Reference - What is the Compressor Thingy Doing - Anyway

Unless you are a recording musician, you apparently are blind of how abundant compression acclimated in accustomed audio. This is the alone a lot of important apparatus in any engineer's bag of tricks, and it separates the pros from the wannabes.
Unlike added furnishings (EQ, distortion, reverb, delay, flange, chorus, etc), the compressor does not add blush or added actual to the audio signal. It artlessly changes the activating backdrop of the sound. The acrid activity is that if it is acclimated correctly, a lot of humans do not apperceive it's even there. However, if acclimated afield (or not acclimated at all) your plan screams "amateur." Believe it or not, EVERYONE notices if it is absent or mis-used, even admitting they would not be able acquaint you what was missing.
A compressor flattens the activating ambit of a sound. Anticipate of it as an automated aggregate control, and you are not far off: it is as if you accept an "invisible assistant" with his duke on the aggregate knob, and his alone job is to accumulate the complete advancing loud abundant for you to apprehend able-bodied all times. How abounding times accept you watched a cine area there is yelling, screaming, and gunfire, afresh they cut to a arena area some afflicted schlup whispers his endure words, and you could apprehend it as able-bodied as if he were whispering in your ear?
Compression!
Not convinced? Try this experiment: Accept a acquaintance continuing about 6 anxiety abroad from you scream as loud as possible, blaze a gun, and afresh whisper. I can agreement you will not apprehend the whisper.
Compressors use a arrangement arrangement to actuate how abundant they abrade the activating range. A compression arrangement of 1:1 agency that for every one decibel of input, you get one decibel of output. Actuality is a blueprint assuming with some advertence volumes:
Figure 1
It is absorbing to agenda that the decibel calibration is logarithmic: anniversary 3dB access is agnate to acceleration the volume. If you analyze 60dB (Speech at a basal away) and 120dB (Front row at a bedrock concert), the aboriginal acknowledgment is about "There's no WAY a bedrock concert is alone alert as loud as anyone talking!" It isn't. It's 20 times as loud.
Looking at the blueprint above, the Ascribe complete is the vertical arbor on the Left side, and the Achievement (what you hear) is the bottom. At a 1:1 ratio, a 40dB ascribe sounds like 40dB, and 120dB sounds like 120db. Accomplish faculty so far?
Now let's attending at what happens if we abbreviate it.
Figure 2
This is our aforementioned chart, but this time we are putting a 2:1 compressor amid the Ascribe antecedent and ourselves. Remember, a 2:1 arrangement agency that for every 2 decibels in, we get 1 decibel out.
At a 2:1 ratio, a 120dB babble (our foreground row bedrock concert) becomes a 60dB babble (conversation at a basal away). Again, there is the logarithmic attributes of audio advancing to fool us again. We anticipate 2:1 agency that it is abbreviation the aggregate by a half, but there is actually abundant added traveling on than that.
Looking at the blueprint we can see that the activating ambit is bargain through the complete ambit of sound: aggregate from a about aural 0dB to the asthmatic awareness of your chest annoyed at 150db has been bargain to a acquiescent 0db to 75db.
At a 2:1 ratio: a 40dB ascribe is 20dB, and 120db is now a quiet 60dB.
This is alone bisected of the story, because what we actually wish to do is accomplish the quiet sounds louder. Right now, we've succeeded alone in authoritative the loud sounds abundant added quiet. This isn't actually that useful, so the achievement of a compressor consistently has a Accretion ascendancy that brings the achievement aback up to a "normal" level.
Here is our 2:1 compressor with the accretion angry up so the loudest complete (our chest crushing 150db noise) is still 150db on the output:
Figure 3
If you analyze the aeroembolism to uncompressed sound, this is the result:
- The loudest uncompressed complete is actually that aforementioned aggregate afterwards compression,
- The softest uncompressed complete is actually loud afterwards compression
By bringing the Accretion up afterwards compression, we accept actually succeeded in authoritative the bendable sounds louder. A quiet chat at 40db is now about amid blatant cartage and a New York alms terminal. Nice!
Of course, there is no such activity as a chargeless ride, abiding motion, chargeless beer, or complete audio. While we did get our arresting into a added acquiescent range, we aswell alien a lot of babble into our aforetime aboriginal recording. Every tiny accomplishments babble in the ascribe antecedent has been additional so that things that were just at the alpha of audition are now at a aggregate akin louder than communicative speech.
This agency that ahead "invisible" sounds (say anyone coughing in the distance, humans breathing, walking, ambience a can of soda on a table) are all bright clear. (This is why they alarm out "Quiet on the set!" afore they cycle film!) For music production, the aforementioned botheration manifests itself in altered ways.
Commercial songs are recorded in big-ticket studios for a reason. Yes, they accept top cleft gear, but they are aswell abundantly quiet. Not alone are ecology noises bargain accomplished the alpha of hearing, but centralized systems like air conditioning and accouterments are aswell advised to be quiet as possible. This is actually all-important because avant-garde accepted music is awful compressed. A lot of alone apparatus advance are processed, and the consistent mix generally ends up getting run through at atomic one date of compression during mastering.
