Audio level compression
Encyclopedia
Dynamic range compression, also called DRC (often seen in DVD and car CD player settings) or simply compression reduces the volume of loud sound
s or amplifies quiet sounds by narrowing or "compressing" an audio signal
's dynamic range
. Compression is commonly used in sound recording and reproduction
and broadcasting
.
The dedicated electronic hardware unit or audio software used to apply compression is called a compressor. Compressors often have attack and release controls that vary the rate at which compression is applied and smooth the effect.
An expander performs the opposite function, increasing the dynamic range of the audio signal. Expanders are generally used to make quiet sounds even quieter by reducing the level of an audio signal
that falls below a set threshold level. A noise gate
is a type of expander.
The signal entering a compressor is split, one copy sent to a variable-gain amplifier
and the other to a side-chain where a circuit controlled by the signal level applies the required gain to an amplifier stage. 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 feeding the control circuit was taken after the amplifier.
There are a number of technologies used for variable gain amplification, each having different advantages and disadvantages. Vacuum tube
s are used in a configuration called 'variable-mu': the grid-to-cathode voltage changes to alter the gain. Also used is a voltage controlled amplifier (VCA), which has its gain reduced as the power of the input signal increases. Optical compressors use a light sensitive resistor
(LDR) and a small lamp (LED
or electroluminescent
panel) to create changes in signal gain. This technique is believed by some to add smoother characteristics to the signal because the response times of the light and the resistor soften the attack and release. Other technologies used include field effect transistors and a diode bridge
.
When working with digital audio, digital signal processing
techniques are commonly used to implement compression via digital audio editor
s, or dedicated workstations
. Often the algorithms used emulate the above analog technologies.
over the threshold, the output signal level will be 1 dB over the threshold. The gain (level) has been reduced by 3 dB:
The highest ratio of ∞:1 is often known as 'limiting'. It 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").
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.
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.
) on the input signal before its level is compared to the threshold. This allows a more relaxed compression that also more closely relates to our perception of loudness.
Stereo linking can be achieved in two ways: Either the compressor sums to mono the left and right channel at the input, then only the left channel controls are functional; or, the compressor still calculates the required amount of gain reduction independently for each channel and then apply the highest amount of gain reduction to both (in such case it could still make sense to dial different setting on left and right channels as one might wish to have less compression for left-side events).
Engineers wishing to achieve dynamic range reduction with few obvious effects might choose a relatively low threshold and low compression ratio so that the source material is being compressed very slightly most of the time. To deliberately soften the attack of a snare drum, they might choose a fast attack time and a moderately fast release time combined with a higher threshold. To accentuate the attack of the snare, they might choose a slower attack time to avoid affecting the initial transient. It is easier to apply these controls with a basic knowledge of acoustics.
Compression and limiting are not different in process but in degree and perceived effect. A limiter
is a compressor with a high ratio and, generally, a fast attack time. Most engineers consider compression with ratio of 10:1 or more as limiting. Engineers sometimes refer to soft and hard limiting which are differences of degree. The "harder" a limiter, the higher its ratio and the faster its attack and release times.
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 to be 'brick wall'. The sonic results of more than momentary and infrequent hard/brick-wall limiting are harsh and unpleasant, thus it is more common as a safety device in live sound and broadcast applications.
Some modern consumer electronics devices incorporate limiters. Sony
uses the Automatic Volume Limiter
System (AVLS), on some audio products and the PlayStation Portable
.
When side-chaining, the compressor uses the volume level of an input signal to determine how strongly the compressor will reduce the gain on its output signal. This is used by disc jockey
s for ducking
– lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the compressor's sidechain 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. A de-esser helps reduce high frequencies that tend to overdrive preemphasized
media (such as phonograph records and FM radio). Another use of the side-chain in music production serves to maintain a loud bass track without the bass drum
causing undue peaks that result in loss of overall headroom.
A linked stereo compressor without a sidechain input can be used as a mono compressor with a sidechain input. The "key", "trigger" or sidechain signal is sent to the left input of the stereo compressor while the signal that is to be compressed is routed through the right channel.
This technique is not to be confused with Parallel compression
, which was referred to as "side chain" compression in a 1977 Studio Sound article by Mike Bevelle.
and can give a measure of dynamic control without significant audible side effects, if 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 (since the compressor will significantly add to the combined gain at low levels only). This will often be beneficial when compressing transient content, since high-level dynamic liveliness is still maintained, despite the overall dynamic range reduction.
Multiband (also spelled multi-band) compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth (full-band, or single-band) compression is that unneeded audible gain changes or "pumping" in other frequency bands is not caused by changing signal levels in a single frequency band.
