Side-Chain - what it is and what it isn’t, Part 1

  • 09/18/2013
  • Michael Krusch
  • Story
  • Basics
  • Compression

Compression

General

Compression generally serves a better level control through the reduction of the level-difference that appears within the signal. Connected with that is a raise of the signal-output respectively the timely energy-density, provided that one compensates the level-reduction again at the exit by raising the gain makeup.

As a general rule, compression does in the first place mean the reduction of the level, as soon as a certain value has been reached over. All the levels that reach above this value will be reduced in relation to the original level. This relation is commonly known as the Ratio. With most compressors the level, from which the level-reduction is supposed to start, can be set manually. It is called threshold.

In compressors the level reduction does not start immediately, the reaction time is settable. Therefore the level does only gradually go back after it has overreached the threshold (Attack time). The level-reduction does likewise only gradually go back if the level overreached the set threshold again (decay respectively release time).

Through the settable reaction time (attack and decay/Release) it is definable, how long the peak of the signal is supposed to be, who has not yet been (completely) influenced by the level-reduction respectively how long the signal-parts are, which after cutting under the threshold are reduced inside the level. If the reaction would happen immediately the sound behavior of the compression would be similar to a distorted amplifier, which can no longer transmit the signal peak that is reaching over a certain value. Signal levels which lie above the upper limits are therewith restricted within the level.

Most of you can imagine from our own experience, how the over blasting sounds. Usually, this sound is not wanted, unless one uses it as a silent remedy or for the sound-design (guitar-amp or similar).

Therefore, the timely reaction of a compressor is mainly determining the sound. Besides the electronically dimensioning of the time-setting links, are especially the topology (Forward- or backward-control) as well as the usage of certain building-parts, for example tubes (vary-µ), photoconductive resistors as well as voltage-controlled amplifiers (VCAs) or field-effect transistor (FETs) determining the sound.

Feed Forward

With the feed forward, the input signal is used to set the amplification of the compressor. Behind the pre-amp (therefore before the automatic gain control amplifier) the entry-signal is tapped twice: once for the actual signal-way and second for the creation of the steering-signal for the amplification of the value of the signal. The reaction of the compression can, in theory, happen without any delay. In the praxis, it also here aimed for a rather slow attack and release.

Depending on the principle, the forward control needs to work through a bigger dynamic-range and is always reactant to the original input signal. In contrast to the feed back, the compressor is unable to react to the deformations that are caused by the compression itself. Kompressor Feed Forward

Feed back

During feedback, the steering-signal of the control amplifier is created from the signal behind the control amplifier. Therefore, the compressor is only able to react to signal-changes that have been through the control amplifier. He thereby gets slower. His reaction is, so to say, walking behind the signal curve. Because in classic compressors, as said in the outset, we don’t mind the delayed reaction-time, this characteristic does not create a setback.

Further the transiting-display is changed, because during the feed back impulse-peaks, which (especially through the time-leaps) are not yet worked with, and are passing the automatic gain control amplifier, are bound into the creation of the lead-signal. Because there already is a compresses signal behind the control amplifier, the dynamic of the signal, from which the lead-signal is formed, is restricted. Kompressor Feed Back
Falls du Teil 2 und Teil 3 unseres Side-Chain-Spezials noch nicht gelesen hast, hole es unbedingt nach.
Side-Chain - what it is and what it isn’t, Part 1
Side-Chain - what it is and what it isn’t, Part 2
Side-Chain - what it is and what it isn’t, Part 3