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.
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.