Multiband compressors work by first splitting the signal through some number of bandpass filters or crossover filters. The frequency ranges or crossover frequencies may be adjustable. 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 effects do not create unwanted peak levels.
Software plug-ins or DSP
emulations of multiband compressors can be complex, with many bands, and require corresponding computing power.
Multiband compressors are primarily an audio mastering
tool, but their inclusion in digital audio workstation
plug-in sets is increasing their use among mix engineers. Hardware multiband compressors are also commonly used in the on-air signal chain of a radio station
, either AM or FM, in order to increase the station's apparent loudness
without fear of overmodulation
. Having a louder sound is often considered an advantage in commercial competition. However, adjusting a multiband output compressor of a radio station also requires some artistic sense of style, plenty of time and a good pair of ears. This is because the constantly changing spectral balance between audio bands may have an equalizing effect on the output, by dynamically modifying the on-air frequency response. A further development of this approach is programmable radio output processing, where the parameters of the multiband compressor automatically change between different settings according to the current programme block style or the time of day.
and mixing
. Serial compression is achieved by using two fairly different compressors
in a signal chain. One compressor will generally stabilize the dynamic range
while the other will more aggressively compress 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) would be directly followed by a fast peak sensing limiter (for overload protection). Done properly, even heavy serial compression can sound very natural in a way not possible with a single compressor. It is most often used to even out erratic vocals and guitar
s.
Compression is often applied in this manner in audio systems for restaurants, retail, and similar public environments, where background music is played at a relatively low volume and needs to be compressed not just to keep the volume fairly constant, but also in order for relatively quiet parts of the music to be audible over ambient noise, or audible at all.
Compression can be used to increase the 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.
or pop music
are usually compressed in order to make them stand out from the surrounding instruments and to add to the clarity of the vocal performance.
Compression can also be used on instrument sounds to create effects not primarily focused on boosting loudness
. 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 in order to obtain 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; see Side-Chaining above.
('ess' sounds) in vocals by feeding the compressor with an EQ set to the relevant frequencies, so that only those frequencies activate the compressor. If unchecked, sibilance could cause distortion even if sound levels are not very high. This usage is called 'de-essing
'. http://alesis.com/support/notes/Signal_Processing/Compapp.html
Compression is used in voice communications in amateur radio
that employ SSB modulation
. Often it is used 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 occurs especially in pileup
s where amateur radio stations are competing for the opportunity to talk to a DX station. Since an SSB signal's amplitude
depends on the level of modulation
, the net result is that the average amplitude of the signal and hence average transmitted power would be stronger than it would be had compression not been used. Most modern amateur radio SSB transceivers have speech compressors built in.
Compression is also used in land mobile radio
, especially in transmit audio of professional walkie-talkie
s and in remote control dispatch consoles
.
to boost the perceived volume of sound while reducing the dynamic range of source audio (typically CDs) to a range that can be accommodated by the narrower-range broadcast signal. Broadcasters in most countries have legal limits on instantaneous peak volume they may broadcast. Normally these limits are met by permanently inserted hardware in the on-air chain (see multiband compression above).
As was alluded to above, the use of compressors to boost perceived volume is a favorite trick of broadcasters who want their station to sound "louder" at the same volume than comparable stations on the dial. The effect is to make the more heavily compressed station "jump out" at the listener at a given volume setting.
But loudness jumps are 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 which are known to be 'too loud'. One complicating factor is that many broadcasters use (quasi-)peak meters and peak-levelling. Unfortunately the peak level reading does not correlate very well with the perceived loudness. It basically should only be used to prevent overmodulation.
The European Broadcasting Union has been addressing this issue in the EBU PLOUD Group, which consist of over 240 audio professionals, many from broadcasters and equipment manufacturers. In 2010, the EBU published EBU Recommendation R 128, which introduces a new way of metering and normalising audio. The Recommendation is based on ITU-R BS.1770 and could be called a real 'audio revolution'. Several European TV stations have already announced to support the new norm and over 20 manufacturers have announced products supporting the new 'EBU Mode' Loudness meters (EBU Tech 3341).
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 Descriptor (EBU Tech 3342).
Heavy compression also complemented the style of 60s DJs who talked/shouted over the music. With the proper setting, a DJ could be "mixed" into the music, rather than being heard over it. This demanded that DJs deliver their patter with a very loud voice to be heard over the music, which added to the energy of the broadcast sound. This allowed DJs to talk "in" rather than over the music without being as intrusive.
As rock became prevalent on FM in the mid-60s, the CBS Volumax/Audimax was one legendary compression rig used, favored because it only "expanded" (lifting soft volume) if any existed. Consequently, it would not expand an unmodulated signal, avoiding the boosting of the noise floor (hiss) as many previous units did. However, it could create an annoying "sucking and pumping" effect (compression and expansion) if driven too severely.
