There are many types of situations in which compressors come in extremely handy. In the case of a soft knee, the ratio will increase slower. After the compressor detects a peak exceeding the threshold the ratio will increase rapidly from 1:1 to 8:1. For example, let’s imagine a ratio set at 8:1. Per short, it controls the response curve of the ratio parameter. The kn ee parameter controls how quickly the ratio gets to the set level after a signal exceeds the threshold. Similarly, the release sets the amount of time, also in milliseconds, after which the compressor stops applying gain reduction after the initial detection of a peak. The attack parameter controls the amount of time, usually in milliseconds, after which the compressor starts applying gain reduction after it detects a peak exceeding the threshold. These are separate parameters but I will explain them together as they serve similar purposes. Why would you do that, some may ask? Because, in order to flatten out the dynamic range you need to reduce the gain of the spurious peaks, but by doing so you also reduce the average level of the signal thus, by applying make-up gain you get a flatter signal coming out of the compressor that has the same amplitude as the incoming signal. This parameter enables the user to amplify the signal that is coming out of the compressor in order to match the incoming level and make up for the gain reduction applied by the compressor. For example, if the ratio of a compressor is set at 2:1 and the incoming signal is 4 dB over the threshold the signal coming out of the compressor will be 2 dB over the threshold.
It is called ratio because it is expressed as a division between the amount of dB that the incoming signal is over the threshold and the amount of dB over the threshold the signal coming out of the compressor is. Ratio is the parameter which lets the user decide how much gain reduction the compressor will apply to the signal. For example if the threshold of a compressor is set at -30 dB all the peaks of the signals that exceed -30 dB will be reduced in gain according to the gain reduction ratio. This is the control that let’s the user decide upon a limit, usually in decibels, above which the signal will be reduced in gain. The threshold parameter is one of the most important ones. Let’s take a look at the basic parameters found on most compressor units: Threshold
Nevertheless all compressors, virtual or hardware, share about the same set of parameters. Most compressor plugins out there are emulations of hardware devices but lately more and more innovative plug-in designs are coming out that go beyond just simulating a hardware device and have all sorts of features that are impossible to implement in analog circuitry.
So what would be an application of a compressor in mixing? There’s endless applications for using a compressor but some of the most usual are things like controlling the dynamic range of a vocal, so that you can easily integrate it in a mix, flattening the decay, sustain and release of a kick drum or snare in order to emphasize the attack or flattening out a bass performance that has spurious peaks.
There are many types of compressors but in essence they all do the same thing – they lower the dynamic range (the “distance” between the lowest amplitude of the signal and the highest) of a signal in order to make that signal more easy to control. But what does a compressor actually do to sound? In short, a compressor is a device that receives an audio signal, compares it to a threshold and reduces the gain of every peak of the incoming signal that exceeds that threshold. It’s not hard to see why when you look at the artistic implications of this technology. Tony Visconti, the producer of David Bowie amongst others, has once said that compression is the sound of rock’n roll.
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