Types of Compressors
Hi folks. Welcome back to The Corner. This week we’re going to discuss types of compression. Compressors are an oft misunderstood and misused effect. Hopefully this article will enlighten those that aren’t quite sure what a compressor is. We’re going to start off with describing what compression is, some basic terminology, and artifacts of compression. Then we’ll go through the major types of compression.
What is a Compressor?
The most basic explanation of an audio compressor is a specialized amplifier that reduces the gap between the loudest part (peak) and quietest part of a signal. The peak-to-average proportion of an audio signal (difference in volume between the peak level and average level) is called the dynamic range. A compressor reduces dynamic range. For example, the dynamic range of a guitar is roughly 20db. A compressor has the ability to reduce the difference in order for the quiet notes to be louder and the peak notes to be quieter, in effect producing a more even sounding signal. This can also be boosted in order to raise the average loudness of the signal. This is common in broadcast; the dynamics of a recording are reduced in order to raise the average loudness. This helps compete with the assumed noisy environment associated with driving a car.
Now that we have a basic definition of a compressor, let’s talk about some common terminology associated with compressors.
Ratio: The ratio controls the amount of dynamic range reduction the compressor is initiating. For example, a ratio of 4:1 means that it takes an increase of 4db to the input signal in order to raise the output of the compressor by 1db. 4:1, 8:1, 12:1, and 20:1 are common, preset ratios with anything over 8:1 considered by most to be the line at which a compressor becomes a limiter.
Threshold: The threshold is the level of signal required to “open” the compressor. In other words, you would set the threshold to a certain db level then only the parts of the incoming signal that crossed that threshold would be affected. Any part of the signal below that threshold would not be affected by the compressor but any signal above it would be “compressed” down to the chosen ratio.
Knee: The knee is the exact moment the compressor begins reducing gain. “Hard Knee” is a sudden shift from unaffected to compressed, there is no transition just instant compression the moment the threshold is crossed. “Soft Knee” broadens the range of the threshold and provides a less noticeable shift into compression.
Attack Time: This refers to the amount of time that elapses between the signal crossing the threshold and the compressor kicking in. Measured in milliseconds, typical attack times range from 1ms to over 100ms. This has an effect on the overall perceived brightness of the tone. The front end transient information conveys brightness character. Using fast attack times immediately reduces the transient and can result in a duller sound, especially with percussive sounds.
Release Time: This is the amount of time the compressor uses to return to unity gain once the signal falls below the threshold. Typical release times range from 5ms to over 5 seconds. The release time can have a pretty major effect on the sound. Very short release times can distort low frequencies. Very long release times can result in “pumping”, a sudden gain reduction that is usually unwanted or “breathing”, a slow return to unity level with a noticeable increase in the noise floor.
Compression and Sustain
Compressors are commonly used in the guitar world as sustainers. There is a common misconception that sustain is what a compressor was intended for but the truth is, sustain is an aftereffect of compression. The job of a compressor is the keep the output level more-or-less the same even while the input level is decreasing. This is what causes the compressor to increase sustain.
One note I would like to make about compressors in general before we talk about types. A compressor reduces the gap between the softest note and the loudest note. It does this by reducing the output level of the loudest notes. When we adjust the volume of an amplifier, we do so with the loudest notes in mind. When those loudest notes are compressed down to a level closer to average, some sort of gain make-up is required in order to retain the original level. This is usually done with the Output (volume) control of the compressor. Now, we just reduced the loudest notes in volume which means the average volume being output to the amplifier is going to seem like the quietest notes are being amplified. This includes the noise floor. This means that any sort of noise in the signal is ultimately going to sound amplified at the output of the compressor. It isn’t actually amplified but since the levels are brought closer together and the quiet notes aren’t as far away from the loud notes, the background noise at the lowest levels will also be closer to the loudest notes, therefore standing out more. This is not completely understood by people that have little or no experience with compressors and often times this natural artifact of the compressor is mistaken for the compressor being “noisy”. If a compressor sounds noisy, often times it is the result of noise induced somewhere in the signal chain that is unnoticeable until the signal is compressed. In my experience this is usually caused by an overdrive or fuzz pedal, bad guitar wiring, or bad patch cable.
Types of Compressors
These use an optical isolator circuit made up of a light bulb or LED and a photocell. The light source glows brighter or dimmer depending on the input level. The photocell reads the varying brightness of the light source and changes gain accordingly. The response time of the optical compressor is slower than other types but I’ve found them to be very natural sounding with a smooth attack and release that can be hardly noticeable unless they are pushed hard. The Diamond Compressor and the EHX White Finger are two examples of optical compressors.
Probably the most common type of compressor is the VCA (Voltage Controlled Amplifier). These are super versatile and can respond to the toughest, most specific conditions needed. A VCA compressor uses an integrated circuit (IC) to give precise control over gain reduction. These usually are less colored and have very little distortion making them one of the most popular compressor types on the market. I believe the Ross and Dyna-Comp compressors belong to this category (if not, please correct us on this one).
These use Field Effect Transistors to vary gain. FET compressors use transistors to emulate tubes with more reliability. They tend to have a unique sound that is fast and clean. There really aren’t many of them out there due to the extra circuitry needed.
Valve compressors are sort of a misnomer in that they do not use a tube for compression. Most valve compressors are based on one of the above circuits with a tube or tubes in the signal path for a warmer, creamier tone. The Electro-Harmonix Black Finger is an optical-based valve compressor.
While not normally associated with pedal-based compressors, the multi-band is nevertheless worth mentioning. A multi-band compressor is exactly what it sounds like, a compressor that works differently on different frequency bands. This allows one to dial in more or less compression on the lows than the highs or vice versa, by frequency. The advantage is that compression artifacts on certain frequencies can be dialed out by reducing or increasing the amount of compression in that frequency band. They work by splitting the input through several filters. Each signal is then fed through its own compressor then recombined at the end. These will mostly be found in mastering suites and DAW plug-ins.
So there you have it. Compression in a nutshell. For those of you who were in the dark, we hope this article sheds some light on it. Compression isn’t tonal “fairy dust” and for guitar related applications it can be downright tonally degrading if not used properly. When used properly, compression can add nuances that are more felt than heard. One basic rule of thumb with compression that a lot of studio guys use is this…If you can hear it, you’re using too much. Thanks for reading folks. We’ll see you next time, in The Corner.