Master your understanding of compression:
COMPRESSION DEFINITION || OPTICAL || FET || VARI-MU (Tube) || VCA || DIGITAL || DIODE-BRIDGE || MASTER BUS COMPRESSION
Contents
What Does Dynamic Range Mean?
Dynamic range in audio refers to the range between the quietest and the loudest sounds that an audio system is capable of producing. It is typically measured in decibels (dB). The dynamic range of a system is looked into when recording, mixing, or mastering audio, as it states the level of detail and clarity that can be captured and reproduced.
A wide dynamic range allows for greater contrast between the loud and quiet parts of the audio, which can make it more interesting and engaging to listen to. On the other hand, a narrow dynamic range can make the audio sound monotonous or flat, as there is less variation in volume.
Dynamic range is an important factor in the quality and clarity of an audio recording or mix. It is typically measured in decibels (dB). For example, an audio system with a dynamic range of 100 dB can produce sound levels that range from very quiet (e.g. 20 dB) to very loud (e.g. 100 dB).
The larger the dynamic range, the greater the range of volume levels that can be captured or reproduced.
How Is Dynamic Range Typically Represented?
The difference between the noise floor and the loudest peaks represents the dynamic range of an audio system or recording.
A system or recording with a high dynamic range will have a large difference between the noise floor and the loudest peaks, while a system or recording with a low dynamic range will have a smaller difference between the two.
The Noise Floor in dynamic range refers to the level of background noise present in an audio system or device.
It is expressed in decibels (dB) and represents the lowest level of signal that can be reliably measured or detected.
In audio systems, the noise floor is determined by a lot of different factors, like the quality of the parts, the design of the unit, and the physical environment in which the system is being used. These values also change depending on whether we’re dealing with analogue or digital audio.
The Peak Level in dynamic range refers to the highest level of volume or intensity reached in a piece of audio or music. It is often measured in decibels (dB) and is used to determine the overall loudness or dynamic range of a recording.
It is an important value to be aware of, as it helps to prevent distortion and ensures that the signal remains within the dynamic range of the system or device it is being played on.
What Is Dynamic Range Compression?
Dynamic range compression is a process that reduces the difference between the loudest and softest parts of an audio signal. It does this by increasing the level of the quiet parts of the signal and decreasing the level of the loudest parts, effectively “compressing” the range of volume levels present in the signal.
Dynamic range compression can be categorized into two broad types based on how they function:
- Downward Compression
- Upward Compression
Downward Compression, also known as “Compression,” in everyday terms, is the process of reducing the dynamic range of a sound or recording by making the loudest parts quieter and the softest parts louder.
This can be done using a compressor, which is a type of audio processing device or software that reduces the volume of the loudest parts of a signal while simultaneously increasing the volume of the quietest parts.
Downward compression is often used to even out the volume levels of different instruments or vocals in a mix or to add sustain to a guitar or other instrument.
Upward Compression, also known as “Expansion,” is the opposite of downward compression. It is the process of increasing the dynamic range of a sound or recording by making the loudest parts louder and the softest parts quieter.
This can be done using an expander, which is a type of audio processing device or software that increases the volume of the quietest parts of a signal while simultaneously decreasing the volume of the loudest parts. Upward compression is often used to add punch or clarity to a sound or to bring out the details in a recording.
For example, compression on vocals can make the overall volume of the vocals more consistent, as it reduces the dynamic range of the vocals. It can also add punch and clarity to the vocals, as it increases the sustain and attack of the vocal performance.
However, if used excessively, compression on vocals can sound unnatural and squeeze the life out of the performance, leading to a “squashed” or “flat” sound. It is important to use compression on vocals with care and moderation and to carefully adjust the compression settings to suit the specific needs of the vocals.
Dynamic Range Compression Controls
Threshold
The threshold control in a compressor is a setting that determines the level at which the compressor starts to reduce the gain of an audio signal. When the level of the input signal exceeds the threshold, the compressor begins to reduce the gain of the signal, effectively “compressing” the dynamic range of the audio.
The amount of gain reduction applied to the signal is determined by the ratio control, which specifies the ratio of input to output signal level.
The threshold level is typically expressed in decibels (dB). For example, if the threshold is set to -22 dB, the compressor will begin to reduce the gain of the signal whenever the input level exceeds -22 dB.
The threshold control can be used to shape the character of the compression by setting the level at which the compressor starts to take effect. A lower threshold will result in more aggressive compression, while a higher threshold will result in less compression.
The threshold control can be thought of as a “Gate” that determines when the compressor starts to take effect. When the input signal level is below the threshold, the compressor is “open,” and the signal passes through without being affected. When the input signal level exceeds the threshold, the compressor is “closed,” and the signal is compressed according to the ratio setting.
