How Audio Compression Works

So how does a compressor actually work? I'm going to start out by talking about a conventional four-control compressor, which is pretty much the norm. The four standard controls are THRESHOLD, RATIO, ATTACK, and RELEASE, or occasion- ally variants thereof. Makeup gain, included on virtually all compressors, is just a simple gain (volume) control that comes after the compressor and that is completely independent of the action of the compressor. I will also refer to things like “circuits,” pretending that we are still in the analog realm, but the principles apply to plugins as well.

How Audio Compression Works

There are also simpler two-knob compressors, and more complex ones such as reacomp that actually give you control over the detection circuit, and there are also idiosyncratic things like “time constants” and so on that some compressors offer, but let's set those aside for the moment. If you want a straightforward freeware compressor to play along with then Kjaerhus classic compressor is pretty good.

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 ex- ample, 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.

So, armed with that knowledge, you could, if you want, take a second look at the example project posted above. Or better yet, you could start to mess around with your own settings and material. 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 release 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, killing your attacks. With a slower attack, in- creasing 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 meaning-less. 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. So let's talk about some guidelines for where to set these settings...

THRESHOLD approaches:

• Set the threshold just above the “average” signal level if you just want trans- parentish peak compression, like a limiter.

• Set the threshold deeper, below the “average” signal level but well above the noise foor if you want to actually modulate the sound or performance dynamics.

(I cannot give numbers, because it depends totally on what your signal is doing. Look at the meters.)

RATIO approaches:

• Any ratio above say 10:1 is basically acting like a limiter – there will be VERY little dynamic variation above the threshold with these settings, EXCEPT as you allow via the “attack” window, or force via the “release.” Ask if this is not making sense.

• Ratios of 2:1 or 3:1 will be very gentle compression, basically inaudible as processing effects, just giving a slight evening out of the signal levels.

• Ratios of around 4:1~8:1 will offer medium compression with some pumping

• As said above, ratio is totally dependent on the threshold Attack and release later.

Quote:

Originally Posted by shemp ok, two questions for me: 1. Does a limiter compress? Meaning, I sometimes use the Kjaerhus classic Limiter and I think I can hear some compression but there are no threshold and ratio settings on it. Please explain? 2. Please explain the 2 knob compressors. Is it more of a pre-set threshold/ratio/attack/release in one knob? Thanks!!!!!!!!!

1. "Limiter" is a bit of a fuzzy term. A pure, unadulterated brickwall instant limiter would be a clipper. I.e. It would simply clip the top off anything that exceeded the limit, like digital clipping. And this approach can actually be very transparent for short overs.

But most “limiters” on the market are actually very high-or infinite-ratio compressors with a fast or instantaneous attack and carefully-tuned release curves de- signed to have as little sonic impact as possible without actually squaring off the tops of the wave forms. How the designer approaches the release is what determines the sound and response of the limiter.

Digital look-ahead limiters actually slightly delay the output signal, which allows them to start compressing BEFORE the signal reaches threshold, which in turn al- lows them to modulate the very top of the waveform in ways that keep a microscopic smidgen of level variation, allowing extremely heavy limiting without the kind of obvious harmonic distortion that would come from a conventional instantaneous attack.

1. 2. Yeah, exactly. For example, in optical compressors, the signal is passed through an LED or lightbulb that varies in brightness according to the signal strength. This in turn fires on a photovoltaic element of some sort (like a solar cell) that modulates the signal (i.e. Reduces the gain) according the intensity of the light. Because the light element does not respond instantly and has a certain delay before it achieves full bright- ness and another delay as it goes dark, there is a sort of built-in attack/release that varies according to the intensity of the light.

By selecting a “just so” combination of light source and photocell, a designer might achieve a continuously-variable response that becomes faster and slower according signal intensity and speed of change that sounds musical and natural at a variety of compression settings and on a variety of material.

The designer might not need to add any additional attack and release delays. And a simple control to adjust the relative voltage sent to the light source could control whether it generally responded more quickly or more slowly.

Please note that there are also very fast-response, four-knob optical compressors, and slow-response two-knob VCA compressors. The optical type is just a little easier to visualize the operation of, I think, so that's the example I used.

You could also have 3-knob or 8-knob compressors, depending on how the de- signer decided to approach it. The famous LA-2A is basically a one-knob com- pressor plus gain (no wonder people like it!), as is the old Ross guitar compressor. More controls have been added over the years to make compressors more versat- ile for different kinds of signal and specific technical or creative goals.

Quote: Originally Posted by stupeT Yep, can you talk about the feedback compressor design and what it does to the sound? Cheers stupeT In most modern technical compressors, the design is feedforward through a side-chain. If you take the opto compressor example above, it would work this way: The signal comes into the compressor, and is split off into two separate circuits. The main signal is fed right into the gain-modulated compressor circuit for processing, and a separate “side chain” is fed to the LED or light bulb. This way, the plain unprocessed signal, complete with dynamics intact, is used to TRIGGER the compression that happens in the main compression circuit. That is feed-forward, and when you hear talk of side-chaining, it just means the ability to feed some other signal into the compressor's side-chain, so that for example you could use kick drum hits to trigger compression on a bass-line to “lock” the two instruments together.

Feedback designs are actually much simpler. The signal only passes through the compressor once, and the level-detection circuit uses the output of the com- pressor. This is less precise, but some people like the slower, squishier sound for some kinds of applications. The sonic differences might not be very pronounced until you get into fairly heavy compression settings, but try it both ways if your compressor has a switch.

For technical compression such as targeted control of peaks, feedforward is usually better.

Aside:

The acoustics thread that I referenced at the very beginning of this thread has a lot less hits than this one does. I really meant what I said – studio acoustics is an absolute basic. Anybody who is following this thread who has not read through the acoustics thread is missing a gigantic part of this stuff. You can find it here, very top thread:

http://forum.cockos.com/forumdisplay...aysprune=&f=29

<em>The pre-ceeding post was some compiled opinions and excerpts from threads written by some great folks on the Reaper forum, the moderator mentioned their was no problem with us creating our own version with the information, so the following is some of the best information and concepts that i believe you, my readers will find especially helpful. I couldn’t have said it any better, these concepts and ideas are top notch, and pure gold. You can read the entire 53 page massive thread if you’d like, here. <a href="http://forum.cockos.com/showthread.php?t=29283">http://forum.cockos.com/showthread.php?t=29283</a></em>