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There's always the fader on the mixer to control volume.But what is the difference between it and the compressor? I know you can add attack release etc on the compressor, but if Ι only want a sound to sound quieter whats the difference then?

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A fader and a compressor are two different things.

Compressor

compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds thus reducing or compressing an audio signal's dynamic range. 1

Fader

In audio engineering, a fade is a gradual increase or decrease in the level of an audio signal. [...]

A fader is any device used for fading, especially when it is a knob or button that slides along a track or slot. 2

You might have confused them because the compressor might lower the volume of an audio input if it is set at a low limit, something that you can do with a fader, but it's not exactly the same.

To understand exactly what compression is, think it of a process that boosts the quiet signals and attenuates the louder ones, whereas the fader simply controls the volume of the input audio.

Here is a great article explaining many aspects of compression for beginners:

The Beginner’s Guide to Compression

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    One addition. Fader i.e. gain change is a linear process, meaning that for each fader setting, there exists a gain change that negates it, so the operation can be reversed. (minus infinity / muting excluded) But compression is a nonlinear process, in other words, it is a form of distortion. Distortion, including compression and other dynamic processing, cannot be "undone". – piiperi May 18 at 9:05
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    @piiperi though by that logic, actually using a fader to fade is a nonlinear process - the fader is only linear when it's not moving, if I understand you correctly? – topo Reinstate Monica May 18 at 10:19
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    so basically fader controls the all the sound sent into it. equally making them louder and quieter. While the compresser can take certain db points and make them quieter and louder. Am i right? – doodler doodle May 18 at 11:24
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    It might be useful to point out that a compressor is an "active" device, while a fader is a "passive" device. A fader is a slider or knob that the audio engineer moves to change the volume. A compressor is an active device that listens to a signal and makes changes to the volume without the audio engineer's intervention. – Robert Harvey May 18 at 14:25
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    @topomorto Yes exactly. You can't reverse a fade i.e. varying gain-change with any linear time-invariant system, and so in theory a fade adds distortion. – piiperi May 18 at 17:54

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