A Compressor is an effect which influences the dynamics of an audio signal. Specifically, a Compressor decreases the dynamic range of an audio signal. Dynamic range is the difference between the loudest(Peaks), and the quietest amplitude value in a waveform, measured in decibels. By reducing the dynamic range of the signal, the elements of an audio signal which were lesser in amplitude than the Peaks of the signal are brought closer to those Peaks, providing more focus to those quieter elements. The action of the compressor allows the compressed signal to be amplified much louder than before, without the occurrence of clipping, or distortion of the signal. The reason for is is if a signal is amplified, the parts of that signal highest in amplitude that hit the Unity Gain* are the peaks, and then Clipping occurs. By bringing the Peaks of the signal closer in amplitude to the rest of the signal, the signal can then be amplified much further without clipping. For obvious reasons, one should never mix elements to full unity gain, and a compressor can certainly help in the mixing process with just that.

List of Compressor functions and terminology:

Threshold: The Db level at which compression takes effect.

Attack: The time(in milliseconds) before the compression takes effect after crossing the Threshold.

Release: The amount of time the compression effect continues after processing a peak over the Threshold.

Gain Reduction: The amount of Gain lost via compression.

Makeup Gain: A control used to makeup amplitude of the signal, to counteract the Gain Reduction.

*Unity Gain: 0db.

Ratio: The Ratio(See Above), in decibels, describing the amount of compression to occur to the signal above the Threshold. A 4:1 ratio means that for every 4 decibels of signal over the Threshold, 1 decibel of that signal comes through. So, if 20db of signal goes over the Threshold, only 5db of that signal would remain.

There are two kinds of Compression, “upwards” compression, and “downwards” compression. Both of these compression types have to do with how the signal is processed above or below the Threshold setting of the compressor.

In Downwards Compression(see diagram above), a DB Threshold is set in the Compressor, and the Compressor then brings the signal over that Threshold down by the Ratio setting, leaving the signal below the Threshold unaffected.

In Upwards Compression(see above), the opposite is achieved, and the signal falling from the Threshold is brought back up. This type of compression usually results in a much louder signal and offers much less control and precision over the dynamics of the signal, and is used much less in production. As a result of this, there aren’t many commercially available upwards compressors on the market. A solution to the amplitude problems of upwards Compression are solved in Steve Duda’s OTT compressor from Xfer. This free plugin offers upwards compression, followed by downwards compression in a series. This allows you to control the loudness of the signal after the upwards compression has been achieved.

A technique called Parallel Compression(see below) can be employed to recreate the effect of upwards compression using a regular downwards compressor, and offers much more control over the amplitude of the signal after the upwards compression….

Parallel Compression: In Parallel Compression, a signal is split into two, one path being routed through a downwards compressor and then to the master bus, and one path continuing unaffected to the master bus. Upwards compression is achieved based upon the settings of the compressor. The object is to completely smash the peaks with an incredibly high Ratio, so that the volume of the recently quieter elements is now much more apparent. The compressed signal mixes with the quieter signal of the uncompressed material, making that material that much louder, and leaving the peaks of the original signal intact, because they are not being compressed along the straight path to the master bus. The amount of the compressed signal that is mixed with the original is of course controllable via the compressor.

Upwards Compression without Parallel Processing: If the above mentioned technique seemed laborious, and you don’t feel like trying to find a dedicated Upwards Compressor, you can still achieve upwards compression using a normal compressor. By reducing the amplitude of the Peaks using a high ratio setting, and then boosting the makeup gain of the compressor, it effectively does the same as Parallel Compression. This technique arguably gives you less control over the upwards compression process. Personally, I use Parallel Compression when I don’t use a dedicated upwards compressor, because I feel it gives me more control over mixing the signals together.

Peak & RMS:

sine wave

In the above sine wave, the highest amplitude of the sine represents the Peak of that signal. The RMS(root mean square) of the signal is below the Peak, and represents the average amplitude of the waveform. Through compression, the RMS value of the signal is increased as the peaks of the signal are brought down, and thusly we perceive the signal as louder. This is useful to remember for mastering, and if you analyze a professionally mastered track, you will find that the RMS value of the waveform(the track) is very high.