What is Compression?
Compression is triggered when a signal reaches your equipment's threshold and gives you the ability to manipulate dynamic range to make it smoother and add contrast to audio signals.

-Input Level
-Gain Reduction (GR)
-Output Level

Every input channel has an initial input level of the source. Compression is triggered when a signal reaches your equipment's threshold. The threshold is the amount of decibels the compression impacts the input level. "The knee" is an adjustable slope of compression that controls signal transitions from uncompressed to compressed signal. You can control that by how fast the signal moves from uncompressed to compressed and back again. This is known as "attack" and "release.Attack is the adjustable amount of time (fast/slow) it takes for the compression ratio to react with the line signal. (The time it takes for the compressor to complete the gain reduction).  Release is the adjustable amount of time(fast/slow) it takes for the compression to release the compressed signal and the compressor reaches its full range of gain reduction. Ratio is the amount of compression applied. The GR is how much Gain Reduction is happening when the input level enters through the compressor. The out put level is the level after the line level has gone through the compressor.

Types Of Compressors

4 common types of audio compressor

2. FET
3. Optical
4. Variable-Mu

Each compressor has a different gain reduction circuit and they each react to audio signals differently. The following explains different types of compressors.

VCA – Abbreviated as “voltage-controlled amplifier”. This compressor type is versatile for multiple applications. VCA provides the most transparent gain reduction but can also achieve aggressive compression. The VCA compressor is fast, smooth and is colorful. This is highly recommended for buss compression and mastering applications.

FET - Abbreviated as “Field Effect Transistor”. FET is more aggressive to input signal and has a unique bright, clear sound quality. It has  claimed the title as fastest attack and release reaction time of these four designs. FET is not used for soft compression, therefore this would not be suitable for buss compressions. It excels as a hard peak limiting compression or if you want to add more transients or color to your mix. FET is primarily used on drums to get more punch, especially the kick and snare.

Optical-  This compressor reacts slower to input signal than the compressors ,giving it a more “natural” sounding gain reduction. It has a softer & slower attack/release time, which makes it smoother when it reduces signal.  Popular compressor because of its subtle, transparent, and musical sound quality. Used to smooth audio waves subtly.

Variable-Mu-  Its speed response is the slowest compare to other compressor types.
Variable-Mu compressor concept is as an “ultra-long” soft knee effect, the compressor is used as gradual gentle compression on the audio signal. The effect of this is a warm and smooth. Again not an ideal compressor for intense, aggressive gain reduction.

Dynamic Range Compression vs. Tonal Compression

Tonal Compression: This is the lightest use of compression which will smooth out your audio input level and give them just a bit of dynamic control. You don't want to crush the transients because that will push the vocals further back in the mix. Instead, you'll want to add some musical tone to the performance. This helps to control the tonal qualities of a track.

Dynamic Compression: Used to make the loud stuff quieter and the quiet stuff louder. This saps all of the impact out of the sound, but you don't have to keep reaching for the volume control.

Parallel Compression: Parallel compression is form of upward compression which is achieved by mixing an unprocessed 'dry' audio signal, or a light tonal compressed signal with a heavily compressed version of the same signal. This is usually ran by bussing an input track in pre fader and adding heavier ratio or threshold to compressor plug in on buss or aux. The purpose of having a bus in prefader is to be able to blend the two faders of (one regular input channel with a heavily buss compressed signal) to desired liking. This way you keep the essence of the natural tone of a track and you can blend with a heavy compressor. This gives you more control of your dynamics in a mix.

Tips for Compressing Audio- 

1. Use light tonal compression with a soft ration, light threshold, long attack time, and long release time while tracking. This way provides the fullest level and is practically not altered. Threshold should only hit -2 to -4 on GR reduction level meter. This compression is so light and you might not hardly notice it/ it shouldn't be notice.

2. Parrell compress by using group bussing. Send input levels of desired input channels to an AUX/Buss. Group drums, keyboards, guitars , vocals and so on to specific classifications (aux). On the groups aux send you can compress harder to get more transients or desired tones out of your compressor types. This keeps raw integrity of a track and integrates with a more processed signal on a fader that you can blend. This encourages compression across wide range of groups with out altering the original state of the audio signal.

3. Master compressor is at the end of the whole audio chain. Ratios should be between a 1 to 4 ratio with a light ratio hitting 2-6 db of GR reduction. Add make up gain to add volume to what you took away. Use a long attack and long release to not suck the life out of the whole mix but gently touch your whole mix up.

4. Pyramid Scheme Of Compression

The principle of mixing parallel compression to intertwine between single AUX groups or different AUX groups taking up different harmonic content in a frequency spectrum. The main idea is keeping low ratios while tracking source , light input compression on initial track, compress harder on groups, compress even harder on parallel compressor groups, compress intertwined groups, and a light compression ratio at the end of mastering chain.Parallel compressors will be in prefader and about -20 to -40db on fader. This is so you will not over power original compression but instead compliment it underneath at a lower level. This is how you control the overall "breath" or "life" of a track with compression.

Limiters- Has same characteristics as compressors but it does not exceed threshold. Has ratio, threshold, attack, release , in put level, make up gain, output level, knee,  and GR. Make up gain is used to boost signal after compression reduction.