Dynamic range compression (DRC) or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or “compressing” an audio signal‘sdynamic range. Compression is commonly used in sound recording and reproduction and broadcasting and on instrument amplifiers.
Audio compression reduces loud sounds which are above a certain threshold while quiet sounds remain unaffected. The dedicated electronic hardware unit or audio software used to apply compression is called a compressor. In recorded and live music, compression parameters may be adjusted by an audio engineer to change the way the effect sounds.
A compressor reduces the level of an audio signal if its amplitude exceeds a certainthreshold. It is commonly set in decibels 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 e.g. −5 dB).
Attack and release
The attack and release phases in a compressor
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.
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 RMS) on the input signal before its level is compared to the threshold. This allows a more relaxed compression that also 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. 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 applies the highest amount of gain reduction to both (in such case it could still make sense to dial different settings on left and right channels as one might wish to have less compression for left-side events).
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.
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 amplifierby 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 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 inrock music 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.
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 conflicting, and provide a pulsating, rhythmic dynamic to the 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.
A compressor can be used to reduce sibilance (‘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.
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 pileups where amateur radio stations are competing for the opportunity to talk to a DX station. Since an SSB signal’samplitude 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-talkies and in 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 (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.
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.
The trend of increasing loudness as shown by waveform images of the same song 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 loudness is achieved by using higher degrees of compression and limiting during mixing andmastering; 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”.
Most television commercials are compressed heavily (typically to a dynamic range of no more than 3 dB) 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.
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 Companding.
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, manydance 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 for hearing protection in some active sound protectionearmuffs and earplugs, to allow sounds at ordinary volumes to 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 clashes.