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How a compressor works (from must-read Reaper forum thread)
I'm reading through this classic thread by someone called Yep about recording/mixing/mastering audio on the Reaper forum - full thread here
Here's a great example post explaining the basics of compression
"How a compressor works
Inside the compressor is a little gremlin that turns down the volume. That's it. Really. HOW and WHEN he turns down the volume is determined by the instructions you give him with the compressor controls.
THRESHOLD sets the gremlin's alarm clock. It is what tells him to wake up and start doing what he does, i.e. turning down the volume. If you set the threshold at -10dB then the gremlin just sleeps his lazy ass off, doing nothing at all until the signal level goes above that threshold. A signal that peaked at anything lower than -10dB will never wake up the gremlin and he'll never do a damn thing. (see why presets could be problematic?) But once the signal goes above the threshold, the gremlin rips off the sheets and springs into explosive action.
RATIO decides HOW MUCH the gremlin turns down the volume, and it acts completely in relation to the threshold. If the ratio is set to 2:1, and the signal goes ABOVE the THRESHOLD, then the gremlin will cut that signal in half. For example, with -10 threshold, a signal that hits -5 (which is 5dB ABOVE -10) will be turned down 2.5dB for an output of -7.5dB. Negative values can be confusing if you're not used to thinking in such terms so re-read and ask questions if you're stuck. This is important, and it does get instantly easier once you "get" it.
ATTACK is like a snooze button for the Gremlin's alarm clock. It lets the gremlin sleep in for a little while. So if the THRESHOLD is set for -10dB, and the ATTACK is set to, say, 50ms, then once the signal goes above -10dB, the gremlin will let the first 50ms pass right by while he rubs his eyes and makes coffee. An attack of zero means the gremlin will respond instantly, like a hard limiter, and will allow nothing above threshold to get through unprocessed. Any slower attack means the gremlin will allow the initial "punch" to "punch through" and will only later start to act on the body of the signal.
RELEASE is like a mandatory overtime clock for the gremlin. It tells him to keep working even after the signal has dropped below threshold. A release of zero means strict Union rules-- once the signal drops below threshold, the whistle blows, and the gremlin drops whatever he's doing and goes back to sleep. But a slower release means the gremlin keeps compressing the signal even after it has dropped below the threshold. This can lead to smoother tails and less "pumping" or "sucking" artifacts that come from unnatural and rapid gain changes."
Comments
Will just post the rest of the compressor bit as it goes together -
"Here are some things to think about:
A compressor with a SLOW attack and a FAST release could give a very punchy, lurchy sound, as the compression lets the initial attack through and then clamps down on the "body" of the note, bringing it down in level, and then lets go as soon as the note starts to decay. This would actually INCREASE the dynamics in the track, and would probably require a limiter on the output after makeup gain was applied.
A compressor with VERY SLOW release times could overlap the release into the next note, compressing the initial attack even further, leading to a time-dragging feel.
A compressor with a high threshold and a heavy ratio will flatten out the peaks of the notes, but will leave the body and decay unaffected.
A compressor with a very low threshold will compress the entire sound, and will make the attack and body blend into the decay, ambience, and noise of the track.
If you "tune" the compressor by setting the threshold low and the ratio high so that it catches every note, you can adjust the attack and decay times so that gain reduction "bounces" along with each note in a way that complements the natural dynamics of the track. Then you can back off the threshold or ratio to get more natural sound.
If you instead "tune" the compressor by setting a slowish attack and realease time, and then tweaking the threshold and ratio to get the right kind of pumping and breathing, you can then adjust the attack and release so that the the impact and decay sound natural and well-balanced.
Practicing both approaches will quickly give you an ear for the subtle ways that compression affects the sound, and you will be able to achieve the best results by tweaking everything in tandem. But remember that certain settings can have opposite effects-- with a longer release time, lowering the threshold could cause the release to overlap into the next note, killng your attacks. With a slower attack, increasing the ratio and lowering the threshold for heavier compression could actually produce MORE dynamic swing. And so on.
