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In this first installment of the Foundations of Audio series, author Brian Lee White shows how to improve the sound of a mix with compressors, limiters, gates, de-essers, and other dynamic processors. The course explains the fundamentals of sound waves, and amplitude, explores common compressor controls, and shows how to eliminate unwanted noise using gates and expanders. The course also demonstrates best practices in compression and limiting in a variety of audio applications and covers sculpting the attack and decay of individual notes with transient shapers and applying frequency specific dynamics control with multiband compressors. Exercise files accompany the course and include special Get in the Mix session files.
There are two major classes of limiters. The first type is one we're already familiar with, as any compressor with a ratio control can be used as a limiter, simply by increasing the ratio above 10 to 1. These types of limiters are useful for controlling signals that need more aggressive gain reduction than compressors with lower ratios, such as very dynamic vocal performances. The second type of limiter is a special limiter whose ratio is essentially infinite. These limiters are often called brickwall limiters because no matter what the input amplitude is, the single will never pass over the threshold.
Brickwall limiters are typically used to maximize loudness and average level while protecting the signal from clipping. Because no signal is allowed to pass the threshold, these types of limiters generally do not have attack controls; the attack time is instant. Actually the limiter buffers the incoming signal and instantly reduces any high- amplitude signal that'll overshoot the threshold. This feature is often referred to as look-ahead processing and is why most brickwall limiters introduce a small amount of latency or delay on the signal they're processing in order to fill the look-ahead buffer and anticipate any overtures.
Look-ahead processing is one of the main reasons these types of limiters are used to maximize the overall loudness of a signal or entire mix, as the peaks are reduced instantly with no overshoot. Also, the average level or body of the signal can be raised up without fear of clipping, effectively increasing the perceived loudness while reducing the dynamic range. Because many brickwall limiters are designed to work transparently and on full multi-track mixes, the release controls are often adaptive or automatic.
This allows the limiters to react to peaks very quickly without digging into the material that follows. While limiters can be extremely powerful tools for honing the signals dynamic range because of their aggressive ratio an instant attack and release times, this brute-force method of peak control can become very nasty when over-used or abused. There is a very fine line between a nice amount of dynamic control and totally squashing the life out of your signal. If the limiter's threshold digs too deeply into the signal, grabbing and releasing the signal too quickly and aggressively, this creates unwanted artifacts, including distortion and what some call pumping.
Let's talk about why this is the case. Because brickwall limiters often use extremely fast release times, low- frequency material may cause the limiter to distort as it attempts to trace the waveform's oscillation. For example, 100 Hertz tone cycles through its waveform at a hundred times per second. It takes the wave from 10 milliseconds to fully develop as one complete compression, or push, and rarefaction, or pull. If the limiter's release time is set faster than the waveforms oscillation speed, there is a risk that the limiter will follow the literal push and pull of that waveform, grabbing the signal any time the waveform has any amplitude as either a compression or rarefaction.
With a deep threshold setting, the insanely fast attacking release times of the brickwall limiter can easily distort the signal. Using a slower release setting can sometimes alleviate the artifacts of distortion by allowing the low-frequency waveforms to complete their full cycles before the limiter releases the signal from gain reduction; however, these artifacts are generally a component of deep threshold settings and changing their release to slower settings may simply trade distortion for audible pumping or breathing, where the attack and releases now slow enough to hear going in and out, like heavy breathing, which in most cases doesn't sound much better.
We'll hear examples of distortion and pumping in the next movie. So to avoid distortion on low- frequency material like bass and kick drum use the processor's automatic release control or set the release times to greater than 20 milliseconds. Remember that retaining some dynamics in your mix is a good thing; they're what make the speakers move and the kick drum punches in the chest. So when working with any kind of limiter, remember that a little can go a long way and always make sure you evaluate the signal with no net gain change so you can tell just how far you've gone.
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