In its effort to deliver a constant modulation (volume level) to the listener, compression works against the wider dynamic range of FM (as compared to AM) which was traditionally one of FM's obvious strong points. Consequently, the so-called "album rock" stations of the 70s and classical music and "easy listening" stations of that era in particular, avoided heavy compression. Classical stations hardly use any, which explains why a classical listener, particularly in the car, must keep turning the volume up and down, constantly fighting the ambient noise prevalent in car listening.
The same recording can have very different dynamics when heard via AM
, FM
, CD, or other media (although frequency response
and noise
are large factors as well).
Record companies, mixing engineers and mastering engineers have been gradually increasing the overall volume of commercial albums. The greater loudness 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 wars".
Most television commercials are compressed heavily (typically to a dynamic range of no more than 3dB) in order to achieve near-maximum perceived loudness while staying within permissible limits. This is the explanation for a problem that TV viewers and listeners have noticed for years: While commercials receive heavy compression for the same reason that radio broadcasters have traditionally used it (to achieve a "loud" audio image), TV program material, particularly old movies with soft dialogue, is comparatively uncompressed by TV stations. This results in commercials much louder than the television programs, since users turn up the volume to hear soft program audio.
.
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.
A compressor is used in hearing aids to bring the audio volume into the listener's hearing range. To enable the patient to perceive the direction from which the sound is coming, binaural
compression may be required.
Compressors are also used in some active sound protection earmuffs, to allow sounds at ordinary volumes to be heard normally while attenuating louder 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.
support plugins which implement compression. These can be used to increase the perceived volume of audio tracks, or to even out the volume of highly-variable music (such as classical music, or a playlist spanning many music types). This improves the listenability of audio when played through poor-quality speakers, or when played in noisy environments (such as in a car or during a party). Such software may also be used in micro-broadcasting or home-based audio mastering.
Available software includes:
To achieve volume-compressed playback on devices other than computer-based audio players, files may need to be processed via the above software then output as wav
s, mp3
s, or other audio formats.
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
s or amplifies quiet sounds by narrowing or "compressing" an audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
's dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
. Compression is commonly used in sound recording and reproduction
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
and broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
.
The dedicated electronic hardware unit or audio software used to apply compression is called a compressor. Compressors often have attack and release controls that vary the rate at which compression is applied and smooth the effect.
Types
Downward compression reduces loud sounds over a certain threshold while quiet sounds remain unaffected. Upward compression increases the loudness of sounds below a threshold while leaving louder passages unchanged. Both downward and upward compression reduce the dynamic range of an audio signal.An expander performs the opposite function, increasing the dynamic range of the audio signal. Expanders are generally used to make quiet sounds even quieter by reducing the level of an audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
that falls below a set threshold level. A noise gate
Noise gate
A Noise Gate or gate is an electronic device or software that is used to control the volume of an audio signal. In its most simple form, a noise gate allows a signal to pass through only when it is above a set threshold: the gate is 'open'. If the signal falls below the threshold no signal is...
is a type of expander.
Design
Variable-gain amplifier
A variable-gain or voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage .VCAs have many applications, including audio level compression, synthesizers and amplitude modulation....
and the other to a side-chain where a circuit controlled by the signal level applies the required gain to an amplifier stage. 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 feeding the control circuit was taken after the amplifier.
There are a number of technologies used for variable gain amplification, each having different advantages and disadvantages. Vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s are used in a configuration called 'variable-mu': the grid-to-cathode voltage changes to alter the gain. Also used is a voltage controlled amplifier (VCA), which has its gain reduced as the power of the input signal increases. Optical compressors use a light sensitive resistor
Photoresistor
A photoresistor or light dependent resistor is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor or CdS device, from "cadmium sulfide," which is the material from which the device is made and that actually exhibits the...
(LDR) and a small lamp (LED
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
or electroluminescent
Electroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
panel) to create changes in signal gain. This technique is believed by some to add smoother characteristics to the signal because the response times of the light and the resistor soften the attack and release. Other technologies used include field effect transistors and a diode bridge
Diode bridge
A diode bridge is an arrangement of four diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input. When used in its most common application, for conversion of an alternating current input into direct current a output, it is known as a...
.
When working with digital audio, digital signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
techniques are commonly used to implement compression via digital audio editor
Digital audio editor
A digital audio editor is a computer application for audio editing, i.e. manipulating digital audio. Digital audio editors are the main software component of a digital audio workstation.-For use with music:...
s, or dedicated workstations
Digital audio workstation
A digital audio workstation is an electronic system designed solely or primarily for recording, editing and playing back digital audio. DAWs were originally tape-less, microprocessor-based systems such as the Synclavier and Fairlight CMI...
