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Sunday, June 7, 2009

Users Guide To Compression

By Peter Wetherbee

The only thing that'll glue together pulsing drums, gnarly synths, gargantuan bass lines and your favorite spatial ear-candy tricks into a perfectly balanced, ready-for-prime-time mix is compression. Although in the wrong hands it'll suck the life out of your song, compression can be a psychoacoustic elixir that magically smooths out janky sounds, and it will even bring forth texture and harmonic richness from a track that was previously “unfixable” with EQ, reverb or any other effects.

Once you get the hang of compression — which is as addictive as it is satisfying — you will start to integrate it frequently into your recording process, resulting in more professional, solid tracks that will require less work in mixdown. It'll help you take fuller advantage of your recorder's specs and improve signal-to-noise ratios. And you will also capture more of the harmonic and dynamic complexities of your source material with a well dialed-in compressor, resulting in deeper tracks with more life and soul.

Although my pitch may sound sort of like a drug dealer or a holy-rolling televangelist, these are not the idle promises of a music-store salesman or marketing exec. Our only goal here is better-sounding music, and you probably already have underused compressors waiting for you in your DAW or gathering dust in the studio. This article should help neophytes fly fearlessly into the wild world of dynamics processing, while hopefully inspiring experienced squashologists to stay fresh and sweet in their gain-structure alchemy. This knowledge will make your tracks pump, slam and breathe with deeper vibe than ever before.

WARMING UP

There are several aspects involved in the unassuming yet enigmatic arts of compression — and why our ears crave it whether we know it or not. For one thing, most everything recorded before the '80s (before digital recording became widespread) was recorded to analog tape, which imparts its own series of very nonlinear compression and equalization characteristics, especially to hot low-end signals that hit the tape hard, such as kick drums and bass tracks. It is not to be underestimated how much our ears have been tuned — through '80s pop music, especially — to the sound of sweet compression, simply because that stuff was recorded and mixed to tape.
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Furthermore, the gulf between perceived loudness and actual levels — and all that Fletcher-Munson and other nonlinear wave and aural esoterica found in the realms of deep physics — is aided, abetted and transcended by ear-friendly compressors. It just isn't very pleasant to listen to someone singing and playing an acoustic guitar if they are standing next to someone slamming a drum kit. That would be an impossibly painful and ridiculous proposition to our naked ears, but it is a pop-music staple thanks to the magical mysteries of our friend the compressor.

We are so accustomed to hearing the individual elements of trap drums balanced with compressors to varying and layered degrees that few of us know what a drum kit really sounds like. Furthermore, compressors allow us to bring out the subtleties and complexities of a drummer's performance that even the musician playing could never possibly hear.

There is much more to compression than simply taming wild sounds and reining in sonic mayhem. A good compressor will actually help bring out the animal in a performance. Dial in the right settings, and your track gets in your face, threatening to burst out of the speakers like an uncaged beast. Fatness, punch, thickness, aggressive texture, width and groove are all fundamental goals when inserting your hardworking dynamics processor into the signal path.

Originally designed in the mid-20th century to keep music from overloading radio transmissions or from grinding through acetate plates, some of the early compressors were as unwieldy as a refrigerator. Like a Harley-Davidson or a tube amplifier, the design of a good compressor is not based on efficiency or frugal use of power. The Fairchild 670, with its 14 huge transformers, is still one of the rarest and most sought-after processing sounds many decades after it was built, but it will cost you as much as the fully loaded SUV you'll need to transport it to the studio.

Meanwhile, two of my favorites are the dbx 165 and Urei LA3A. Like many of the best compressors, they have very weird electronics that transform the audio signal to light (no joke) and read the brightness of that light with a photo-optical cell that determines how and when they kick in. Others use tubes, simple voltage-controlled circuits or combinations of both, and they vary greatly depending on age, accumulated grime and breakdown of components — all of which contribute to how each unit works and sounds. Even if they have consecutive serial numbers, two will rarely sound the same. Then there is the plethora of DAW plug-in emulations that are meticulously crafted to capture the characteristic wheezing inefficiencies of each of the above.

THE BIG SQUEEZE

Unless you are a trained audio engineer, you probably don't really know what compressors do, and plenty of people who actually understand their function still don't know what to do with them. The paradox is that compression is a subtle signal processor — as opposed to reverb, distortion, a filter sweep, etc. — that is nevertheless more powerful psychoacoustically than just about any other single type of gear you can use on a track. It is likely that a vast majority of the sounds that make up the music you listen to are significantly compressed — often multiple times — to make them throb smoothly in your earholes.

