Compressor / expander (of dynamics)

Take a song that has loud and soft parts. It would perhaps be difficult to hear the soft parts. You could turn the volume up to hear the soft parts but then perhaps the loud parts will become too loud. You could play with the volume knob to turn it up during the soft parts and turn it down during the loud parts, making the soft parts louder, or the loud parts softer, or both. A compressor does just that: it changes the amplitude dynamics of the sound to bring the softer and louder parts closer together. An expander does the opposite.

A compressor / expander of dynamics changes the amplitude dynamics of sound, usually bringing the amplitude of soft and loud parts together (compressor), or taking them further apart (expander).

For example, if you are recording the vocals to a song and these vocals have quiet and loud parts, you could do of the following: 1) you would come closer to the microphone during the soft parts and pull back during louder parts; 2) you would have your engineer change your gain up and down during the song; 3) you would run your vocals through a compressor. All these are examples of compression.

Examples of compressors and sound samples

See Orinj Compressor, Orinj Limiter, Orinj Side chained compressor, and Orinj Simple compressor to hear sound samples before and after compression.

Characteristics of compressors

Compressors have:

• Threshold: The threshold is the amplitude level above which the compressor changes the amplitude of the sound and below which the compressor does nothing. The threshold is noted as compared to the maximum amplitude permitted by the system (e.g., "-10 dB"). If, for example, the threshold is -10 db, the compressor will decrease the amplitude of the signal when that amplitude is over -10 dB and will do nothing if the amplitude of the signal is under -10 dB.
• Compression ratio: This is the ratio by which the amplitude of the sound over the threshold is changed. Suppose that the threshold is -10 dB and the compression ratio is 2:1. If the amplitude of the signal is -6 dB, then the signal exceeds the threshold by 4 dB. Since the compression ratio is 2:1, the compressor will change amplitude of the signal, so that it exceeds the threshold by 2 dB instead of 4 dB. The new amplitude of the signal will be -8 dB.
• Attack: The attack is the time it takes for the compressor to reach the compression threshold. If, for example, the attack time is set at 10 milliseconds, the first 10 ms after the signal exceeds the threshold will be used to smoothly increase the desired compression ratio from having no compression.
• Release: The release the time it takes for the compressor to stop working after the sound level drops below the threshold. If, for example, the release time is 50 ms the compressor will decrease the compression ratio over 50 ms after the sound drops under the threshold until it reaches no compression.

The compression attack and release prevent distortion. If the compressor has attack and release that are equal to zero, it will act too quickly on the signal. It will then distort the peaks of the signal wave. As described below, the attack and release can also control how the signal transients pass through the compressor and can change the tone of the recording.

Characteristics of more complex compressors

More complex compressors have other characteristics.

• A compressor may have "separate compression / or expansion ratios above and below the threshold".
• A compressor may transition slowly from one compression ratio to another, which is known as having a soft knee, or the transition may be fast, which is known as having a hard knee (see below).
• A compressor may have "more than one threshold".
• A compressor may have attack and release times that are dependent on the level of the input signal.
• A compressor could have some "input or output gain" that will additionally amplify all sound that comes in or goes out (sometimes called "compensation gain").
• A compressor may respond to instantaneous peaks in the input signal, or may average the input gain over a short time, before computing whether the input gain is above or below the threshold.
• A compressor may link the various channels of the input signal and use compression on all channels whenever one channel needs to be compressed. Alternatively, a compressor may treat each channel independently.
• A "parallel" compressor is one that adds the original signal to the processed signal. This is useful when heavy compression with large compression ratios changes the characteristics of loud, compressed parts very audibly. By adding the original signal, some of these characteristics are preserved. There is still some compression of loud parts. The loud original signal parts are added to compressed parts, but quiet original signal parts are added to uncompressed (original) signal parts. Thus, loud and quiet parts are brought closer together. The amount of compression is reduced, but the result does not sound as processed. If more compression is needed, the output of several compressors can be added to a single original signal.
• Compression can make changes to one signal / track based on how another signal / track compares to the set thresholds ("side chaining", "side chained gate or compressor"; see below).
• Certain compressors may "look ahead." They will compress not based on what the signal looks like now but based on what the signal will look like after a few milliseconds. This allows the compressor to act quickly and even preemptively, thus creating much smoother dynamics.

Dynamic compression

Compressors control the amplitude of instruments and vocals. The compression threshold is set so that the loudest peaks of the recording exceed the threshold. The amplitude of these loud peaks is then reduced. The quieter parts of the recording pass through the compressor unchanged.

This dynamic compression may introduce distortion or accentuate different frequencies in the mix. For example, the amplitude of the signal computed by digital compressors is often not precise and does not increase or decrease smoothly (see Hilbert transform. The compressor quickly switches between compressing the signal and passing the signal uncompressed and distorts the peaks of the wave in the signal.

Dynamic compression is therefore used carefully, where the threshold is set so that only the very loud peaks are compressed. Gain staging (gain automation) is applied to the signal before the compressor so that the work of the compressor is minimal. The compression ratio can be significant (e.g., over 4:1), but multiple compressors, one after the other and with more gentle ratios, can also be used. The attack and release can be fast (e.g., 1-10 ms and 20 ms).

A very fast attack (e.g., 1 ms) or a look ahead that squashes all peaks of the signal is said to push the recording into the background. This is appropriate for some music styles (e.g., rock) but not typical of most other styles.

Tonal compression

Tonal compression controls how the signal transients pass through. Examples of transients can include the initial hit on a guitar string or vocal consonant peaks. The compression ratio is gentle (e.g., 1.5:1 or 2:1). The attack is slower (e.g., 15-20 ms), where an even slower attack (e.g., 30 ms) passes more of the transients and is said to make the recording more aggressive, and a faster attack passes less of the transients and is said to make the recording thicker and less aggressive. The release is also slow (e.g., 40 ms) and often set to match the tempo of the song. A soft knee can be used to make the compressor less pronounced.

De-essing

A multiband compressor is used to change the amplitude of specific frequency bands. This is typically used for de-essing, which can occur in several frequency intervals including around 3-4 Hz and 6-8 kHz but may be as low as 1.5 kHz and as high as 10 kHz. The compressor splits the signal into frequency bands and acts on these individual bands separately, only when the band itself exceeds a threshold.

Ducking

Compression can be used for "ducking". For example, a side chained compressor can decrease the volume of the bass during the hits of the drums, as is done in electronic dance music to make the drums more pronounced and the drum and bass mix less muddy. This compressor will begin to act every time the drums exceed some amplitude threshold, but instead of compressing the drums, will compress the bass. A similar side chained compressor can be used to decrease the amount of vocal reverb during pronounced vocal parts (syllables), thus making sure that the reverb does not smear the vocals. Such compressors effectively separate two instruments or two signals in the mix.

Limiting

A limiter is simply a compressor with a very high compression ratio and with very fast attack and release. A limiter usually has a compression ratio higher than 10:1, attack of, say, less than 5 ms, and a release of up to, say, 20 ms, although limiters vary. A limiter that has a compression ratio of 20:1 or over is known as a "hard" limiter or a "brick wall" limiter. Limiters are typically used to quickly reduce large signal peaks, so that the overall amplitude of the signal can be increased without overload and distortion.

Compressors in the mix chain

A typical mixing chain will include several compressors. For example, a vocal chain can add gain automation, a narrow equalizer, tonal compression, a wider tonal equalizer, dynamic compression, and a de-esser.