. Often the algorithms used emulate the above analog technologies.
Compressor controls and features
Threshold
A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. 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 amount of gain reduction is determined by ratio: a ratio of 4:1 means that if input level is 4 dBDecibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
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)
The highest ratio of ∞:1 is often known as 'limiting'. It 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
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
Peak vs RMS sensing
A peak sensing compressor responds to the instantaneous level of the input signal. While providing tighter peak control, peak sensing might yield very quick changes in gain reduction, more evident compression or sometimes even distortion. Some compressors will apply an averaging function (commonly RMSRoot mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
) on the input signal before its level is compared to the threshold. This allows a more relaxed compression that also more closely relates to our 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 both channels are compressed individually. It becomes noticeable when a loud element that is panned 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: Either the compressor sums to mono the left and right channel at the input, then only the left channel controls are functional; or, the compressor still calculates the required amount of gain reduction independently for each channel and then apply the highest amount of gain reduction to both (in such case it could still make sense to dial different setting on left and right channels as one might wish to have less compression for left-side events).
Makeup gain
Because the compressor is reducing the gain (or 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 level can be used.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 a misnomer in that the future is not actually observed. Instead, the input signal is split, and one side is delayed. The non-delayed 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 that the signal is delayed.Use
An audio engineer might use a compressor to reduce the dynamic range of source material in order to allow it to be recorded optimally on a medium with a more limited dynamic range than the source signal or else in order to change the character of an instrument being processed.Engineers wishing to achieve dynamic range reduction with few obvious effects might choose a relatively low threshold and low compression ratio so that the source material is being compressed very slightly most of the time. To deliberately soften the attack of a snare drum, they might choose a fast attack time and a moderately fast release time combined with a higher threshold. To accentuate the attack of the snare, they might choose a slower attack time to avoid affecting the initial transient. It is easier to apply these controls with a basic knowledge of acoustics.
Limiting
Compression and limiting are not different in process but in degree and perceived effect. A limiter
Limiter
In electronics, a limiter is a circuit that allows signals below a specified input power to pass unaffected while attenuating the peaks of stronger signals that exceed this input power....
is a compressor with a high ratio and, generally, a fast attack time. Most engineers consider compression with ratio of 10:1 or more as limiting. Engineers sometimes refer to soft and hard limiting which are differences of degree. The "harder" a limiter, the higher its ratio and the faster its attack and release times.
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 to be 'brick wall'. The sonic results of more than momentary and infrequent hard/brick-wall limiting are harsh and unpleasant, thus it is more common as a safety device in live sound and broadcast applications.
Some modern consumer electronics devices incorporate limiters. Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
uses the Automatic Volume Limiter
Automatic Volume Limiter
Automatic volume limiter , or sometimes automatic volume limiter system is an option that limits the maximum volume level and is enabled through software or hardware in music players such as, cassette players, MP3 players, CD players or other devices used with headphones such as the Sony PSP...
System (AVLS), on some audio products and the PlayStation Portable
PlayStation Portable
The is a handheld game console manufactured and marketed by Sony Corporation Development of the console was announced during E3 2003, and it was unveiled on , 2004, at a Sony press conference before E3 2004...
.
Side-chaining
Disc jockey
A disc jockey, also known as DJ, is a person who selects and plays recorded music for an audience. Originally, "disc" referred to phonograph records, not the later Compact Discs. Today, the term includes all forms of music playback, no matter the medium.There are several types of disc jockeys...
s for ducking
Ducking
Ducking is an effect commonly used in radio and pop music, especially dance music. It is an effect where the level of one signal is reduced by the presence of another signal, through the use of side chain compression....
– lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the compressor's sidechain so that whenever the DJ speaks the compressor reduces the volume of the music. A sidechain with equalization
Equalization (audio)
Equalization is the process commonly used in sound recording and reproduction to alter the frequency response of an audio system using linear filters. Most hi-fi equipment uses relatively simple filters to make bass and treble adjustments. Graphic and parametric equalizers have much more...
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. A de-esser helps reduce high frequencies that tend to overdrive preemphasized
Preemphasis
In processing electronic audio signals, pre-emphasis refers to a system process designed to increase the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as...
media (such as phonograph records and FM radio). Another use of the side-chain in music production serves to maintain a loud bass track without the bass drum
Bass drum
Bass drums are percussion instruments that can vary in size and are used in several musical genres. Three major types of bass drums can be distinguished. The type usually seen or heard in orchestral, ensemble or concert band music is the orchestral, or concert bass drum . It is the largest drum of...
causing undue peaks that result in loss of overall headroom.
