Dynamic range compression
In recorded and live music, compression parameters may be adjusted to change the way they affect sounds.
[5] Optical compressors use a photoresistor stimulated by a small lamp (incandescent, LED, or electroluminescent panel)[6] to create changes in signal gain.
[citation needed] A number of user-adjustable control parameters and features are used to adjust dynamic range compression signal processing algorithms and components.
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.
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.
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.
Some compressors apply a power measurement function (commonly root mean square or RMS) on the input signal before comparing its level to the threshold.
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.
[citation needed] For paging and evacuation systems, this adds clarity under noisy circumstances and saves on the number of amplifiers required.
[13] 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.
Compression is also used in land mobile radio, especially in transmitted audio of professional walkie-talkies and remote control dispatch consoles.
[15] Compression is used extensively in broadcasting to boost the perceived volume of sound while reducing the dynamic range of source audio.
[19] Most television commercials are heavily compressed to achieve near-maximum perceived loudness while staying within permissible limits.
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.
Instrument amplifiers often include compression circuitry to prevent sudden high-wattage peaks that could damage the speakers.
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.
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,[23] and similarly for musicians to hear quiet music but be protected from loud noises such as drums or cymbal crashes.
[25] The sonic results of more than momentary and infrequent brick-wall limiting are harsh and unpleasant, thus it is more common as a safety device in live sound and broadcast applications.
Some bass amps and PA system amplifiers include limiters to prevent sudden volume peaks from causing distortion or damaging the speakers.
The side-chain input is used by disc jockeys for ducking – lowering the music volume automatically when speaking.
On the other hand, a high compression ratio with significant audible artifacts can be chosen in one of the two parallel signal paths.
The TC Electronic Finalizer included a three band compressor and was a popular audio mastering tool around year 2000.
[30] On-air signal chains of radio stations commonly use multiband compressors to increase loudness while avoiding overmodulation.
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.
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.
[31] Five signal descriptors were considered: RMS power, EBU R 128 integrated loudness,[18] crest factor, R 128 LRA,[19] and density of clipped samples.
[18] The crest factor, which is the difference between the signal's peak and its average power,[31] is on occasions considered as a basis for the measure of micro-dynamics, for instance in the TT Dynamic Range Meter plug-in.
