is low level noise from RF interference, circuitry noise or mains hum. The EXPANDER/GATE section cuts out the audio below a set volume
level to eliminate these unwanted noises when the audio should be silent.
Connection
The CL22 can be used either as 2 independent mono compressor/limiters (e.g. for channel inserts on vocal microphones) or as the main stereo
compressor/limiter for a sound system or recording setup. When the CL22 is to be used to control the main stereo mix, it should be connected
via the master inserts on the mixer or between the mixer and power amplifier/recording device. If settings for each side of the stereo are to be
identical, the STEREO LINK button can be pressed in and all settings for Channel A will apply to Channel B and controls for Channel B will be
ineffective.
Connect the L+R outputs from the mixer or insert sends to the Channel A and Channel B inputs of the CL22 using good quality XLR or 6.3mm
jack leads (balanced or unbalanced). Select the correct operating level (+4dBu or -10dBV) for high or low impedance type signals via the
OPERATING LEVEL switches on the rear panel. Connect the L+R outputs from the CL22 to the amplifier/recording device inputs or insert
returns. Connect mains via the supplied IEC, ensuring that the voltage selector is the same as supply voltage.
In certain scenarios, it may be preferred to make the compressor respond to the volume of an external signal (e.g. to duck the audio when a
vocal announcement is detected). Plugging an external control signal into the DETECTOR LOOP via mono 6.3mm jack will achieve this effect.
Alternatively, in some cases, it may be desired to reduce volume
only
when the high frequencies are loud. This can be useful to reduce volume
of “sibilance” (the “S” and “T” sounds of a vocal passage) and can be achieved by placing a high pass filter win the DETECTOR LOOP using an
insert lead (TRS jack to 2 x mono jack). The Ring connection carries the SEND of the loop and the TIP carries the RETURN of the loop, both
sharing a common ground or earth. The compressor becomes a “De-esser” in this configuration.