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Audio signal processing operation

Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or compressing an audio signal's dynamic range. Compression is commonly used in sound recording and reproduction, broadcasting, live sound reinforcement and some instrument amplifiers. 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 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 and, generally, a short attack time. Compression is used to improve performance and clarity in public address systems, as an effect and to improve consistency in mixing and mastering. It is used on voice to reduce sibilance and in broadcasting and advertising to make an audio program stand out. It is an integral technology in some noise reduction systems.

There are two types of compression: downward and upward. Both types of compression reduce the dynamic range 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 makes 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. There are a number of technologies used for variable gain amplification, each having different advantages and disadvantages. Vacuum tubes are used in a configuration called variable mu where the grid to cathode voltage changes to alter the gain. Optical compressors use a photoresistor stimulated by a small lamp (incandescent, LED, or electroluminescent panel) to create changes in signal gain. Other technologies used include field effect transistors and a diode bridge. When working with digital audio, digital signal processing (DSP) techniques are commonly used to implement compression as audio plug ins, in mixing consoles, and in digital audio workstations. Often the algorithms are used to emulate the above analog technologies.

A number of user adjustable control parameters and features are used to adjust dynamic range compression signal processing algorithms and components.

A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. Threshold is commonly set in decibels (dBFS for digital compressors and dBu for hardware compressors), where a lower threshold (e. g.−60 dB) means a larger portion of the signal is treated. When the signal level is below the threshold, no processing is performed and the input signal is passed, unmodified, to the output. Thus a higher threshold of, e. g.,−5 dB, results in less processing, less compression. Threshold timing behavior is subject to attack and release settings (see below). When the signal level goes above threshold, compressor operation is delayed by the attack setting. For an amount of time determined by the release after the input signal has fallen below the threshold, the compressor continues to apply dynamic range compression.

The amount of gain reduction is determined by ratio: a ratio of 4:1 means that if input level is 4 dB over the threshold, the output signal level is reduced to 1 dB over the threshold. The gain and output level has been reduced by 3 dB. Another way of stating this is that any input signal level over the threshold will, in this case, be output at a level which is only 25% (i. e. 1 over 4) as much over the threshold as its input level was. The highest ratio of :1 is often known as limiting, and effectively denotes that any signal above the threshold is brought down to the threshold level once the attack time has expired.

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 the 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.

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.

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

A compressor in stereo linking mode applies the same amount of gain reduction to both the left and right channels. This is done to prevent image shifting that can occur if each channel is compressed individually.

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

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

Compression is often applied in audio systems for restaurants, retail, and similar public environments that play background music at a relatively low volume and need it compressed, not just to keep the volume fairly constant, but also to make quiet parts of the music audible over ambient noise. Compression can increase average output gain of a power amplifier by 50 to 100% with a reduced dynamic range. For paging and evacuation systems, this adds clarity under noisy circumstances and saves on the number of amplifiers required.

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). In electronic dance music, side chaining is often used on basslines, controlled by the kick drum or a similar percussive trigger, to prevent the two from conflicting, and provide a pulsating, rhythmic dynamic to the sound.

A compressor can be used to reduce sibilance ('ess' sounds) in vocals (de essing) by feeding the compressor's side chain an equalized version of the input signal, so that specific, sibilance related frequencies (typically 4000 to 8000 hz) activate the compressor more. Compression is used in voice communications in amateur radio that employ single sideband (SSB) modulation to make a particular station's signal more readable to a distant station, or to make one's station's transmitted signal stand out against others. This is applicable especially in DXing. An SSB signal's strength depends on the level of modulation. A compressor increases the average level of the modulation signal thus increasing the transmitted signal strength. Most modern amateur radio SSB transceivers have speech compressors built in. Compression is also used in land mobile radio, especially in transmitted audio of professional walkie talkies and remote control dispatch consoles.

Compression is used extensively in broadcasting to boost the perceived volume of sound while reducing the dynamic range of source audio. To avoid overmodulation, broadcasters in most countries have legal limits on instantaneous peak volume they may broadcast. Normally these limits are met by permanently inserted compression hardware in the on air chain. Broadcasters use compressors in order that their station sounds louder than comparable stations. The effect is to make the more heavily compressed station jump out at the listener at a given volume setting. This is not limited to inter channel differences; they also exist between programme material within the same channel. Loudness differences are a frequent source of audience complaints, especially TV commercials and promos that seem too loud. The European Broadcasting Union (EBU) has been addressing this issue in the EBU PLOUD group, which consists of over 240 audio professionals, many from broadcasters and equipment manufacturers. In 2010, the EBU published EBU R 128 which introduces a new way of metering and normalizing audio. The Recommendation uses ITU R BS.1770 loudness metering. as of 2016, several European TV stations have announced their support for the new norm 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.

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.

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

Noise reduction systems use a compressor to reduce the dynamic range of a signal for transmission or recording, expanding it afterward, a process called companding. This reduces the effects of a channel or recording medium with limited dynamic range. Instrument amplifiers often include compression circuitry to prevent sudden high wattage peaks that could damage the speakers. Electric bass players often use compression effects, either effects units available in pedal, rackmount units, or built in devices in bass amps, to even out the sound levels of their basslines. Gain pumping, where a regular amplitude peak (such as a kick drum) causes the rest of the mix to change in volume due to the compressor, is generally avoided in music production. However, many dance and hip hop musicians purposefully use this phenomenon, causing the mix to alter in volume rhythmically in time with the beat. Hearing aids use a compressor to bring the audio volume into the listener's hearing range. To help the patient perceive the direction sound comes from, some hearing aids use binaural compression. Compressors are also used for hearing protection in some electronic active hearing protection earmuffs and earplugs, to let sounds at ordinary volumes be heard normally while attenuating louder sounds, possibly also amplifying softer sounds. This allows, for example, shooters wearing hearing protection at a shooting range to converse normally, while sharply attenuating the much louder sounds of the gunshots, 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.

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. 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 jockeys 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 causing undue peaks that result in loss of overall headroom. Multiband compressors work by first splitting the signal through some number of band pass filters, 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 tool, but their inclusion in digital audio workstation plug in sets is increasing their use among mix engineers. The TC Electronic Finalizer included a three band compressor and was a popular audio mastering tool around year 2000. On air signal chains of radio stations commonly use multiband compressors to increase loudness while avoiding overmodulation. Having a louder sound is often considered an advantage in commercial broadcasting.

Serial compression is a technique used in sound recording and mixing. Serial compression is achieved by using two fairly different compressors in a signal chain. One compressor generally stabilizes the dynamic range 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 vocals and guitars.

Some software audio players support plugins 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).

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, Finally, R 128 LRA has been repeatedly considered as a measure of macro dynamics or dynamics in the musical sense.