The threshold control is an important parameter in shaping the character of the compression, and it can be used to achieve a wide range of effects, from subtle and transparent to heavy and aggressive.
The equation for calculating the amount of gain reduction applied to a signal by a compressor is:
Gain Reduction = (Threshold – Input Level) x Ratio
This equation shows that the amount of gain reduction applied to a signal is determined by the difference between the threshold level and the input level, as well as the ratio setting.
So if the threshold is set to -22 dB and the ratio is set to 4:1, the compressor will apply 5.5 dB of gain reduction for every 1 dB that the input level exceeds the threshold.
Ratio
The ratio control in a compressor is a setting that determines the amount of gain reduction applied to the audio signal. It is expressed as a ratio, with the input level being the numerator and the output level being the denominator. However, the ratio doesn’t affect the threshold or the attack & release, only the amount of gain reduction applied.
For example, if the ratio is set to 4:1, this means that for every 4 dB of input above the threshold, the compressor will only allow 1 dB of output. This reduces the dynamic range of the signal and helps to even out loud and soft passages.
The equation for calculating the amount of gain reduction applied by a compressor with a given ratio is:
Gain Reduction = Threshold – (Input Level – Threshold) / Ratio
For example, if the threshold is set at -10 dB and the input level is -5 dB with a ratio of 4:1, the gain reduction would be calculated as:
Gain Reduction = -10 – (-5 – (-10)) / 4
= -10 – (-5) / 4
= -10 – (-1.25)
= -8.75 dB
This means that the compressor will reduce the gain of the signal by 8.75 dB in order to bring it back down to the threshold level.
Attack
The attack time of a compressor determines how quickly it responds to the level of a signal and can be used to shape the character of the sound being compressed. It can technically be referred to as the amount of time it takes for the compressor to reduce the level of a signal once it exceeds the threshold.
Though the attack is generally seen as the steepness of a slope in relation to an ADSR curve in synthesizers, they’re technically calculated in milliseconds.
For example, if you have a guitar track that you want to compress. You set the threshold at -12dB, meaning that any signal above this level will be compressed. You set the attack time to 5ms, meaning that if the guitar signal exceeds -12dB, the compressor will take 5ms to reduce the level of the signal.
If you set the attack time to a shorter value, such as 1 ms, the compressor will react more quickly to the signal exceeding the threshold. This can be useful in situations where you want to catch fast transients, such as in a drum bus where you want to capture the details of the snare drum and hi-hats in time.
On the other hand, if you set the attack time to a longer value, such as 50ms, the compressor will take longer to react to the signal exceeding the threshold. This can be useful in situations where you want to allow the signal to peak before compressing it, such as in a vocal mix where you want to preserve the initial impact of the singer’s voice.
Release
Release time in a compressor is the amount of time it takes for the compressor to stop reducing the level of the audio signal after the threshold has been crossed. Basically, it is the time it takes for the compressor to “release” the gain reduction it has applied to the signal. This time can be adjusted to control the amount of dynamic range reduction applied to the signal.
For example, if the threshold is set at -10 dB and the release time is set at 100 milliseconds, the compressor will reduce the level of the audio signal as long as the signal is above -10 dB. Once the signal falls below -10 dB, the compressor will take 100 milliseconds to return to its normal gain.
Here are some examples of how release time can be used in a compressor:
- Fast release time (e.g. 20 milliseconds) can be used to create a punchy sound with fast transients, such as drums or vocals.
- Slow release time (e.g. 500 milliseconds) can be used to smooth out long, sustained notes, such as a sustained guitar chord or a saxophone solo.
- Medium release time (e.g. 100-200 milliseconds) can be used as a default setting for most cases where an extremely punchy or drawn-out sustained sound isn’t needed.
Overall, the release time setting can significantly affect the dynamics and character of an audio signal and is important to keep in mind while using a compressor.
Knee
The knee shows the way a compressor reacts as the audio goes from an uncompressed state to a compressed. The knee denotes the speed at which the compressor comes down on the signal, as it crosses the threshold. While the knee is denoted in digits, it’s experienced as a quality, i.e. soft or hard knee.
There are two types of knee settings: hard knee and soft knee.
A Hard Knee setting means that the compressor will immediately begin applying gain reduction as soon as the signal reaches the knee threshold. This results in a more abrupt and noticeable reduction in volume.
A Soft Knee setting means that the compressor will gradually begin applying gain reduction as the signal approaches the knee threshold. This results in a more subtle and gradual reduction in volume.
The choice between a hard knee or soft knee setting depends on what you’re going for. A hard knee setting may be more suitable for drums or percussion, while a soft knee setting may be more suitable for vocals or bass.