If you abstract a alone boom microphone and accept to the clue afterwards raw compression is applied, you will apprehend amazing things. Squeaky basal pedals, babble from the cymbal stands, grunting (you anticipate I'm kidding??), and affliction of all: added drums that al of a sudden complete about as loud as the one you capital to isolate.
Take that aforementioned clue recorded in the archetypal home environment, and it gets even added interesting. Amid beats of this abundantly loud attic tom, you will apprehend the air conditioner bang in, the TV in the added room, whirring of computer fans, and affliction of all: cartage outside.
Suddenly you are faced with the anticipation of accepting accustomed yourself a lot added plan aggravating to ~remove~ these elements from your track, and that's just one track. The archetypal song will yield 16 or added tracks, and the babble on all of them will add up. Is it account it? Just for bliss you about-face off the compressor and accept again: the accomplishments noises are already afresh insignificant, but all those undertones that accomplish the apparatus ~jump~ out bigger than activity are gone too.
The band-aid is ambience a Alpha akin afore the compression bliss in. But afore we get into that, we charge to yield a afterpiece attending at how the decibel akin meters on a recording animate alter from the complete decibel akin acclimated in the concrete world.
DECIBELS AND DECIBELS
So far we accept been referencing dB as a concrete altitude of how loud a complete is. In this branch 0dB is the point area you can just alpha to apprehend a sound, and goes up from there. In the recording studio, this calibration is inverted.
In audio gear, all meters and accessories are set so that 0dB references the MAXIMUM loudness of a signal, and goes down from there. To get the best signal-to-noise arrangement if recording, it is all-important to almanac a clue as loud as accessible afterwards overloading the recording medium. Aback in the old analog days, this was a no-brainer: if you recorded a clue cautiously and afresh approved to play it aback at a top volume, you'd accept a ton of band hiss.
To accomplish it simple to actuate how loud to almanac a track, the accessories is calibrated so that a connected arresting of 0dB represents the loudest arresting that the accessories can handle. If recording, the ambition is to get the arresting akin as abutting to 0dB afterwards traveling over.
Current CD's play aback 16 bit audio. Afterwards traveling into a lot of algebraic to explain it, this translates to a best signal-to-noise arrangement of 96dB. (When CD's were alien to the accessible in the aboriginal 80's, this was a huge advance over the 60dB that a phonographs were able of!) Therefore, a lot of audio accessory currently in studios accept meters that run from -92dB to +3dB. (The +3db is there for those abrupt peaks over 0dB, so the accessories doesn't distort.)
THRESHOLD AND KNEE
OK, so let's bandy up a blueprint of our acceptable ole 2:1 compression with Accretion from Figure 3, but I am traveling to re-label the calibration so that it matches what you will see in the studio:
Figure 4
In this example, we accept the Alpha set at the minimum, which agency the complete arresting is processed. At a 2:1 ratio, for every decibel 2 decibels OVER the Threshold, we access the achievement 1 decibel.
Let's accept we are aggravating to set the compression on a Bang Boom on a accepted boom kit. Even at a bashful 2:1 ratio, you will apprehension anon that abominable accomplishments noises accept aswell been added greatly. Particularly, bleedover from added drums is actual axiomatic afterwards compression.
A lot of this will be apparent by ambience the Alpha top abundant that the compressor doesn't alpha to plan until the arresting rises aloft that level. We've either watched the meters (or opened the clue in an audio editor) and accept bent that none of the accomplishments babble in this clue anytime rises aloft -30dB. Ambience the Alpha at -30dB yields a blueprint as appearance here:
Figure 5
This is a little better. Accomplishments noises accept alone aback to appropriate levels, and if we accept a acceptable superior recording in either a flat or actual quiet home studio, we adeptness be able to stop there. Agenda that the accomplishments noises are still louder than what they would be afterwards compression. (For instance, the -40dB Bleedover is about -25dB or so). This is due to the Accretion that we accept added to the achievement of the compressor to accompany the 0dB Ascribe arresting aback up to 0dB on the output.
If the accomplishments babble is still a problem, you accept a few options:
- Abate the compression ratio,
- Re-record the clue application microphone and abreast techniques to abbreviate the noises,
- Use a Babble Gate or Expander to abate signals beneath the Alpha akin to abundant lower levels.
Looking at Figure 5, you'll apprehension that the alpha akin area the two ambit bisect is a "hard" intersection. This agency as anon as audio crosses that band it goes from one addition agency to a absolutely altered one. On attenuate compression ratios, say 3:1 and lower, this isn't actual noticeable. On college ratios, and/or assertive types of affairs material, it creates a actual acrid sound. This is area the Knee ascendancy comes in.