A linked stereo compressor without a sidechain input can be used as a mono compressor with a sidechain input. The "key", "trigger" or sidechain signal is sent to the left input of the stereo compressor while the signal that is to be compressed is routed through the right channel.
This technique is not to be confused with Parallel compression
Parallel compression
Parallel compression, also known as New York compression, is a dynamic range compression technique used in sound recording and mixing. Parallel compression, a form of upward compression, is achieved by mixing an unprocessed 'dry', or lightly compressed signal with a heavily compressed version of...
, which was referred to as "side chain" compression in a 1977 Studio Sound article by Mike Bevelle.
Parallel compression
One technique is to insert the compressor in a parallel signal path. This is known as parallel compressionParallel compression
Parallel compression, also known as New York compression, is a dynamic range compression technique used in sound recording and mixing. Parallel compression, a form of upward compression, is achieved by mixing an unprocessed 'dry', or lightly compressed signal with a heavily compressed version of...
and can give a measure of dynamic control without significant audible side effects, if 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 (since the compressor will significantly add to the combined gain at low levels only). This will often be beneficial when compressing transient content, since high-level dynamic liveliness is still maintained, despite the overall dynamic range reduction.
Multiband (also spelled multi-band) compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth (full-band, or single-band) compression is that unneeded audible gain changes or "pumping" in other frequency bands is not caused by changing signal levels in a single frequency band.
Multiband compressors work by first splitting the signal through some number of bandpass filters or crossover filters. The frequency ranges or crossover frequencies may be adjustable. 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 effects do not create unwanted peak levels.
Software plug-ins or DSP
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
emulations of multiband compressors can be complex, with many bands, and require corresponding computing power.
Multiband compressors are primarily an audio mastering
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 source from which all copies will be produced...
tool, but their inclusion in digital audio workstation
Digital audio workstation
A digital audio workstation is an electronic system designed solely or primarily for recording, editing and playing back digital audio. DAWs were originally tape-less, microprocessor-based systems such as the Synclavier and Fairlight CMI...
plug-in sets is increasing their use among mix engineers. Hardware multiband compressors are also commonly used in the on-air signal chain of a radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
, either AM or FM, in order to increase the station's apparent loudness
Loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . 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."Loudness, a subjective measure, is often...
without fear of overmodulation
Overmodulation
Overmodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always considered a fault condition. In layman's terms, the...
. Having a louder sound is often considered an advantage in commercial competition. However, adjusting a multiband output compressor of a radio station also requires some artistic sense of style, plenty of time and a good pair of ears. This is because the constantly changing spectral balance between audio bands may have an equalizing effect on the output, by dynamically modifying the on-air frequency response. A further development of this approach is programmable radio output processing, where the parameters of the multiband compressor automatically change between different settings according to the current programme block style or the time of day.
Serial compression
Serial compression is a technique used in sound recordingSound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
and mixing
Audio mixing (recorded music)
In audio recording, audio mixing is the process by which multiple recorded sounds are combined into one or more channels, most commonly two-channel stereo. In the process, the source signals' level, frequency content, dynamics, and panoramic position are manipulated and effects such as reverb may...
. Serial compression is achieved by using two fairly different compressors
Audio level compression
Dynamic range compression, also called DRC or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or "compressing" an audio signal's dynamic range...
in a signal chain. One compressor will generally stabilize the dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
while the other will more aggressively compress 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) would be directly followed by a fast peak sensing limiter (for overload protection). Done properly, even heavy serial compression can sound very natural in a way not possible with a single compressor. It is most often used to even out erratic vocals and guitar
Guitar
The guitar is a plucked string instrument, usually played with fingers or a pick. The guitar consists of a body with a rigid neck to which the strings, generally six in number, are attached. Guitars are traditionally constructed of various woods and strung with animal gut or, more recently, with...
s.
Public spaces
Compression is often used to make music sound louder without increasing its peak amplitude. By compressing the peak (or loudest) signals, it becomes possible to increase the overall gain (or volume) of a signal without exceeding the dynamic limits of a reproduction device or medium. The net effect, when compression is applied along with a gain boost, is that relatively quiet sounds become louder, while louder sounds remain unchanged.Compression is often applied in this manner in audio systems for restaurants, retail, and similar public environments, where background music is played at a relatively low volume and needs to be compressed not just to keep the volume fairly constant, but also in order for relatively quiet parts of the music to be audible over ambient noise, or audible at all.