Make-Up Gain
Since the compressor reduces the level of the loudest signals in order to achieve the desired amount of compression, the resulting signal is much quieter than the original input. The Make-Up Gain control is typically measured in decibels (dB) and can be adjusted to increase the level of the signal to match the level of the original, uncompressed signal.
Offered as a control in the output section, Make-Up Gain allows you to get back the decibels lost during compression.
The equation for calculating the Make-Up Gain required to match the original signal level is as follows:
Make-Up Gain (dB) = Original Signal Level (dB) – Compressed Signal Level (dB)
For example, if the original signal level is 0 dB and the compressed signal level is -10 dB, the Make-Up Gain needed to match the original signal level would be 10 dB.
It is important to note that the Make-Up Gain should not be used to try to increase the overall level of the signal beyond the original level, as this can lead to distortion and other unwanted effects.
Instead, the Make-Up Gain should be used to simply bring the compressed signal back up to the original level, allowing the compressor to perform its intended function without reducing the overall level of the signal.
Input Gain
The Input Gain parameter adjusts the level of the incoming signal before it reaches the threshold.
For example, if the threshold is set too low, the compressor may be constantly engaged and result in a heavily compressed signal.
Increasing the Input Level can help to bring the signal back up to a more natural level, while still allowing the compressor to function effectively.
Output Gain
The output Gain determines the overall volume level of the audio signal after the compressor has processed it.
It is an important control in a compressor as it allows you to fine-tune the volume of the compressed audio to match the desired level in the mix.
What Are The Different Dynamic Range Compression Types?
There are several different types of dynamic compressors that are commonly used in music production and mixing, depending on what purpose you need them for. We’ll be going through the five major types and outline what they are generally used for:
VCA Compression:
A VCA or Voltage Controlled Amplifier is used to control the level of the audio signal based on a control signal(CV), which is usually generated by a separate compressor circuit.
This control signal is used to adjust the gain of the VCA in response to the level of the audio signal, while the VCA is used to amplify or attenuate the audio signal as needed to achieve the desired level of compression.
FET Compression:
FET compression works by applying a voltage to the gate of the FET, which modulates the resistance of the transistor. This changes the amount of current that flows through the transistor, which in turn affects the level of the audio signal passing through it.
The FET compressor is known for its fast attack and release times, which make it suitable for use on instruments that have fast transients, such as drums and percussion. It is also often used on electric guitars and bass, as well as on vocals and other sources that need a tight, punchy sound.
It is also used to add sustain and clarity to guitar and bass parts, as well as to bring out the details in vocals and other sources.
Opto Compression:
An optical compressor is a type of compressor that uses a light-dependent resistor (LDR) or a photoresistor to control the amount of gain reduction applied to a signal.
The LDR is typically placed in the sidechain of the compressor, meaning that it responds to the level of the signal being processed rather than the main output of the compressor.
Primarily, Opto compressors are known for providing smooth and transparent gain reduction without adding audible artefacts or colouration to the sound. Opto compressors are also used to add sustain to guitar and bass signals and to control the dynamic range of a signal in a recording or mixing situation.
Opto compressors are quite revered as they tend to be more musical and transparent than other types of compressors.
Diode-Bridge Compression:
A diode-bridge compressor uses a diode-bridge circuit to control the amount of gain reduction applied to the signal. It works by allowing the input signal to go through a series of diodes in a bridge format. The diodes cause the AC to convert to DC, which eventually leads to compression.
Diode-bridge compressors are known for their smooth, transparent sound and are often used to add subtle compression to a signal without altering its character.
Like Opto compressors, diode bridge compressors can be used on a variety of instruments, including vocals, electric guitar and bass, and are often a preferred choice on the mix bus, working well on groups of instruments.
Vari-Mu Compression:
A variable mu compressor is a type of compressor that uses a valve or tube to reduce the gain. The built-in tube circuit changes the bias in proportion to the incoming signal, resulting in variable gain attenuation. While tubes are not uncommon in the makeup gain section, Vari-Mu compressors use tubes specifically as their gain reduction element.
Vari-Mu compressors are known for their smooth, warm, and musical sound, which is often described as “gluey” or “sticky.”
They are often used on vocals, drums, and bass to add density and sustain to the sound, and they can also be used on other instruments to add character and thickness to the tone. Vari-Mu compressors are also used in mastering situations to add punch and clarity to a mix.
Sai is a full-time music producer located in India, and is head of Faculty at D7 Media Institute. He is the most passionate music production guru I’ve ever had the pleasure of meeting. Fantastic at sound design, mixing, and recording, Sai heads most of the review content, as well as dabbling in some mixing and mastering content too here at WCS. Give Sai any topic and he could write forever about it. He has over 10 years of experience working in the industry and has earned both Music Production and Music Composition & Piano degrees.
