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A rack of audio compressors in a recording studio. From top to bottom: Retro Instruments/Gates STA level; Spectra Sonic; Dbx 162; Dbx 165; Empirical Labs Distressor; Smart Research C2; Chandler Limited TG1; Daking FET (91579); and Altec 436c.Dynamic range compression ( DRC) or simply compression is an operation that reduces the volume of loud or amplifies quiet sounds thus reducing or compressing an 's.

Compression is commonly used in, and in some.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 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 is a compressor with a high and, generally, a short. Upward compressionDownward compression reduces loud sounds over a certain threshold while quiet sounds remain unaffected. A is an extreme type of downward compression. Upward compression increases the loudness of sounds below a certain threshold while leaving louder sounds unaffected.

Both downward and upward compression reduce the of an audio signal.An expander increases the dynamic range of the audio signal. Expanders are generally used to make quiet sounds even quieter by reducing the level of an that falls below a set threshold level. A is a type of expander.

A feed-forward compressor design (left) and feedback design (right)The signal entering a compressor is split; one copy is sent to a 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. Are used in a configuration called variable-mu where the grid-to-cathode voltage changes to alter the gain. Optical compressors use a and a small lamp (, or ) to create changes in signal gain. Other technologies used include and a.When working with digital audio, techniques are commonly used to implement compression as, in, and in.

Often the algorithms used emulate the above analog technologies. Controls and features.

Different compression ratios for a signal level above the thesholdA 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 ( for digital compressors and 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 ). 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 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.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 is brought down to the threshold level once the attack time has expired.Attack and release. The attack and release phases in a compressorA 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).

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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 these cannot be adjusted by the user. Sometimes the attack and release times are automatic or program dependent, meaning that the behavior may change dependent on the input signal.Soft and hard knees. Hard Knee and Soft Knee compressionAnother 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 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.

The trend of increasing loudness as shown by waveform images of the song ' by mastered on CD four times since 1983.Record companies, mixing engineers and mastering engineers have been gradually increasing the overall volume of commercial albums. The greater is achieved by using higher degrees of compression and limiting during and; compression algorithms have been engineered specifically to accomplish the task of maximizing audio level in the digital stream. Hard limiting or can result, affecting the tone and timbre of the music. The effort to increase loudness has been referred to as the.Other uses Some applications use a compressor to reduce the dynamic range of a signal for transmission, expanding it afterward. This reduces the effects of a channel with limited dynamic range. See and.and often include compression circuitry to prevent sudden high-wattage peaks that could damage the speakers. Players often use compression effects, either available in pedal, units, or built-in devices in bass amps, to even out the sound levels of their., 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 and hip-hop musicians purposefully use this phenomenon, causing the mix to alter in volume rhythmically in time with the beat.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 compression.Compressors are also used for in some electronic 'active sound protection' and, 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.

Main article:Compression and limiting are identical in process but different in degree and perceived effect. A 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 hard/brick-wall limiting are harsh and unpleasant, thus it is more common as a safety device in live sound and broadcast applications.Some and amplifiers include limiters to prevent sudden volume peaks from causing distortion and/or damaging the speakers. Some modern consumer electronics devices incorporate limiters. Uses the (AVLS), on some audio products and the.Side-chaining. The first 8 bars are without side-chaining applied, the second 8 are with side-chaining.Problems playing this file?

See.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. The compressor behaves in the conventional manner when both inputs are supplied with the same signal. The side-chain input is used by for – 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 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, reducing the level of vocal in the range of 6–9 kHz. A de-esser helps reduce high frequencies that tend to overdrive 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 causing undue peaks that result in loss of overall. Parallel compression One technique is to insert the compressor in a parallel signal path. This is known as, a form of upward compression that facilitates 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 significantly adds to the combined gain at low levels only).

This is often beneficial when compressing transient content, since it maintains high-level dynamic liveliness, despite reducing the overall dynamic range.Multiband compression Multiband compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth 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. 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 emulations of multiband compressors can be complex, with many bands, and require corresponding computing power.Multiband compressors are primarily an tool, but their inclusion in plug-in sets is increasing their use among mix engineers. On-air signal chains of commonly use hardware multiband compressors to increase apparent without fear of. Having a louder sound is often considered an advantage in commercial competition.

However, adjusting a radio station's multiband output compressor requires some artistic sense of style, plenty of time, and good ears. This is because the constantly changing 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 used in. Serial compression is achieved by using two fairly different compressors in a signal chain.

One compressor generally stabilizes the 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 and.Software audio players Some support that implement compression. These can increase perceived volume of audio tracks, or even out the volume of highly-variable music (such as, or a playlist that spans multiple music types).

This improves listenability of audio 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 or home-based audio mastering.Objective influence on the signal In an article released 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:, EBU3341/R128 integrated loudness, EBU3342 LRA, and density of clipped samples. RMS power accounts for the signal's physical level, EBU3341 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, EBU3342 LRA has been repeatedly considered as a measure of macro-dynamics or dynamics in the musical sense.