Compression can be used to increase the average output gain of a power amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
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 performances more consistent in dynamic range so that they "sit" in the mix of other instruments better and maintain consistent attention from the listener. Vocal performances in rock musicRock and roll
Rock and roll is a genre of popular music that originated and evolved in the United States during the late 1940s and early 1950s, primarily from a combination of African American blues, country, jazz, and gospel music...
or pop music
Pop music
Pop music is usually understood to be commercially recorded music, often oriented toward a youth market, usually consisting of relatively short, simple songs utilizing technological innovations to produce new variations on existing themes.- Definitions :David Hatch and Stephen Millward define pop...
are usually compressed in order to make them stand out from the surrounding instruments and to add to the clarity of the vocal performance.
Compression can also be used on instrument sounds to create effects not primarily focused on boosting loudness
Loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . 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."Loudness, a subjective measure, is often...
. 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 in order to obtain 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; see Side-Chaining above.
Voice
A compressor can be used to reduce sibilanceSibilant consonant
A sibilant is a manner of articulation of fricative and affricate consonants, made by directing a stream of air with the tongue towards the sharp edge of the teeth, which are held close together. Examples of sibilants are the consonants at the beginning of the English words sip, zip, ship, chip,...
('ess' sounds) in vocals by feeding the compressor with an EQ set to the relevant frequencies, so that only those frequencies activate the compressor. If unchecked, sibilance could cause distortion even if sound levels are not very high. This usage is called 'de-essing
De-essing
De-essing is any technique intended to reduce or eliminate excess sibilant consonants such as "s", "z" and "sh" in recordings of the human voice. Excess sibilance can be caused by compression, microphone choice and technique, and even simply a singer's mouth...
'. http://alesis.com/support/notes/Signal_Processing/Compapp.html
Compression is used in voice communications in amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
that employ SSB modulation
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
. Often it is used 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 occurs especially in pileup
Pileup (disambiguation)
A pileup can refer to:* Multiple-vehicle collision* In particle physics, a situation where a particle detector being affected by several events at the same time....
s where amateur radio stations are competing for the opportunity to talk to a DX station. Since an SSB signal's amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
depends on the level of modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, the net result is that the average amplitude of the signal and hence average transmitted power would be stronger than it would be had compression not been used. Most modern amateur radio SSB transceivers have speech compressors built in.
Compression is also used in land mobile radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
, especially in transmit audio of professional walkie-talkie
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...
s and in remote control dispatch consoles
Tone remote
Remote controls are used anytime a two-way radio base station is located away from the desk or office where communication originates. For example, a dispatch center for taxicabs may have an office downtown but have a base station on a distant mountain top...
.
Broadcasting
Compression is used extensively in broadcastingBroadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
to boost the perceived volume of sound while reducing the dynamic range of source audio (typically CDs) to a range that can be accommodated by the narrower-range broadcast signal. Broadcasters in most countries have legal limits on instantaneous peak volume they may broadcast. Normally these limits are met by permanently inserted hardware in the on-air chain (see multiband compression above).
As was alluded to above, the use of compressors to boost perceived volume is a favorite trick of broadcasters who want their station to sound "louder" at the same volume than comparable stations on the dial. The effect is to make the more heavily compressed station "jump out" at the listener at a given volume setting.
But loudness jumps are 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 which are known to be 'too loud'. One complicating factor is that many broadcasters use (quasi-)peak meters and peak-levelling. Unfortunately the peak level reading does not correlate very well with the perceived loudness. It basically should only be used to prevent overmodulation.
The European Broadcasting Union has been addressing this issue in the EBU PLOUD Group, which consist of over 240 audio professionals, many from broadcasters and equipment manufacturers. In 2010, the EBU published EBU Recommendation R 128, which introduces a new way of metering and normalising audio. The Recommendation is based on ITU-R BS.1770 and could be called a real 'audio revolution'. Several European TV stations have already announced to support the new norm and over 20 manufacturers have announced products supporting the new 'EBU Mode' Loudness meters (EBU Tech 3341).
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 Descriptor (EBU Tech 3342).
Rock stations
The use of compressors began with competitive AM rock stations of the 1960s. AM broadcasters had no qualms about heavy compression since AM radio had such poor dynamic range anyway. The Gates Sta-level was an often used compressor that would reduce "highs" and boost "lows" to yield a very "punchy" sound with the perceived increased volume energy mentioned above.Heavy compression also complemented the style of 60s DJs who talked/shouted over the music. With the proper setting, a DJ could be "mixed" into the music, rather than being heard over it. This demanded that DJs deliver their patter with a very loud voice to be heard over the music, which added to the energy of the broadcast sound. This allowed DJs to talk "in" rather than over the music without being as intrusive.