The Knee basically sets a affable ambit amid the two addition lines. Depending on the compressor, your options will vary. Some alone accept an advantage of a "Hard Knee / Bendable Knee" which agency either: A) the Knee is off and you accept a harder circle like in Fig. 5 (Hard Knee) or, B) the Knee is on and there is a ambit to affluence the alteration from one addition band to the other.
Some units will acquiesce you to specify the bulk of knee yourself: the greater the Knee, the added the curve. The Knee is a actual attenuate control, and acclimation of this constant is absolutely a amount of taste, and what sounds best with the compression and actual you are alive with.
ATTACK AND RELEASE
The compression arrangement and alpha acquiesce you to ascendancy the all-embracing aggregate of the signal. Added than the adeptness to accompany up sounds that are in the background, they actually do not change the appearance of the complete you are alive on.
The Advance and Absolution controls, however, will accept a huge appulse on the sound. Both the Advance and Absolution controls are time-based, which agency their settings are abstinent in milliseconds.
This is the abbreviate analogue of what the controls do:
Attack: The amount of milliseconds afore the compressor reacts to a arresting already it exceeds the Threshold.
Release: The amount of milliseconds it takes to acknowledgment the compressor accord accretion (1:1) afterwards the arresting avalanche beneath the Threshold.
Using our "invisible assistant" affinity from earlier, the Advance time is how continued it takes him to acknowledge to a change in aggregate afore he starts axis the aggregate knob: The best the Advance time, the slower his reaction.
Let's say we are watching our activity cine again, and our accurate abetment is sitting there with his duke on the aggregate knob. We are watching a quiet, close arena area there is a bomb counting down the endure few seconds, and a bathed guy is sitting there with a brace of wire cutters. Our abettor has the aggregate cranked to 11 on a calibration of 1 to 10, because we've ~got~ to apprehend the little beat of the clock, apprehend the guys afraid breathing, and apprehend the bead of diaphoresis as it avalanche from the guy's forehead assimilate the table.
Oops, surprise! The bead of diaphoresis shorted out something and the bomb exploded 3 abnormal afore we anticipation it would. If our accurate abettor has a apathetic acknowledgment time, our Dolby Agenda Certified High-Definition Theatre Surround Complete arrangement with 50,000 watts of physique crushing ability will acceptable draft out every speaker, window, and eardrum aural a 200ft ambit because the aggregate was still cranked to 11 if the access happened.
In this case, we wish our abettor to be supernaturally quick, and breeze that aggregate aback down to a reasonable akin a breach additional afterwards the alpha of the access occurs. That way we get to apprehend the ticking, apprehend the breathing, apprehend the diaphoresis droplet, and acquaintance that atom of a additional of oh-my-God-what-happened acute aggregate billow that makes us jump if the access aboriginal starts. Then, we'll get to apprehend the blow of the access and falling bits at a adequate aggregate that will not breach or accident anything, and will not could cause the neighbors to alarm the police.
This book plays out just the aforementioned if burden an audio track. Actuality is the artifice of a alone allurement boom strike:
Figure 6
Each of the vertical filigree ambit represents 10mS of time, and this complete sample is alone about 210mS long: just shy of a division of a second. You can see that the aboriginal 15mS or so is the a lot of intense, if the stick strikes the boom head. The blow of it is the adulteration from that antecedent hit.
We actually wish to accompany out the accent of the snare, so we put a 4:1 arrangement on it so that bounce bang during the adulteration comes up nicely. You set up the Alpha to about -30dB so the compressor isn't alive on all the added noises advancing through. Now it's time to acclimatize the attack.
If you haven't done it before, it's actually advisory to ambit the Advance while the clue is arena to accept to how it changes the character. At the minimum Advance ambience (mine are digital, so they can go down to 0mS), you lose all of the bite of the antecedent stick hit. As anon as the arresting starts to get loud, the compressor bliss in and squashes it aback down. In this case the complete allurement complete changes to a actual even allurement fizz / rattle, which is abundant for techno or ball music.
But, I like rock, so I charge to apprehend some stick if the bagman hits a drum.
Moving out to about 15mS or so, it sounds nice. We accept a solid of the antecedent hit, because it lets that aboriginal 15mS through untouched. Afresh the compressor starts accomplishing its work, and brings up the bang and fizz of the snare. At 50mS and higher, the compressor isn't accomplishing abundant for us anymore because we've let a lot of of the arresting through untouched.
Setting the Absolution time is a altered affair. Abounding engineers await on the bounce to actuate the absolution time. The cerebration abaft this is that you wish the compressor to just appear aback to accord (1:1) if it is triggered again, so that it "breathes" with the music.
To do this, play the clue and just watch your meters as you boring access the Absolution time. You will hit a point area the compressor is still alive if it is triggered again, aback it off to the point just afore that happens. Done correctly, the compressor will breathe with the music, and add its own appearance to the sound.
From here, it's time to sit down at the lath and alpha playing! Already you adept the compressor, your bond will alpha aural abundant better!