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What is an audio compressor?

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What is an audio compressor?

An audio compressor can be a piece of software or hardware that takes a digital audio signal and applies an algorithm to the data to reduce the size of the signal for storage or transmission.

The term “audio compressor” also describes hardware or software used to modify an audio channel to remove or normalize frequencies above a defined threshold, usually to improve perceived sound quality. When data compression is described, an audio compressor can compress digital data into a lossless format, so all audio data remains intact, or into a lossy format, so some data may be changed or removed to improve the efficiency of the compression algorithm. Almost all digitally transmitted audio data uses some form of compression, leading to the development of hardware dedicated to compressing audio in real time. Similarly, many forms of digital audio recording, such as sound on compact discs (CDs) or digital video discs (DVDs), use compression to save space and allow more information to be stored.

There are many common algorithms that are commonly used to compress different types of digital data. However, audio signals are very unstable and do not benefit from traditional compression in the same way that images or other data do. Instead, an audio compressor may choose to create very large files with minimal compression, or it may be designed to modify the audio data to make it more suitable for compression, although this results in lossy compression where some audio elements are raw. The signal will be altered. or lost.

One way an audio compressor can prepare audio data for compression is by removing sound elements whose existence is not necessarily determined in the first place. This includes frequencies outside the range of human hearing, as well as sounds that produce data, but are actually masked by louder sounds in the same time period. Additionally, audio compressors sometimes attempt to modify rapid changes in the signal to be smoother, smoother, or more predictable. All of these methods allow the use of different compression techniques to ensure that the audio signal or file achieves a good amount of compression and still maintains acceptable quality.

When recording or editing music, an audio compressor can be hardware or software that helps level out the sound of an audio channel or track. The software will finally make weak sounds seem louder and unusual sound peaks softer. Channel compressors can also simply remove these peaks from the audio channel, increasing the perceived clarity of the remaining sound.

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Difference between compressor and limiter

As we saw in previous articles on the operation of the compressor, the latter acts as an automatic volume, or more accurately, decreases the volume from a certain threshold, which has the consequence of reducing the dynamic range of a signal, attenuating the loudest sounds, bringing them closer to the weakest sounds.

Compressor / Limiter

This makes the sound of an instrument more balanced and helps you maintain a stable level throughout a song, even if the musician varied their sound intensity during their performance.

The compressor:

A compressor uses the threshold setting to define the level from which compression will take place. Weaker signals or signals below this threshold will not be altered, while signals that exceed this threshold will simply decrease in volume.

The amount of signal attenuation depends on what is called the compression ratio or rate, which simply indicates the number of decibels that has been reduced compared to the input signal after crossing the threshold and, by the way, the number db to add to the output gain to have equal volume.

Difference between compressor and limiter

Without compression, a 1 dB increase in input is always equivalent to a 1 dB increase in output, but if you set a 4: 1 ratio, you will need a 4 dB increase in input level to get a gain of 1 dB at the output. The higher the ratio, the greater the degree of compression. A 1: 1 ratio means there is no compression. It is important to remember that the gain reduction only applies to signals that exceed the threshold. The weakest signals remain unchanged.

After the threshold and ratio, the other common compressor parameters are attack and release times, although some compressors do not provide these parameters. In other words, attack adjusts the compressor reaction time (how long it takes to reduce the gain) once the threshold is crossed, while the release time determines the time it takes for the gain to return to its original volume.

These controls must be carefully adjusted for different types of compressors to keep the compression effects transparent, although it is also possible to use more pronounced compression to create a particular effect. For example, setting very fast attack and release times causes an audible artifact known as the bulge effect, which can be used to give some dynamics or rhythm effects.

Difference between compressor and limiter
Some compressors only provide switchable fast / slow attack times, but the release time is generally adjustable. The only obvious exception concerns compressors that automatically adjust their release time according to the dynamics of the processed signal. Compressors with an “automatic” mode like the famous SSL are useful for sources whose dynamics vary during a song.

Since compression works by reducing the gain of strong signals, the compressor’s output is always weaker than its input. That’s why gain control is provided to bring the output to an appropriate level. Even if the output level is the same as the input level, the compressed signal will continue to appear stronger and stronger compared to the original signal, because even if the levels are the same, the average signal level (RMS) will increase and The weakest sounds that were originally below the threshold. Although the compressors only reduce the signals that exceed the threshold, the gain effect grants the role of the compressor as a device that increases the level of low sounds.

The limiter

The limiter works in exactly the same way as a compressor, except that it has an extremely high compression ratio (100: 1). This means that the peaks of a signal systematically stop at the threshold and cannot exceed it.

Difference between compressor and limiter

In many limiters (as in certain compressors), the threshold system is different from that described above. Rather than moving the threshold to match the input signal level and then using the output gain to find the original level, another approach is to define an output threshold and then vary the input gain to force it to reach the defined threshold or output level.

This allows the maximum maximum level to be set in advance, to limit and increase the average volume of a sound effectively in a single operation. This is the principle used during mastering treatment, for example, to obtain an RMS level without exceeding 0dbfs.

Limiters: Features and Applications.

The limiter is a dynamic processing tool similar to compression. It is known to mastering engineers and is useful for optimizing the overall volume of a song. But this tool can also be useful for mixing when performing certain tasks that its cousin, the compressor, cannot perform as easily.

Audio limiter

Limiter mode compressors and brick wall limiters

A limiter is a processing module that allows you to quickly apply a very high compression ratio. In theory, any compressor that achieves a high compression ratio (say more than 20: 1) can be considered a limiter. In addition, different compressors (LA-2A, Neve 33609, DBX 160 …) provide a selector or have a section that allows them to act as a limiter.

However, these compressors generally do not allow to know exactly the maximum output volume. This is where brickwall type limiters come in. Brickwall limiters allow you to set a volume that will never be exceeded. With their immediate attack, they set up a brick wall that audio can never get through. This important feature, of course, makes it possible to avoid clipping during mastering, but also makes the limiter extremely effective when treating tracks with strong, short tips.

limiter audio

The configuration

The limiter offers fewer settings than its cousin the compressor. First, there is the threshold that determines from which point the signal will be limited. This threshold can be determined by a setting that sets the threshold or can be reached by increasing the input level. Another parameter, called “ceeling” or “output level”, allows you to configure the maximum output volume. With digital audio, this ceiling is expressed in negative dB. Therefore, a ceiling of -0.2 dB indicates that the maximum volume will be 0.2 dB below the digital zero point. Very often it contains a release that, like the compressor, determines the speed at which, by limiting it, it will stop operating. Of course, one or more meters are added to show the input and output volume and the degree of reduction.

Since they are often placed at the end of the general output (master bus, main bus, or mix bus), the limiting plugins often include “wobbly” selectors that provide the ability to reduce the sampling depth (24, 20, 16, 12 and sometimes 8 bits) to correspond to the final medium. For example, we use 16 bits for a CD.