Dynamic range compression

A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. Threshold is commonly set in decibels (dBFS for digital compressors and dBu for hardware compressors),[8] where a lower threshold (e.g. −60 dB) means a larger portion of the signal is treated. When the signal level is below the threshold, no processing is performed and the input signal is passed, unmodified, to the output. Thus a higher threshold of, e.g., −5 dB, results in less processing, less compression.

Threshold timing behavior is subject to attack and release settings (see below). When the signal level goes above threshold, compressor operation is delayed by the attack setting. For an amount of time determined by the release after the input signal has fallen below the threshold, the compressor continues to apply dynamic range compression.

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 is reduced to 1 dB over the threshold. The gain and output level has been reduced by 3 dB. Another way of stating this is that any input signal level over the threshold will, in this case, be output at a level which is only 25% (i.e. 1 over 4) as much over the threshold as its input level was.

The highest ratio of ∞ :1 is often known as limiting, and effectively denotes that any signal above the threshold is brought down to the threshold level once the attack time has expired.

The attack and release phases in a compressor

A compressor may provide a degree of control over how quickly it acts. The attack is the period when the compressor is decreasing gain in response to increased level at the input to reach the gain determined by the ratio. The release is the period when the compressor is increasing gain in response to reduced level at the input to reach the output gain determined by the ratio, or, to unity, once the input level has fallen below the threshold. Because the loudness pattern of the source material is modified by the time-varying operation of compressor, it may change the character of the signal in subtle to quite noticeable ways depending on the attack and release settings used.

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 takes for the gain to change a set amount of dB or a set percentage towards the target gain. There is no industry standard for the exact meaning of these time parameters.[9]

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 cannot be adjusted. Sometimes the attack and release times are automatic or program dependent, meaning that the behavior may change depending on the input signal.

Hard Knee and Soft Knee compression

Another control a compressor might offer is hard knee or soft knee selection. This controls whether the bend in the response curve between below threshold and above threshold is abrupt (hard) or gradual (soft). 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 potentially audible transition from uncompressed to compressed, and is especially applicable for higher ratio settings where the changeover at the threshold would be more noticeable.[10]

A peak-sensing compressor responds to the peak level of the input signal. While providing tighter peak level control, peak level sensing does not necessarily relate to human perception of loudness. Some compressors apply a power measurement function (commonly root mean square or RMS) on the input signal before comparing its level to the threshold. This produces a more relaxed compression that more closely relates to human perception of loudness.

A compressor in stereo linking mode applies the same amount of gain reduction to both the left and right channels. This is done to prevent image shifting that can occur if each channel is compressed individually. This becomes particularly noticeable when a loud element that is panned close to either edge of the stereo field raises the level of the program to the compressor's threshold, causing its image to shift toward the center of the stereo field.

Stereo linking can be achieved in two ways: The compressor uses the sum of the left and right inputs to produce a single measurement that drives the compressor; or, the compressor calculates the required amount of gain reduction independently for each channel and then applies the highest amount of gain reduction to both (in such case it could still make sense to dial different settings on left and right channels as one might wish to have less compression for left-side events[11]).

Because a downward compressor only reduces the level of the signal, the ability to add a fixed amount of make-up gain at the output is usually provided so that an optimum output level is produced.

The look-ahead function is designed to overcome the problem of being forced to compromise between slow attack rates that produce smooth-sounding gain changes, and fast attack rates capable of catching transients. Look-ahead is implemented by splitting the input signal and delaying one side (the audio signal) by the look-ahead time. The non-delayed side (the gain control signal) is used to drive the compression of the delayed signal, which then appears at the output. This way a smooth-sounding slower attack rate can be used to catch transients. The cost of this solution is added audio latency through the processor.

Compression is often applied in audio systems for restaurants, retail, and similar public environments that play background music at a relatively low volume and need it compressed, not just to keep the volume fairly constant, but also to make quiet parts of the music audible over ambient noise.

Compression can increase average output gain of a power amplifier by 50 to 100% with a reduced dynamic range. For paging and evacuation systems, this adds clarity under noisy circumstances and saves on the number of amplifiers required.

An inexpensive guitar compressor

Compression is often used in music production to make instruments more consistent in dynamic range, so that they "sit" more nicely in the mix with the other instruments (neither disappear during short periods of time, nor overpower the other instruments during short periods).[12] Vocal performances in rock music or pop music are compressed for the same reason.

Compression can also be used on instrument sounds to create effects not primarily focused on stabilizing the volume. For instance, drum and cymbal sounds tend to decay quickly, but a compressor can make the sound appear to have a more sustained tail. Guitar sounds are often compressed to produce a fuller, more sustained sound.

Most devices capable of compressing audio dynamics can also be used to reduce the volume of one audio source when another audio source reaches a certain level; this is called side-chaining.[13] In electronic dance music, side-chaining is often used on basslines, controlled by the kick drum or a similar percussive trigger, to prevent the two from conflicting, and provide a pulsating, rhythmic dynamic to the sound.

