Dynamic range compression (DRC) or simply compression is an
audio signal processing
Audio signal processing is a subfield of signal processing that is concerned with the electronic manipulation of audio signals. Audio signals are electronic representations of sound waves— longitudinal waves which travel through air, consist ...
operation that reduces the volume of loud
sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by ...
s or amplifies quiet sounds, thus reducing or ''compressing'' an
audio signal
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...
's
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base- ...
. Compression is commonly used in
sound recording and reproduction
Sound recording and reproduction is the electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recordin ...
,
broadcasting
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began ...
,
live sound reinforcement and in some
instrument amplifier
An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
s.
A dedicated electronic hardware unit or audio software that applies compression is called a compressor. In the 2000s, compressors became available as software plugins that run in
digital audio workstation
A digital audio workstation (DAW) is an electronic device or application software used for recording, editing and producing audio files. DAWs come in a wide variety of configurations from a single software program on a laptop, to an integra ...
software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and limiting are identical in process but different in degree and perceived effect. A
limiter is a compressor with a high
ratio
In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
and, generally, a short
attack time.
Types
There are two types of compression, downward and upward. Both downward and upward compression ''reduce'' the
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base- ...
of an audio signal.
''Downward'' compression reduces the volume of loud sounds ''above'' a certain threshold. The quiet sounds ''below'' the threshold remain unaffected. This is the most common type of compressor. A
limiter can be thought of as an extreme form of downward compression as it compresses the sounds over the threshold especially hard.
''Upward'' compression increases the volume of quiet sounds ''below'' a certain threshold. The louder sounds ''above'' the threshold remain unaffected.
Some compressors also have the ability to do the ''opposite'' of compression, namely ''expansion''. Expansion ''increases'' the dynamic range of the audio signal. Like compression, expansion comes in two types, downward and upward.
''Downward'' expansion make the quiet sounds below the threshold even quieter. A
noise gate can be thought of as an extreme form of downward expansion as the noise gate make the quiet sounds (for instance: noise) quieter or even silent, depending on the floor setting.
''Upward'' expansion make the louder sounds above the threshold even louder.
Design
The signal entering a compressor is split; one copy is sent to a
variable-gain amplifier
A variable-gain (VGA) or voltage-controlled amplifier (VCA) is an electronic amplifier that varies its Gain (electronics), gain depending on a control voltage (often abbreviated CV).
VCAs have many applications, including audio level compression, ...
and the other to a ''side-chain'' where the signal level is measured and a circuit controlled by the measured signal level applies the required gain to the amplifier. This design, known as a ''feed-forward'' type, is used today in most compressors. Earlier designs were based on a ''feedback'' layout where the signal level was measured after the amplifier.
There are a number of technologies used for variable-gain amplification, each having different advantages and disadvantages.
Vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
s are used in a configuration called ''variable-mu'' where the grid-to-cathode voltage changes to alter the gain. Optical compressors use a
photoresistor
A photoresistor (also known as a photocell, or light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The resi ...
stimulated by a small lamp (
incandescent
Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...
,
LED, or
electroluminescent panel) to create changes in signal gain. Other technologies used include
field effect transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs con ...
s and a
diode bridge
A diode bridge is a bridge rectifier circuit of four diodes that is used in the process of converting alternating current (AC) from the input terminals to direct current (DC, i.e. fixed polarity) on the output terminals. Its function is to c ...
.
When working with digital audio,
digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner ar ...
(DSP) techniques are commonly used to implement compression as
audio plug-in
An audio plug-in, in computer software, is a plug-in that can add or enhance audio-related functionality in a computer program. Such functionality may include digital signal processing or sound synthesis. Audio plug-ins usually provide their o ...
s, in
mixing console
A mixing console or mixing desk is an electronic device for mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals from electric or electronic inst ...
s, and in
digital audio workstation
A digital audio workstation (DAW) is an electronic device or application software used for recording, editing and producing audio files. DAWs come in a wide variety of configurations from a single software program on a laptop, to an integra ...
s. Often the algorithms are used to emulate the above analog technologies.
Controls and features
A number of user-adjustable control parameters and features are used to adjust dynamic range compression signal processing algorithms and components.
Threshold
A compressor reduces the level of an audio signal if its amplitude exceeds a certain ''threshold''. Threshold is commonly set in
decibels
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a pow ...
(
dBFS for digital compressors and
dBu for hardware compressors), 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
Below may refer to:
*Earth
* Ground (disambiguation)
* Soil
* Floor
* Bottom (disambiguation)
* Less than
*Temperatures below freezing
* Hell or underworld
People with the surname
* Ernst von Below (1863–1955), German World War I general
* Fr ...
). 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.
Ratio
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% 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.
Attack and release
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.
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.
Soft and hard knees
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.
Peak vs RMS sensing
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
In mathematics and its applications, the root mean square of a set of numbers x_i (abbreviated as RMS, or rms and denoted in formulas as either x_\mathrm or \mathrm_x) is defined as the square root of the mean square (the arithmetic mean of th ...
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.
Stereo linking
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).
Make-up gain
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.