As rock became prevalent on FM in the mid-60s, the CBS Volumax/Audimax was one legendary compression rig used, favored because it only "expanded" (lifting soft volume) if any existed. Consequently, it would not expand an unmodulated signal, avoiding the boosting of the noise floor (hiss) as many previous units did. However, it could create an annoying "sucking and pumping" effect (compression and expansion) if driven too severely.
In its effort to deliver a constant modulation (volume level) to the listener, compression works against the wider dynamic range of FM (as compared to AM) which was traditionally one of FM's obvious strong points. Consequently, the so-called "album rock" stations of the 70s and classical music and "easy listening" stations of that era in particular, avoided heavy compression. Classical stations hardly use any, which explains why a classical listener, particularly in the car, must keep turning the volume up and down, constantly fighting the ambient noise prevalent in car listening.
The same recording can have very different dynamics when heard via AM
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
, FM
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
, CD, or other media (although frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
and noise
Noise (audio)
Noise in audio, recording, and broadcast systems refers to the residual low level sound that is heard in quiet periods of a programme....
are large factors as well).
Marketing
Audio mixing (recorded music)
In audio recording, audio mixing is the process by which multiple recorded sounds are combined into one or more channels, most commonly two-channel stereo. In the process, the source signals' level, frequency content, dynamics, and panoramic position are manipulated and effects such as reverb may...
and mastering; compression algorithms have been engineered specifically to accomplish the task of maximizing audio level in the digital stream. Hard limiting or clipping
Clipping (audio)
Clipping is a form of waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability...
can result, affecting the tone and timbre of the music. The effort to increase loudness has been referred to as the "Loudness wars".
Most television commercials are compressed heavily (typically to a dynamic range of no more than 3dB) in order to achieve near-maximum perceived loudness while staying within permissible limits. This is the explanation for a problem that TV viewers and listeners have noticed for years: While commercials receive heavy compression for the same reason that radio broadcasters have traditionally used it (to achieve a "loud" audio image), TV program material, particularly old movies with soft dialogue, is comparatively uncompressed by TV stations. This results in commercials much louder than the television programs, since users turn up the volume to hear soft program audio.
Other uses
A compressor is sometimes used to reduce the dynamic range of a signal for transmission, to be expanded afterward. This reduces the effects of a channel with limited dynamic range. See CompandingCompanding
In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...
.
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
Electronic dance music
Electronic dance music is electronic music produced primarily for the purposes of use within a nightclub setting, or in an environment that is centered upon dance-based entertainment...
and hip-hop musicians purposefully use this phenomenon, causing the mix to alter in volume rhythmically in time with the beat.
A compressor is used in hearing aids to bring the audio volume into the listener's hearing range. To enable the patient to perceive the direction from which the sound is coming, binaural
Binaural
Binaural literally means "having or relating to two ears." Binaural hearing, along with frequency cues, lets humans and other animals determine direction of origin of sounds...
compression may be required.
Compressors are also used in some active sound protection earmuffs, to allow sounds at ordinary volumes to be heard normally while attenuating louder 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.
Compressors for software audio players
Some software audio playersAudio player (software)
An audio player is a kind of media player for playing back digital audio, including optical discs such as CDs, SACDs, DVD-Audio, HDCD, audio files and streaming audio....
support plugins which implement compression. These can be used to increase the perceived volume of audio tracks, or to even out the volume of highly-variable music (such as classical music, or a playlist spanning many music types). This improves the listenability of audio when played through poor-quality speakers, or when played in noisy environments (such as in a car or during a party). Such software may also be used in micro-broadcasting or home-based audio mastering.
Available software includes:
- Windows (system-wide, all players):
- WinampWinampWinamp is a media player for Windows-based PCs and Android devices, written by Nullsoft, now a subsidiary of AOL. It is proprietary freeware/shareware, multi-format, extensible with plug-ins and skins, and is noted for its graphical sound visualization, playlist, and media library features.Winamp...
plugins (these can also be enabled for other players via ffdshowFfdshowffdshow is a media decoder and encoder mainly used for the fast and high-quality decoding of video in the MPEG-4 ASP and AVC formats, but it supports numerous other video and audio formats as well...
):- Stereo Tool (also available as a command line wavWAVWaveform Audio File Format , is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs...
-to-wav tool) - Rock Steady
- Volume Leveler 15
- Sound Solution
- Compwide (DSP version of the compressor in 1&1 Player available from the same site)
- Stereo Tool (also available as a command line wav
- Plugins with multiple player compatibility:
- iZotope OzoneMP (Windows Media PlayerWindows Media PlayerWindows Media Player is a media player and media library application developed by Microsoft that is used for playing audio, video and viewing images on personal computers running the Microsoft Windows operating system, as well as on Pocket PC and Windows Mobile-based devices...
