Audio compressors are volume controllers used to reduce an audio signal’s dynamic range, which essentially is the difference between the loudest and the quietest part of a signal. Generally speaking, compressors have attack, release, ratio, and threshold controls. You may also find soft or hard knee along with output gain controls in Higher-end compressors.
More often than not, people undervalue compressors and misuse them as a tool. Generally, a well-trained ear is much more valuable than an equalizer in compressing. Adjusting an audio’s dynamic range is a universal/standard concept, yet the actual compressing process varies, depending on the types of boards and the units being used.
Compressors fall under different types and each type has its own set of characteristics that you have to learn. These characteristics are more noteworthy when choosing a compressor rather than its general type. That’s why some audio engineers often have several different brands of compressors including ones of the same type. Yet, they might still prefer a certain type over the other.
No matter what the compressor’s brand is, it usually falls under one of the 4 main different types: VCA, FET, Opto, and Mu Compressors.
Types of Compressors
VCA (Voltage Controlled Amplifier) Compressors
A VCA compressor employs a Voltage-Controlled Amplifier in its amplitude-regulating circuit. This amplifier circuit will vary the voltage gain for its input (usually the audio signal) in proportion to the control voltage (also referred to as modulatory voltage). In essence, VCAs are used to attenuate or amplify the compressor’s input signal.
Using the term voltage-amplifier is somewhat deceptive. In most VCA circuits, this “amplifier” circuit only acts as a buffer; meaning that the voltage gain never actually exceeds the ratio of 1. In simple terms, the higher the control voltage (Amplitude), the more carrier signal is allowed to pass through to the output but at a fixed ratio.
Fundamentally, the audio signal is usually the input of the VCA, while the control voltage is the sidechain. This sidechain is usually what activates the compression and finally, the audio is what gets affected by this compression.
When should you use a VCA compressor?
If you are aiming for transparency, then VCA compressors are your best option. They also have flexible release, attack, and threshold controls and are capable of very high levels of gain reduction.
VCA based compressors have gained a lot of popularity in the market. This could be attributed to their ability to handle very fast attack and release times, while also being typically less expensive than Mu and Opto Compressors. Aside from that, VCA compressors are known to distort if pushed hard enough and may lack high-end clarity.
FET (Field Effect Transistor) Compressors
FET compressors are a type of analog compressor that rely on a FET(Field effect transistor) in its circuit to emulate the valve/tube sound. These compressors are known for their ability to handle “aggressive” sounds. Additionally, since they are fast-acting, they are very capable of handling “snappiness”.
A FET is a semiconductor component that controls the flow of current between its source and drain terminals. By applying a voltage to the gate (control terminal of FETs), you create an electric field that varies the conductivity between the drain and the source (think of it as a voltage-controlled switch). Hence, you can control the current flow across its terminals.
FETs are similar to triode tubes, which are the devices utilized in Mu Compressors. So, you could assume that both FET and Mu compressors are rather similar in how they compress audio. Despite that, there are many notable differences between these 2 different types. A tube’s gain-reduction circuit, for example, reacts slower than a FET’s. Additionally, FETs are known to offer linear compression when calibrated correctly, while tubes have nonlinear compression curves.
When should you use a FET Compressor?
If you are seeking aggression and excitement, then FET-based compressors are your answer. Their fast speed makes them great for handling strong transients. This makes them perfect for adding sound certain effects to your audio. Additionally, FET compressors are especially great if you are looking to add color while also regulating aggression.
Optical (Opto) Compressors
An Opto compressor utilizes a light element and a photocell to regulate the dynamic range of an audio signal. So, if the amplitude of a signal increases, the light bulb starts emitting more light. Consequently, the photocell will detect this increase and regulate the amplitude of the signal.
Opto compressors utilize a resistive component called LDR(Light-dependent resistor) in their electric circuit to achieve compression. As the name suggests, an LDR has variable resistance which is determined by the amount of light it is receiving. This is due to an LDR’s “Photoconductivity”. As the LDR receives more light particles (photons), its resistance decreases , so more current flows in the circuit.
As such, the resistance of the LDR is inversely proportional to the brightness of the light source. Different Optio Compressors utilize different types of light sources. However, having a linear input-to-brightness curve and a fast reaction time is valuable to a compressor’s design. Therefore, most manufacturers opt to use an electroluminescent source (Like LEDs) because of their fast reaction speed and output predictability.
When should you use an Optical compressor?
Optical compressors are not essentially the fastest. Yet, they are mostly used for specific applications where transparency and smoothness are extremely valued. Opto compressors excel in polishing a peaceful and calm type of audio signal. Mainly, these compressors work best on guitar, bass, and vocal type of audio.
Variable Mu Compressors
A Variable Mu compressor, the oldest audio compressor, is another type of analog compressor centered around a valve(vacuum tube). Like other types of compressors, Mu compressors have a varying overall gain depending on the input signal, where the overall gain level increases as the input signal decreases. However, unlike other Compressors, the valve here is particularly used as the gain reduction component.
Variable Mu compressors employ the use of cut-off tubes. These cut-off tubes have natural properties that allow for a wide variation of gain when utilized by Mu compressors. As you increase the input voltage to the control signal, the current flowing through the plate decreases. This results in the attenuation of the overall gain levels.
When should you use Variable Mu Compressors?
Mu Compressors are great at adding warmth, thickness, and depth to a sound. Additionally, with Mu compressors, you can vary the amount of compression depending on your input signal. For the most part, this compressor is best used on the master buss because of its ability to combine sounds into one smooth audio track.
Choosing the Right Audio Compressor
There is no single all-purpose compressor, what compressor you are going to choose really comes down to your specific use case. Being able to accurately produce the sound you want takes years of experience. There are engineers who have with years of experience, yet still struggle with this process.
Before you choose the right compressor, it is important to know what it is you are doing. Let’s go through the four main reasons to compress:
Here are the 4 main reasons to compress:
- Attenuating the dynamic range. Certain audio files might have very loud parts and very quiet parts. Usually, a huge difference could cause a certain part to overpower another. By compressing, you can limit the volume range of an audio file while also increasing the average loudest volume.
- Combining sounds together. If you are layering different sounds, compressing the file at different stages glues these layers into one cohesive sound.
- Adding Different Effects. For example, by using longer attacks and longer releases in your compression you can achieve good effects like an “oomph” effect for vocal tracks.
- Clamping sounds. using compressors to clamp down drum audios or to squish a pristine guitar track will make them stand out.
As mentioned earlier, choosing the right compressor depends on what it is you are trying to achieve, the sort of sound you are going for and so on. However, there will be times when you don’t have access to different types of compressors. Instead, you will be forced to make use of whatever you have available. You can certainly find a lot of emulators that sound almost exactly like their real counterparts. Usually, a lot of research goes into developing this hardware, which is why a lot of this high-end equipment is only accessible to elite studios.
We hope that by reading this, you understand that different types of compressors perform the same tasks differently. Eventually, when you end up comparing multiple compressors, you will start identifying noticeable differences in functionality. In the long run, you will find certain features and functionalities to be more useful than others and those features will become part of your sound, since that is what you will end up using.