Dynamic Range Compression reduces the dynamic range of a signal.

Note this does not necessarily mean a reduction in volume. Effectively compression can be thought of as an automated volume control, raising the level of quiet parts of the part, and lowering peaks.

Why compress?

In medieval times, compression was originally designed as part of the process of producing vinyl. as large swings in volume could cause the cutting stylus to jump or depress too strongly into the vinyl. On playback too, excessive dynamic range could cause needle jumps, as could swift changes in panning.

The usable dynamic range of magnetic is approx 55-60dB and so when tape became the mainstream medium for recording, compression was still necessary to keep musical program material within this range. Overloading would cause analog saturation, a desirable by-product of tape recording, and very low signals would be drowned in the noise inherent to the medium.

This same analogy now applies to digital recording techniques, with overloads causing unpleasant digital clipping, and low signals becoming lost in the noise floor.

In modern times, the technique has come to encompass many different creative reasons for use over and above practical reasons.

Modern & Creative Applications of Compression

The primary use is still dynamic range reduction but for creative purposes. This involves raising the level of quiet parts of the part, and lowering peaks, for example to get an overall higher average volume level on a part, to make vocals more predominant, or to tame volume excesses on instruments with higher dynamic ranges.

For example in a vocal track, quiet passages may be inaudible over full instrumentation, and insufficient microphone control could cause loud passages to be too strident and jarring. Judicious use of a compressor to the vocal track averages out these problems and allows a mix engineer to get a better mean average volume which he can raise and lower within the context of the track, rather than riding the volume fader to try and iron out these inconsistencies.

Similarly modern rock snare is highly compressed to form the "backbeat" of a drum track; this needs to be consistent and powerful, so compression to make the envelope of the sound the same each time while retaining performance characteristics is a universal technique.

There are many other uses and applications of compressors but the scope of these is outwith this article which is designed to describe the concept and parameters of compression to help you gain an understanding. A new article entitled Creative Use of Compressors should be created.

Parameters of Compression

Much of the following content is paraphrased from Wikipedia.

A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. The amount of gain reduction is determined by ratio: a ratio of 4:1 means that, if input level is 4 dB over the threshold, the output signal level will be 1 dB over the threshold. The gain (level) has been reduced by 3 dB:

Threshold = −10 dB
Input = −6 dB (4 dB above the threshold)
Output = −9 dB (1 dB above the threshold)

Compressors are, additionally, often supplied with attack and release controls that can slow down the response speed of the circuit to smooth the effect.

Threshold

Threshold is the level above which the signal is reduced. It is commonly set in dB, where a lower threshold (e.g. -60 dB) means a larger portion of the signal will be treated (compared to a higher threshold of -5 dB).

Ratio

The ratio determines the input/output ratio for signals above the threshold. For example, a 4:1 ratio means that a signal overshooting the threshold by 4 dB will leave the compressor 1 dB above the threshold. The highest ratio of ∞:1 is commonly achieved using a ratio of 60:1, and effectively denotes that any signal above the threshold will be brought down to the threshold level (except briefly after a sudden increase in input loudness, known as an "attack").

Attack and release

A compressor might provide a degree of control over how quickly it acts. The 'attack phase' is the period when the compressor is decreasing gain to reach the level that is determined by the ratio. The 'release phase' is the period when the compressor is increasing gain to the level determined by the ratio, or, to zero dB, once the level has fallen below the threshold. The length of each period is determined by the rate of change and the required change in gain. For more intuitive operation, a compressor's attack and release controls are labeled as a unit of time (often milliseconds). This is the amount of time it will take for the gain to change a set amount of dB, decided by the manufacturer, very often 10 dB. For example, if the compressor's time constants are referenced to 10 dB, and the attack time is set to 1 ms, it will take 1 ms for the gain to decrease by 10 dB, and 2 ms to decrease by 20 dB.

In many compressors the attack and release times are adjustable by the user. Some compressors, however, have the attack and release times determined by the circuit design and these cannot be adjusted by the user. Sometimes the attack and release times are 'automatic' or 'program dependent', meaning that the times change depending on the input signal. Because the loudness pattern of the source material is modified by the compressor it may change the character of the signal in subtle to quite noticeable ways depending on the settings used.

Soft and hard knees

Another control a compressor might offer is hard/soft knee. This controls whether the bend in the response curve is a sharp angle or has a rounded edge. A soft knee slowly increases the compression ratio as the level increases and eventually reaches the compression ratio set by the user. A soft knee reduces the audible change from uncompressed to compressed, especially for higher ratios where the changeover is more noticeable.

Limiting

A limiter is a compressor designed to prevent overloads, whether this is overloading the maximum SPL handling of a speaker, or overloading a digital input. Effectively it is a compressor with an infinite compression ratio and a very fast attack. See limiters for more details.