A compressor can be used to reduce sibilance ('ess' sounds) in vocals (de-essing) by feeding the compressor's side-chain an equalized version of the input signal, so that specific, sibilance-related frequencies (typically 4000 to 8000 hz) activate the compressor more.[14]

Compression is used in voice communications in amateur radio that employ single-sideband (SSB) modulation to make a particular station's signal more readable to a distant station, or to make one's station's transmitted signal stand out against others. This is applicable especially in DXing. An SSB signal's strength depends on the level of modulation. A compressor increases the average level of the modulation signal thus increasing the transmitted signal strength. Most modern amateur radio SSB transceivers have speech compressors built-in. Compression is also used in land mobile radio, especially in transmitted audio of professional walkie-talkies and remote control dispatch consoles.[15]

Compression is used extensively in broadcasting to boost the perceived volume of sound while reducing the dynamic range of source audio. To avoid overmodulation, broadcasters in most countries have legal limits on instantaneous peak volume they may broadcast. Normally these limits are met by permanently inserted compression hardware in the on-air chain.

Broadcasters use compressors in order that their station sounds louder than comparable stations. The effect is to make the more heavily compressed station jump out at the listener at a given volume setting.[12] This is not limited to inter-channel differences; they also exist between programme material within the same channel. Loudness differences are a frequent source of audience complaints, especially TV commercials and promos that seem too loud.

The European Broadcasting Union (EBU) has been addressing this issue in the EBU PLOUD group, which consists of over 240 audio professionals, many from broadcasters and equipment manufacturers. In 2010, the EBU published EBU R 128 which introduces a new way of metering and normalizing audio. The Recommendation uses ITU-R BS.1770 loudness metering. As of 2016, several European TV stations have announced their support for the new norm[16][17] and over 20 manufacturers have announced products supporting the new EBU Mode loudness meters.[18]

To help audio engineers understand what loudness range their material consists of (e.g. to check if some compression may be needed to fit it into the channel of a specific delivery platform), the EBU also introduced the Loudness Range (LRA) descriptor.[19]

Most television commercials are heavily compressed to achieve near-maximum perceived loudness while staying within permissible limits. This causes a problem that TV viewers often notice: when a station switches from minimally compressed program material to a heavily compressed commercial, the volume sometimes seems to increase dramatically. Peak loudness might be the same—meeting the letter of the law—but high compression puts much more of the audio in the commercial at close to the maximum allowable, making the commercial seem much louder.[20]

The trend of increasing loudness as shown by waveform images of the song "Something" by The Beatles mastered on CD four times since 1983.

Record companies, mixing engineers and mastering engineers have been gradually increasing the overall loudness of commercial albums. This is achieved by using higher degrees of compression and limiting during mixing and mastering; compression algorithms have been engineered specifically to accomplish the task of maximizing audio level in the digital stream. Hard limiting or clipping can result, affecting the tone and timbre of the music. The effort to increase loudness has been referred to as the loudness war.

Noise reduction systems use a compressor to reduce the dynamic range of a signal for transmission or recording, expanding it afterward, a process called companding. This reduces the effects of a channel or recording medium with limited dynamic range.

Instrument amplifiers often include compression circuitry to prevent sudden high-wattage peaks that could damage the speakers. Electric bass players often use compression effects, either effects units available in pedal, rackmount units, or built-in devices in bass amps, to even out the sound levels of their basslines.

Gain pumping, where a regular amplitude peak (such as a kick drum) causes the rest of the mix to change in volume due to the compressor, is generally avoided in music production. However, many dance and hip-hop musicians purposefully use this phenomenon, causing the mix to alter in volume rhythmically in time with the beat.[21]

Hearing aids use a compressor to bring the audio volume into the listener's hearing range. To help the patient perceive the direction sound comes from, some hearing aids use binaural compression.[22]

Compressors are also used for hearing protection in some electronic active hearing protection earmuffs and earplugs, to let sounds at ordinary volumes be heard normally while attenuating louder sounds, possibly also amplifying softer sounds. This allows, for example, shooters wearing hearing protection at a shooting range to converse normally, while sharply attenuating the much louder sounds of the gunshots,[23] and similarly for musicians to hear quiet music but be protected from loud noises such as drums or cymbal crashes.

In applications of machine learning where an algorithm is training on audio samples, dynamic range compression is a way to augment samples for a larger data set.[24]

The tested limiters had the following influence on the signal:

• increase of RMS power,
• increase of EBU R 128 loudness,
• decrease of crest factor,
• decrease of EBU R 128 LRA, but only for high amounts of limiting,
• increase of clipped sample density.

In other words, limiters increase both physical and perceptual levels, increase the density of clipped samples, decrease the crest factor and decrease macro-dynamics (LRA) given that the amount of limiting is substantial.

As far as the compressors are concerned, the authors performed two processing sessions, using a fast attack (0.5 ms) in one case, and a slow attack (50 ms) in the other. Make-up gain is deactivated, but the resulting file is normalized.

Set with a fast attack, the tested compressors had the following influence on the signal:

• slight increase of RMS power,
• slight increase of EBU R 128 loudness,
• decrease of crest factor,
• decrease of EBU R 128 LRA,
• slight decrease of clipped sample density.

In other words, fast-attack compressors increase both physical and perceptual levels, but only slightly. They decrease the density of clipped samples, and decrease both crest factor and macro-dynamics.

Set with a slow attack, the tested compressors had the following influence on the signal:

• decrease of RMS power,
• decrease of EBU R 128 loudness,
• no influence on crest factor,
• decrease of EBU R 128 LRA,
• no influence on clipped sample density.

In other words, slow-attack compressors decrease both physical and perceptual levels, decrease macro-dynamics, but have no influence on crest factor and clipped sample density.