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Probably the most difficult signal processor to understand is a compressor. Most people get the idea that it squashes a sound, but that’s as far as it goes. Below is an excerpt from my Recording Audio book. Read through this, think about it, but then spend a lot of time playing with various recorded tracks going through a compressor. Experiment with the controls and get a feel for how they affect the sound. Where to start with the settings? Try this:
Set attack to fast, medium release.
Output gain to zero.
Ratio 3:1
Threshold around -10 or so.
Threshold is the magic one for now. Play your recorded track and turn the threshold down. You’ll see the gain reduction meter light up, indicating that output level is being reduced. Enough of this and you’ll hear it sounding more squashed. Find a balance, so perhaps set it for just a few dB gain reduction. And now, the excerpt:
A compressor is a signal processor that limits how much an audio signal varies from soft to loud. As the incoming signal gets higher, the compressor will reduce this so it sounds more even. Think of it as a volume cruise control for audio.
Compressor functions
Threshold is the signal level at which the device begins to compress. Any signal that is higher than the threshold will get attenuated, meaning it won’t be so loud coming out the other end. Threshold doesn’t directly dictate how much compression will be applied, only the point at which the device starts doing its thing. A high threshold will only compress the highest, loudest levels in the signal, whereas a lower threshold affects more of the entire signal.
Ratio sets how much the signal will be reduced. Once a signal goes over the threshold point, it will be attenuated at a rate set by the ratio. For example, if you set it at 3:1, every 3dB of signal beyond threshold will result in only 1dB coming out of the compressor (smaller dB numbers are quieter). Since these are ratios, multiples apply here, meaning if 9dB of signal goes over threshold, it will output 3dB.
A signal going over threshold triggers the processor to start compressing, but we can control how quickly it gets to its full reduction setting. This is the attack time; a fast attack setting will very quickly reach maximum gain reduction once threshold is reached, whereas a slower attack will take its own sweet time to get there. This is a big deal because it affects the tone of the sound as well—a really fast attack will lose the initial bite or attack of the sound, giving you a more rounded tone. Lengthening the attack time will allow that bite to come through before the overall dynamic range gets limited. Many people misunderstand this concept, believing that a longer attack time means the processor sets a timer before beginning the compression cycle. Not so. The technical term for what you’re shaping is called transients, which are the very first high frequency components of a sound that happen when an instrument is plucked or hit. So, turning the attack time up and down will give you a brighter or duller tone.
Many compressors also provide a knee setting, which effectively broadens the threshold point into more of a range. Once a signal approaches threshold, the processor will initiate a smooth, gradual transition into the compression cycle.
Once the incoming signal begins dying away and falls below threshold, the compressor will “let go” and allow the signal level to return to normal (unity gain). How long does this take? That’s the release time you set. A medium release is usually pretty smooth, whereas a long release may prevent the unit from recovering before the next musical passage begins. This obviously depends on the tempo and what they’re playing. A long release could also extend the life of a note just a bit; as the original sound fades away naturally, the compressor is slowly returning to unity gain, briefly counteracting the decay. A very short release setting can cause the unit to cycle too quickly, resulting in a breathing or pumping sound. Most compressors feature an auto setting option for attack and release, and for the most part it works pretty well. However, go ahead and take control yourself and experiment to make it fit each particular situation.
After a signal goes through a compressor, it’s usually got a lower overall signal level and might sound quieter in your mix. To compensate for this, all compressors have an amplifier at the very end where you can crank it back up a bit (gain). Sounds contradictory, but what’s happening is that once the compressor does its thing, you’ve got a more controlled signal with less dynamic level swings. Now you can take this streamlined signal and crank it up as needed. You set it in the mix and it won’t jump in and out as much.
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