In the previous post I presented an overview of common plug-ins. Two types used in nearly every project are equalization and compression to tame volume levels and sculpt the sound. Let’s take a closer look into how each operates.
All equalizer plug-ins work with several common controls. Some EQ models have more features, but the general concepts are the same. An equalizer will boost (raise) or cut (lower) the volume of a specific frequency within the sound spectrum of the track. Some EQs feature only a single control point, while others include more – three, four, or even unlimited.
As you boost or cut volume, the audio frequencies around the control frequency are also progressively raised or lowered in what is presented as a bell-shaped curve on a frequency graph. The width of this bell is called the Q value. As you widen the curve – the Q value – more of the surrounding frequencies are also affected. A smaller Q – a tighter curve – results in a more surgical adjustment. An extremely tight Q value is often referred to as a notch, because you are only affecting that frequency and very little else. Notch settings and separate notch filters are often used to remove or reduce specific annoying background sounds in the audio.
Most multi-point EQs are designed so that the lowest and highest frequency control points are shelf controls. When you adjust the high frequency and it operates as a shelf, then everything above that frequency is rolled off. The same for a low shelf, except that the roll-off is in the other direction – lower frequencies. The slope of this roll-off can be gradual or sharp, depending on the features on the plug-in.
An extremely sharp slope at the low-end creates a high-pass filter (higher frequencies are allowed, lower frequencies are cut). An extremely sharp slope at the top is a low-pass filter. Depending on the equalizer model, a high-pass filter control may also be referred to as a low-cut control. High-pass versus low-cut is purely a semantics difference as the controls work the same. Some EQs allow the slope of the low-cut to be adjusted along with the frequency, while others leave the slope at a fixed amount.
Compressors come in more varieties and with a wider range of features than EQ plug-ins. For most, the core operation is the same. The intent is to squash audio peak levels to reduce the overall dynamic range of a track between low and high volume levels. The smoother a compressor works, the more natural and unobtrusive the compression effect is.
The threshold control determines the volume level at which the compressor starts to bite into the signal. As you lower the threshold, more of the signal is impacted. Some compressors also include an input gain control to raise the audio coming into the filter ahead of the threshold control.
The ratio control determines the amount of signal compression, i.e. gain reduction above the threshold. A 2:1 ratio means that 2dB of gain over the threshold would be reduced by half to 1dB. A 4:1 ratio would be a reduction from 4dB down to 1dB for any audio peaks that exceed the threshold.
The make-up gain control (when available) is often used to compensate for the gain reduction. When you apply a heavy amount of compression, affecting a larger range of the signal, the overall output will sound lower. Increasing the make-up gain compensates for this volume loss. However, this risks also bringing up the noise floor, since quiet portions of the tracks have been attenuated, too.
When you see the compressor settings displayed graphically, the adjustment appears as a hockey stick standing on its end. The threshold point is displayed on the graph as the point where the line bends. The angle of this bend is the ratio. The higher the ratio, the flatter the bent section of the line. A slightly curved bend is referred to as a soft knee, meaning that compression kicks in more gradually.
The response of the compressor to peaks is controlled by the attack and release adjustments. Set a fast attack time and the compressor will react quickly to peaks. A slow release time means that bite of the gain reduction holds on longer before the compressor returns to a neutral effect. The attack and release times determine the characteristics of how that compressor sounds. As an example, your adjustments would be different for speech than if you were recording drums. The impact of the compressor would sound different in each case.
The lookahead setting determines how far ahead the compressor plug-in is analyzing the track in order to respond to future peak levels. But, you are balancing performance versus precision. Long lookahead times require more processing power for the computer. A very short lookahead value means that some peaks will get through. Lookahead only works when you are working with a recorded track and isn’t applicable to compression on live sources, such as in a recording session.
Multi-band compressors and limiters
There are two special types of compressors. The multi-band compressor divides the sound spectrum into several frequency ranges. This enables the user to control the amount of compression applied to different parts of the signal, such as low versus mid versus high frequencies. As we will see in Part 4, some equalizers can be paired with compression controls to create a combo plug-in of EQ coupled with multi-band compression.
Another variation is the limiter. This is a compressor that’s designed to block all volume peaks above a determined threshold. Limiters are important if you have to deliver files for broadcast or streaming services in order to stay within loudness parameters. Some editors and mix engineers will place a multi-band compressor followed by a limiter on their stereo output mix bus for this reason.
Finally, some compressors include a built-in limiter, often referred to as a brick wall limiter. This is a second stage of compression with a tighter ratio. Graphically, the slope after the knee would appear flat. The limiter threshold is designed to fully compress all peaks that exceed the set level of the compressor. Typically the limiter values would be set somewhat higher than the compressor adjustments in order to allow for some dynamic range between the two.
In Part 3, I’ll check out one of the more popular audio plug-in developers, FabFilter Software Instruments, and their Pro-L2 Limiter plug-in.
©2021 Oliver Peters