Look-ahead
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
Latency refers to a short period of delay (usually measured in milliseconds) between when an audio signal enters a system and when it emerges. Potential contributors to latency in an audio system include analog-to-digital conversion, buffering, d ...
through the processor.
Uses
Public spaces
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.
Music production
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).
[ Vocal performances in ]rock music
Rock music is a broad genre of popular music that originated as "rock and roll" in the United States in the late 1940s and early 1950s, developing into a range of different styles in the mid-1960s and later, particularly in the United States and ...
or pop music
Pop music is a genre of popular music that originated in its modern form during the mid-1950s in the United States and the United Kingdom. The terms ''popular music'' and ''pop music'' are often used interchangeably, although the former describ ...
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. 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.
Voice
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.
Compression is used in voice communications in amateur radio
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communi ...
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
DXing is the hobby of receiving and identifying distant radio or television signals, or making two-way radio contact with distant stations in amateur radio, citizens' band radio or other two-way radio communications. Many DXers also attempt to ...
. An SSB signal's strength depends on the level of modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...
. 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-talkie
A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gros ...
s and remote control dispatch consoles.
Broadcasting
Compression is used extensively in broadcasting
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began ...
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. 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
The European Broadcasting Union (EBU; french: Union européenne de radio-télévision, links=no, UER) is an alliance of public service media organisations whose countries are within the European Broadcasting Area or who are members of the Co ...
(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. , several European TV stations have announced their support for the new norm and over 20 manufacturers have announced products supporting the new ''EBU Mode'' loudness meters.
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.
Marketing
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.
Over-usage
Record companies, mixing engineers and mastering engineers have been gradually increasing the overall loudness
In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relation of phys ...
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
The loudness war (or loudness race) is a trend of increasing audio levels in recorded music, which reduces audio fidelity and—according to many critics—listener enjoyment. Increasing loudness was first reported as early as the 1940s, with ...
.
Other uses
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
In telecommunication and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expanding, ...
. This reduces the effects of a channel or recording medium with limited dynamic range.
Instrument amplifier
An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
s often include compression circuitry to prevent sudden high-wattage peaks that could damage the speakers. Electric bass
The bass guitar, electric bass or simply bass (), is the lowest-pitched member of the string family. It is a plucked string instrument similar in appearance and construction to an electric or an acoustic guitar, but with a longer neck and ...
players often use compression effects, either effects unit
An effects unit or effects pedal is an electronic device that alters the sound of a musical instrument or other audio source through audio signal processing.
Common effects include distortion/overdrive, often used with electric guitar in e ...
s available in pedal, rackmount
A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is wide. The 19 inch dimension includes the edges or "ears" that protrude from each side of the equ ...
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.
Hearing aid
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers su ...
s 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
Binaural literally means "having or relating to two ears." Binaural hearing, along with frequency cues, lets humans and other animals determine the direction and origin of sounds. Similar to diotic which is used in psychophysics to describe an audi ...
compression.
Compressors are also used for hearing protection in some electronic active hearing protection earmuffs
Earmuffs are clothing accessories or personal protective equipment designed to cover a person's ears for hearing protection or warmth. They consist of a thermoplastic or metal head-band that fits over the top or back of the head, and a cushion ...
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, 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.
Limiting
Compression and limiting are identical in process but different in degree and perceived effect. A limiter is a compressor with a high ratio and, generally, a fast attack time. Compression with ratio of 10:1 or more is generally considered limiting.
''Brick wall limiting'' has a very high ratio and a very fast attack time. Ideally, this ensures that an audio signal never exceeds the amplitude of the threshold. Ratios of 20:1 all the way up to ∞:1 are considered ''brick wall''. The sonic results of more than momentary and infrequent brick-wall limiting are harsh and unpleasant, thus it is more common as a safety device in live sound and broadcast applications.
Some bass amps and PA system
A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
amplifiers include limiters to prevent sudden volume peaks from causing distortion or damaging the speakers.
Side-chaining
A compressor with a side-chain input controls gain from main input to output based on the level of the signal at the side-chain input. An early innovator of side-chain compression in an effects unit was the Eventide Omnipressor from 1974. With side-chaining, the compressor behaves in the conventional manner when both main and side-chain inputs are supplied with the same signal. The side-chain input is used by disc jockey
A disc jockey, more commonly abbreviated as DJ, is a person who plays recorded music for an audience. Types of DJs include Radio personality, radio DJs (who host programs on music radio stations), club DJs (who work at a nightclub or music f ...
s for ducking – lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the side-chain input so that whenever the DJ speaks the compressor reduces the volume of the music. A sidechain with equalization controls can be used to reduce the volume of signals that have a strong spectral content within a certain frequency range: it can act as a de-esser, reducing the level of vocal sibilance in the range of 6–9 kHz. Another use of the side-chain in music production serves to maintain a loud bass track without the bass drum
The bass drum is a large drum that produces a note of low definite or indefinite pitch. The instrument is typically cylindrical, with the drum's diameter much greater than the drum's depth, with a struck head at both ends of the cylinder. T ...
causing undue peaks that result in loss of overall headroom.