, Quintessential PlayerQuintessential PlayerQuintessential Player is a freeware, multi-format media player developed by Paul Quinn.Quintessential Player began life in 1997 as a CD-only player for Windows, when it was known as Quintessential CD Player...
, WinampWinampWinamp is a media player for Windows-based PCs and Android devices, written by Nullsoft, now a subsidiary of AOL. It is proprietary freeware/shareware, multi-format, extensible with plug-ins and skins, and is noted for its graphical sound visualization, playlist, and media library features.Winamp...
) - Freeverb3 (XMMSXMMSThe X Multimedia System is a free audio player for Unix-like systems.-History:XMMS was originally written as X11Amp by Peter and Mikael Alm in November 1997. The player was made to resemble Winamp, which was first released in May that year. As such, XMMS has supported Winamp 2 "classic" skins...
and AudaciousAudacious Media PlayerAudacious is a free software audio player with a focus on low resource usage, high audio quality, and support for a wide range of audio formats. It is designed primarily for use on POSIX-compatible systems such as Linux, with limited support for Microsoft Windows...
; includes a linear phase multi band compressor)
- iZotope OzoneMP (Windows Media Player
- Players with built-in compression:
- AudiograbberAudiograbberAudiograbber is a proprietary freeware CD audio extractor/converter program for Microsoft Windows. It was one of the first programs in the genre to become popular. The data extraction algorithm was designed by Jackie Franck and was included in the Xing Technology software package Xing Audio...
appears to have some compression abilities built into its Audio normalizationAudio normalizationAudio normalization is the application of a constant amount of gain to an audio recording in order to bring the average or peak amplitude to a target level ....
feature. - foobar2000Foobar2000foobar2000 is a freeware audio player for Windows developed by Peter Pawlowski, a former freelance contractor for Nullsoft. It is known for its highly modular design and extensive SDK which allows third-party developers to do such things as completely replace the interface...
- Audiograbber
To achieve volume-compressed playback on devices other than computer-based audio players, files may need to be processed via the above software then output as wav
WAV
Waveform Audio File Format , is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs...
s, mp3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
s, or other audio formats.
See also
- SquelchSquelchIn telecommunications, squelch is a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal.-Carrier squelch:...
- Automatic gain controlAutomatic gain controlAutomatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
- Audio & Design (Recording) LtdAudio & Design (Recording) LtdAudio & Design Ltd are an English based company who specialised in the development and production of professional audio processors, in the form of limiters, compressors, expanders & equalisers. The company were one of the first to introduce Field-effect transistors into a commercially available...
- Gain compressionGain compressionGain compression is a reduction in 'differential' or 'slope' gain caused by nonlinearity of the transfer function of the amplifying device. This nonlinearity may be caused by heat due to power dissipation, or by overdriving the active device beyond its linear region...
- Noise gateNoise gateA Noise Gate or gate is an electronic device or software that is used to control the volume of an audio signal. In its most simple form, a noise gate allows a signal to pass through only when it is above a set threshold: the gate is 'open'. If the signal falls below the threshold no signal is...
- De-essingDe-essingDe-essing is any technique intended to reduce or eliminate excess sibilant consonants such as "s", "z" and "sh" in recordings of the human voice. Excess sibilance can be caused by compression, microphone choice and technique, and even simply a singer's mouth...
- LA-2A Leveling AmplifierLA-2A Leveling AmplifierThe LA-2A Leveling Amplifier is an audio compressor originally produced by Teletronix in 1965. Three versions of the LA-2A were made until 1969. The LA-2A was inducted into the TECnology Hall of Fame in 2004.The LA-2A is a hand-wired, tube-based, compressor...
- 1176 Peak Limiter1176 Peak LimiterThe 1176LN Peak Limiter is a compressor introduced by UREI in 1968. The 1176LN was inducted into the TECnology Hall of Fame in 2008. At the time of its introduction, it was the first true peak limiter with all solid-state circuitry.-Uses:...
- Tone mappingTone mappingTone mapping is a technique used in image processing and computer graphics to map one set of colors to another in order to approximate the appearance of high dynamic range images in a medium that has a more limited dynamic range...
, the optical equivalent
External links
- An overview of compression w. a more practical user friendly approach (also a wiki)
- Description of expansion, compression, and limiting in an audio processor
- Dynamic range compression
- Understanding compressors and compression
- Example and explanation of over compression
- Article on Optical Compressors from Sweetwater SoundSweetwater SoundSweetwater Sound, Inc. is one of the United States' largest dealers in musical equipment for musicians, recording studios, schools, houses of worship, concert sound companies and broadcasters...
- Pumping up the volume
- Information on Loudness from the EBU