Every control is interactive, and every control depends on what is going on in the signal. Presets such as "rock bass" or "vocals" are basically completely meaningless. They might as well be labeled "random 1" and "random 2" when it comes to compression. The tempo and source material could make appropriate settings for one song have a completely opposite effect on another song with a different singer."
Absolutely brilliant description
Awww, someone needs to animate that little gremlin.
Oh wow, what an awesome thread that is! I am going to have to PDF that sucker for the train.
Thanks for the hookup!
@gusgranite Thank you this is very helpful!
There is multiband which... i dunno... but there is multiband.
That's what the knee control does on any compressor that has one (dbx calls it "over easy" compression). I believe the La-2A also changes ratio depending on what is going into it, and a vari-mu compressor definitely works that way.
I simply can't get over how well written Yep's posts are on that thread. If he is actually writing these on the fly and not just taking excerpts from a book he wrote he is one of the best writers on the subject I have read. Simply amazing.
I agree. I'm more of a 'make squelchy noises on the sofa' than a 'record the next Adele in my home studio' kinda guy but I can't stop dipping into the Yep thread. Maybe I should change the title of this thread to attract a few more eyeballs to the whole Yep one.
Yah I know what you mean. I am certainly just a small niche hobby slice of the big picture that he has, but even describing all of the nuance that goes into recording and mixing an electric bass really resonated with me. Unintentional pun.
So many gem one liners too...
"An informed understanding is not a substitute for listening and experimentation, it's just an accelerant that speeds up the digestive process."
@AudioGus while I'm reading that bass section
That guy's advice on level matching helped my mixing. Paraphrasing, louder always sounds better, and quieter worse. He had the good advice to be wary of processors that make things seem louder, as a side effect of how they work, and adjust their output levels to match what is going in. That way it is easier to tell something like an eq adjustment it is indeed making it better, rather than merely louder, compared to the bypassed sound.
Soft knee compression makes a lot of sense with higher ratios of compression, because, you have a range of softer, subtler ratios as you get into the knee, and usually, you want something to just stop getting louder, past a certain volume level (above the knee).
Excellent thread, well written. A keeper for sure!
Ha, quote from that thread...
bookmarked, thx
Seems like Pierce Porterfield has a YouTube channel https://www.youtube.com/channel/UC8qv5hZppyGQbe9laVKwg3w that explains music production in very clear and concise terms.
This is a video on compressors
Lol, was just about to post the same link
Nice! Thanks for this! I have to make a built in compressor for Loopy Pro soon; this is a helpful refresher 😄
Cheat a bit and do a 'wrapper' for AUDynamicsProcessor
I considered it! I may yet; but I want multiband!
AUMultiBandCompressor then, it's got 4 freely asignable bands
I was looking into the idea of a wrapper AU for builtin AU's and mostly to get AUMultibandCompressor and AUMatrixReverb. But, they seem to be gone on iOS now.. They are still there on Big Sur. I thought I saw them on iOS before, maybe I'm remembering wrong?
His videos are really good. Thanks for sharing.
I was thinking that too – I’m not at my desk so I couldn’t check, but I’m pretty sure if I had’ve seen multiband compressor then I would’ve just used that straight away.
So compressors add zero saturation to a signal, they do not change the "shape" just the amplitude, so to say? Because I always had the impression this compressor pedal that I own introduced some overdrive/saturation... but I guess it's just because the makeup gain also allows the calmer parts of a note to be loud enough to go into saturation when I'm already around the breakup point?
It depends on the compressor. There are various topologies for compressors and, especially with guitar pedals, some add saturation. Orange flavored compressors are an example. In a general sense, a compressor is still a nonlinear device. But, the compression effect is slow enough that it does act locally linear in time. So, the compression itself isn't going to add nonlinear distortion.