Parallel compression
Inserting the compressor in a parallel signal path is known as parallel compression. It is a form of upward compression that facilitates dynamic control without significant audible side effects so long as the ratio is relatively low and the compressor's sound is relatively neutral. On the other hand, a high compression ratio with significant audible artifacts can be chosen in one of the two parallel signal paths. This is used by some concert mixers and recording engineers as an artistic effect called ''New York compression'' or ''Motown compression''. Combining a linear signal with a compressor and then reducing the output gain of the compression chain results in low-level detail enhancement without any peak reduction; The compressor significantly adds to the combined gain at low levels only.
Multiband compression
Multiband compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth compression is that problems related to a specific frequency range can be fixed without unnecessary compression in the other, unrelated frequencies. The downside is that frequency-specific compression is more complex and requires more processing capacity than full-bandwidth compression and can introduce phase issues.
Multiband compressors work by first splitting the signal through some number of band-pass filters
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects ( attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two- ...
, crossover filters or filter banks. Each split signal then passes through its own compressor and is independently adjustable for threshold, ratio, attack, and release. The signals are then recombined and an additional limiting circuit may be employed to ensure that the combined signals do not create unwanted peak levels.
In music production, multiband compressors are primarily an audio mastering
Mastering, a form of audio post production, is the process of preparing and transferring recorded audio from a source containing the final mix to a data storage device (the master), the source from which all copies will be produced (via meth ...
tool, but their inclusion in digital audio workstation
A digital audio workstation (DAW) is an electronic device or application software used for recording, editing and producing audio files. DAWs come in a wide variety of configurations from a single software program on a laptop, to an integra ...
plug-in sets is increasing their use among mix engineers.
On-air signal chains of radio station
Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radi ...
s commonly use multiband compressors to increase loudness
In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relation of phys ...
while avoiding overmodulation. Having a louder sound is often considered an advantage in commercial broadcasting.
Serial compression
Serial compression is a technique used in sound recording
Sound recording and reproduction is the electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recordin ...
and mixing. Serial compression is achieved by using two fairly different compressors in a signal chain. One compressor generally stabilizes the dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base- ...
while the other aggressively compresses stronger peaks. This is the normal internal signal routing in common combination devices marketed as ''compressor-limiters'', where an RMS compressor (for general gain control) is followed by a fast peak-sensing limiter (for overload protection). Done properly, even heavy serial compression can sound natural in a way not possible with a single compressor. It is most often used to even out erratic vocal
The human voice consists of sound made by a human being using the vocal tract, including talking, singing, laughing, crying, screaming, shouting, humming or yelling. The human voice frequency is specifically a part of human sound producti ...
s and guitar
The guitar is a fretted musical instrument that typically has six strings. It is usually held flat against the player's body and played by strumming or plucking the strings with the dominant hand, while simultaneously pressing selected string ...
s.
Software audio players
Some software audio players support plugins
Plug-in, plug in or plugin may refer to:
* Plug-in (computing) is a software component that adds a specific feature to an existing computer program.
** Audio plug-in, adds audio signal processing features
** Photoshop plugin, a piece of software t ...
that implement compression. These can increase loudness of audio tracks, or level out the volume of highly-variable music (such as classical music, or a playlist that spans multiple music types). This improves the listenability of audio played through poor-quality speakers, or when played in noisy environments (such as in a car or during a party).
Objective influence on the signal
In an article published in January 2014 by the ''Journal of the Audio Engineering Society'', Emmanuel Deruty and Damien Tardieu performed a systematic study describing the influence of compressors and brickwall limiters on the musical audio signal. The experiment involved four software limiters: Waves L2, Sonnox Oxford Limiter, Thomas Mundt’s Loudmax, Blue Cat’s Protector, as well as four software compressors: Waves H-Comp, Sonnox Oxford Dynamics, Sonalksis SV-3157, and URS 1970. The study provides objective data on what limiters and compressors do to the audio signal.
Five signal descriptors were considered: RMS power, EBU R 128 integrated loudness, crest factor, R 128 LRA, and density of clipped samples. RMS power accounts for the signal's physical level, R 128 loudness for the perceived level. The crest factor, which is the difference between the signal's peak and its average power, is on occasions considered as a basis for the measure of micro-dynamics, for instance in the ''TT Dynamic Range Meter'' plug-in. Finally, R 128 LRA has been repeatedly considered as a measure of macro-dynamics or dynamics in the musical sense.
Limiters
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.
Compressors
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.
See also
* Squelch
In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a strong input signal. Essentially, squelch is a specialized type of noise gate designed to suppress weak ...
* Automatic gain control
Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...
* LA-2A Leveling Amplifier
* 1176 Peak Limiter
* Tone mapping, the photographic equivalent
References
External links
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Dynamic range compression
Article on Optical Compressors
from Sweetwater Sound
Sweetwater is the largest online retailer of musical instruments and pro audio equipment in the United States, based out of Fort Wayne, Indiana.
In August 2021, it was announced that its founder had stepped down as CEO after Providence Equity ...
Information on Compression in Home Recording
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Audio engineering
Dynamics processing