To say that a compressor generally works by reducing an audio signal's level by a certain ratio above a specified threshold would be to reduce its magic to a simple truth that doesn't even begin to tell the story. For one thing, the most interesting elements of that gain reduction occur in its attack and release stages, the slopes of which are rarely consistent. Without getting hung up on largely irrelevant technical details, let's just start by saying that compressors lower the levels of peaks and allow us to bring up the overall level of a signal without clipping.

Imagine a guy listening to a track with his finger on a fader, lowering the fader a little when the signal gets too loud, and bringing it back up when the signal gets quieter. How fast he lowers the fader for hot signals would be the “attack” of his gain-reduction process, and how quickly he returns the fader to unity after the signal gets quieter again would be the “release” time of his compression. The personality of this guy moving the fader, and how smoothly he moves it, determines the musicality of the compressor. If the dude has had too much coffee, he might be a little jerky, perhaps even anticipating what's coming musically; if he's been drinking cold medicine and brandy, or if he's never heard the track before, he might be a little slower to respond overall.

Either scenario could be musically pleasing or might cause an unpleasant “pumping” that feels unrelated to the program material. Conventional wisdom says that to avoid pumping or other undesirable artifacts (in other words, for the compression to be as effective yet transparent as possible), the attack should not be fast enough to swallow the natural rise of the signal, and the release should fall most of the way back before the next signal peak. And both should be as consistent as possible.

COLOR BY NUMBERS

Here is a basic approach for getting your feet wet with compressor parameters. If you are not feeling too confident about it, take a conservative approach to find your settings, and crank it up from there.

Insert your compressor into a mixer channel or DAW track that has a drum loop or something similar with consistent peaks or spikes, such as kick and snare drums.
Set the Ratio control somewhere around the 5:1 or 7:1 range.
Set the Attack control as slow as possible, for the longest response time.
Set the Release control at the faster, shortest response time.
Look at the Gain Reduction meter to see how much compression is occurring, and listen to what happens as you lower the Threshold control.
After making a mental note about what this sounds like, bring the Threshold or Compression knob back to where it is just barely triggering the meter slightly with peaks.
Sweep the Attack control from slowest to fastest settings and find the setting where it just starts to clip the beginning of the peak sounds it is triggering — where you can clearly hear it doing something. Now pull back to a slightly slower spot, where you still hear the original sound clearly but the compressor is kicking in a little.
Sweep the Release control from fastest to slowest and find the setting where the meter has time to recover before the next peak triggers it again.
You are now in the ballpark. The next step is to determine whether it can do something useful for your programmed material. If necessary, adjust the Threshold control to a place where the meter is kicking in fairly regularly.
Now go back to the Ratio control and sweep from 1:1 (no compression) through infinity:1 (brickwall limiting). Find a good-sounding setting and check that what you are hearing starts to correspond with what you are seeing on the meter.
Finally, adjust the Output or Makeup Gain control to find an overall level that is as close as possible to the original signal. That allows you to use a Bypass or similar switch to go back and forth between the compressed and unprocessed signal to see if you like what the compressor is doing without being misled by one or the other simply being louder than the other.

COMPRESSION DISCRETION

If one set of settings is louder, it will usually seem like the better setting; our brains and ears generally tend to think that louder is better. Therefore it is absolutely crucial to match the levels of processed and unprocessed signals as closely as possible if we really want to be able to make a useful assessment of what is going on with the compressor. Use your eyes to see the compression happening to help your ears learn what it sounds like, and try not to think too much about the process. One good thing about compressors is that they are somewhat forgiving once you are in the ballpark as far as attack and release settings are concerned, which can be very helpful when you are recording 10 or 20 tracks at a time.

Try going through the previously discussed steps with bass, percussion loops or a James Brown-type of chunky, jangling guitar. If you can play an instrument live into the compressor, you will be surprised at how good it'll make you sound with the right basic settings. The trick is to be able to suss out what's needed quickly, and once you get the hang of it, things'll simply sound better: punchier, solid, smooth and fat.

While tasteful compression lets you pull disparate tracks together into a cohesive relationship in a mix, too much of this good thing will make your track sound dull and will take the vibe right out of even the best performance. So at first, be careful and try to err on the side of subtlety because compression is like distortion: Once it's there, it's pretty hard to get rid of, but you can always add more later.

From: Remix Mag Jun 1, 2007

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