One subject that seems to cause more confusion than any other among beginners and experienced engineers alike, is dynamic control. With thousands of compressors, limiters, leveling amps and maximizers on the market, it's perhaps little wonder that the subtle differences can be hard to grasp. We'll explain what these subtly different tools are, how they work and how they can help you.
The main focus of this month's article will be on limiters, although, as we'll find out later, there isn't really a specific definition of the dividing line between compressors and Limiters.
Limiters, Leveling Amps and Expanders
The difference between a compressor and a limiter (sometimes known as a peak limiter) is essentially very small. Both operate m a similar way, reducing the volume of a signal when its level exceeds a given threshold. The usual rule of thumb is that if the ratio is 10:1 or higher, the unit is considered a limiter. However, there are a huge number of exceptions to this rule, and you'll find many compressors that offer higher ratios and limiters with lower ratios. A 'brickwall' limiter uses a ratio of infinity:1 in order to ensure that the signal never exceeds the threshold.
Occasionally you may also come across a compressor described as a leveling amplifier. Rather than describing a specific topology or circuit design, leveling amplifier is really just an old-fashioned term for a compressor, most famously used on the legendary Teletronix LA-2A of the 1960s and cropping up occasionally on new equipment. Thanks to the vintage link, it's more closely associated with retro sounding compressors which color the sound of any signal they compress. However, this is only a very vague rule of thumb. In reality, the differences between compressors, limiters and leveling amplifiers are so small that they should really be considered different variations on the same basic idea. The good news is that this means the controls tend to work in pretty much the same way. If you know how the threshold, attack and release settings work on a compressor you'll have no trouble with a limiter or leveling amp.
Expanders, which seem to have fallen out of fashion in recent years, are effectively the opposite of compressors. Rather than reducing the level of a signal above the threshold, expanders reduce the level of any signal below the threshold. The result is to allow louder parts of the signal to pass through untouched while reducing the volume of quieter parts. You could think of it as a more flexible version of a noise gate whereas a noise gate completely mutes any signal below the threshold, an expander simply makes it quieter and allows you to adjust how dramatic the gain reduction is.
Multi-band versions of compressors and limiters work in precisely the same way as their single-band equivalents but use filters to split the signal into a series of contiguous frequency ranges before processing each one individually. The signals from each band are then summed back together.
Maximizing is the term most often used to describe the process of increasing the volume (or perceived volume) of a signal. The key to the process is reducing the dynamic range. We can amplify any signal until its loudest peak just hits 0 dB, but depending on the headroom between this peak and the rest of the signal, the track still might not seem loud.
Maximizing with a limiter or compressor effectively increases the average level of a signal by reducing the level of the peaks, allowing us to push the gain up and make the whole track seem louder.
The subtle distinction between general compression and using a limiter to maximize a signal is that maximizing specifically aims to push the overall level of the input signal as high as possible. This can be achieved in two main ways. The first is to route your signal into the limiter, pull the threshold down until the desired level of gain reduction is achieved, and then add makeup gain to push the output up as high as possible (again, until the loudest peak is at 0 dB). The other method is to set the limiter threshold at or close to 0 dB and amplify the signal before it hits the limiter in order to achieve the desired level of gain reduction on the peaks. The downside to this process is the fact that dynamic range is essential to keep a track musical.
Limiting and Dynamic Range
The effects of compression and limiting on dynamic range always seem to cause confusion so it's worth thinking about them again. Understanding what you're doing to a signal's dynamic range is one of the keys to getting good results from dynamics processors. Compression and limiting effectively reduce the difference in level between the quieter parts of your signal and the louder parts, so let's consider three different ways this can occur and look at the implications for whole track limiting, drum buss compression and kick drum compression.
Firstly, consider the track as a whole. For argument's sake, let's say we have a verse which IS quieter than the chorus. If we push the whole track through a limiter m order to make it seem louder, we'll eventually pull the threshold down to a point where the loud parts (the chorus) are being limited and the quieter parts (the verse) are being amplified. As a result, the dynamic balance between the two sections is being affected. If the overall level of the track can't be increased without detrimentally sacrificing some of the track's intended dynamic variation, then it suggests there's a problem that should be addressed in the mix rather than with broad, sweeping application of a limiter. Transient shaping Secondly, on a smaller scale, let's consider the situation when we compress a bussed drum mix. When a drummer plays, some hits will be louder than others due to the drummer's emphasis on certain beats and potentially due to some degree of inaccuracy (accidentally hitting the snare harder than intended on certain beats or fluffing a kick drum hit.If we use a compressor to tame the dynamic range of the hits, we can smooth out the variation in level between each one. Louder hits can be squashed, making the level more consistent, and this allows us to push the level of the quieter hits up to match. But as the drums are compressed harder and harder, the dynamic range between hits is reduced. This may be desirable in some cases, but care should be taken to avoid squashing the dynamics to the point where the drummer's expression is lost. On an even smaller scale again, consider a single kick drum hit.
At shorter attack and release times, a compressor on a drum hit can dramatically affect its overall amplitude envelope. For most of us, the envelope-shaping effect of a compressor on a drum track is an invaluable tool - one of the characteristic sounds of Electronic music is the effect of a drum machine being pushed through heavy compression - but if you don't think and listen carefully to the effects of your compressor it's easily possible to end up with a drum hit which doesn't sound anything like you want it to.
The desired amount of dynamic range may vary according to genre. Jazz tracks or more gentle acoustic music would typically benefit from the highest levels of dynamic range in order to retain a realistic, lifelike sound, whereas the (slightly controversial) trend in recent years has been for Pop, Rock and Dance music to be more squashed, so lower dynamic range might be acceptable if it meant the track would have the loud, squashed sound which we've become used to. The temptation with limiters and compressors is often to go overboard, but care should be taken avoid the squashed, lifeless sound which often occurs.