The convention for analog is to use time constants. Here's a demonstration of that (these are all the models offered by Logic's native compressor, same settings as the first test above): Notice how the compressor still releases when the signal level changes, even though the new level is also exceeding the threshhold! It doesn't simply "keep compressing" because the new signal level also passes the threshold, the attack and release settings are still in play!Įach compressor type has its own way of responding, which is part of the reason why VCA compressors sound different than FETs, which sound different than Opto, and so on. So once the compression starts, it should never stop, right? here's the same test, but with the threshold dropped to -30 which is below both signal levels of the test tone. So for a brief moment (500ms in this example), the signal that is below the threshold is actually being compressed!īut wait. You can see how the compression begins immediately after the signal crosses the threshold, at a rate determined by the attack setting, and then releases once the signal drops below the threshold at a rate determined by the release setting.īut at the moment the signal drops below the threshold the compressor is still compressing the release time is how long the compressor takes to return the signal to its uncompressed level. The two test tones are set to -20dBFS and -10, and the threshold of the compressor is set to -15, so the -10 signal should be compressed but the -20 signal left alone. (Well, sort of - I'll get to that in a minute.) The compressor isn't "waiting", the release time is the opposite of attack - it's how long the compressor takes to ramp back from the compressed level to the original level, once the signal stops exceeding the threshold. The important thing is that it represents a transition from "no compression" to "full compression". The first is a pretty hard limiter, the second is going to be a very transparent effect, something perhaps for larger dynamics rather than trying to squash a drumkit.Īs beeps-n-boops points out, knees don't have to start above the threshold, they can start below, or be centered around the threshold too. Maybe you have a 1.5:1 compressor and you set it at -25 with a 15 dB knee. Maybe you have a 20:1 compressor and you set it at -3 dB with a 3 dB knee. It's in the name - a hard knee has a more pronounced effect and a soft knee has a more "natural" or "transparent" effect. Four decibels above the threshold in that case might only be compressed by half, because it's still in the knee. So a 6 dB knee with a threshold of -20 means that you won't have full compression until the signal hits -14 or higher. So with a 4:1 ratio if you're 4 dB above the threshold you will end up only 1 dB up.Ī soft knee (knee > 0 dB) means that the compressor will transition from no compression (1:1) to full compression (ratio:1) at knee decibels above the threshold. A hard knee (a knee of 0 dB) is when a compressor goes from 1:1 at the threshold immediately to the full ratio above the threshold.
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