Noise gate 1039345 223928749 2008-07-06T15:43:08Z Lightbot 7178666 Units/dates/other {{Refimprove|date=August 2007}} A '''Noise Gate''' or '''gate''' is an [[electronic device]] or [[software logic]] 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 allowed to pass (or the signal is substantially attenuated): the gate is 'closed'. <ref>{{Citation | last = Davis | first = Gary | last2 = Jones | first2 = Ralph | last3 = Yamaha International Corporation | title = The Sound Reinforcement Handbook | publisher = Hal Leonard | year = 1989 | isbn = 0881889008 }}.</ref>A noise gate is used when the level of the 'signal' is above the level of the '[[noise]]'. The threshold is set above the level of the 'noise' and so when there is no 'signal' the gate is closed. A noise gate does not remove noise from the signal. When the gate is open both the signal and the noise will pass through. They are commonly used in the [[recording studio]] and [[sound reinforcement]]. Rock musicians may also use small portable units to control unwanted noise from their amplification systems. Band-limited noise gates are also used to eliminate background noise from audio recordings by eliminating [[frequency]] bands that contain only [[static]]. [[Image:Noise Gate Attack Hold Release.svg|thumb|400px|right|The Attack, Hold, and Release functions of a noise gate.]]Noise Gates have a ''Threshold'' control to set the level at which the gate will open. More advanced noise gates have more features. The ''Release'' sets the amount of time for the gate to go from open to fully closed. A fast release abruptly cuts off the sound once it has fallen below the threshold, a slower release smoothly changes from open to closed, much like a slow fade out.If the release time is too short a click can be heard when the gate re-opens.{{Fact|date=June 2008}} Release is the most common control to find on a gate, after Threshold. The ''Attack'' control sets the time for the gate to change from closed to open, much like a fade-in. The ''Hold'' control which allows you to define the amount of time the gate will stay open after the signal falls below the threshold. This is useful during short pauses between words or sentences in a speech signal.<ref>http://drawmercom.virtual31.nebula.fi/uploads/File/download/op_manuals/user_ds201.pdf</ref> The amount of [[attenuation]] when the gate is closed can be set by the ''Range'' control. Often there will be complete attenuation, that is no signal will pass when the gate is closed. In some circumstances complete attenuation is not desired and the range can be changed. Advanced gates have a ''[[Dynamic range compression#Side-chaining|sidechain]]''. This is an additional input that allows the gate to be triggered by another audio signal. A variation of a sidechained noise gate used in [[electronic music]] production is a ''trigger gate'', ''trancegate'' or just simply ''gate'', where the noise gate is not controlled by audio signal but a preprogrammed pattern resulting in a precisely controlled chopping of a sustained sound. Noise gates often implement [[hysteresis]], that is, they have two thresholds. One to open the gate and another, set a few [[Decibel|dB]] below, to close the gate. This means that once a signal has dropped below the close threshold, it has to rise to the open threshold for the gate to open, so that a signal that crosses over the close threshold regularly does not open the gate and cause [[chattering]]. A longer hold time as described above also helps avoid chattering. ==Audio noise reduction== In audio [[post-processing]], noise gating reduces steady noise sources such as [[rumble (noise)|rumble]] from [[gramophone record|LP record]]s, [[hiss]] from [[audio tape]], [[static]] from a [[radio]] or [[amplifier]], and [[hum]] from a power system, without greatly affecting the source sound. An audio signal such as [[music]] or [[Speech communication|speech]] is broken up into many [[passband|frequency band]]s by a collection of overlapping [[band-pass filter]]s, and if the signal amplitude in any one band is lower than a preset threshold then that band is eliminated from the final sound. This greatly reduces perceptible background noise because only the frequency components of the noise that are within the gated [[passband]]s survive. The technique was implemented in real-time electronics in some [[audiophile]] [[record player]]s as early as the 1980s, and is now commonly used in audio production [[post-processing]], where software to [[Fourier transform]] the audio signal can yield a very detailed [[spectrum]] of the background noise. Common [[digital audio]] editing software packages such as [[CoolEdit]] and [[Audacity]] include easy-to-use digital noise gating code: the user selects a segment of audio that contains only static, and the amplitude levels in each frequency band are used to determine the threshold levels to be applied across the signal as a whole. Noise gating works well when the static is steady and either narrowly confined in frequency (e.g. hum from AC power) or well below the main signal level (15 dB minimum is desirable). In cases where the signal merges with the background static (for example, the brushed drum sounds in the [[Sun King (song)|Sun King]] track on the [[Beatles]] album [[Abbey Road (album)|Abbey Road]]) or is weak compared to the noise (as in very faint [[Sound recording#Magnetic recording|tape recording]]s), the noise gating can add [[Sonic artifact|artifacts]] that are more distracting than the original static. In the context of a multi microphone recording session, noise gating is employed to reduce the leakage of sound into a microphone from sources other than the one the microphone was intended for. One example involves the mic-ing up of a [[drumkit]]. In most multi-mic drum recordings, amongst others, a microphone will be used to capture the [[snare drum]] sound and another to capture the [[kick drum]] sound. The snare microphone will output a signal comprising of a high level snare signal and a lower level kick drum signal (due to the further distance of the kick drum from the snare microphone). If the threshold level of the noise gate is set correctly a snare drum signal can be [[isolated]]. To fully isolate the snare drum signal the release rate has to be quite fast which can cause the tail end of the snare sound to be 'chopped off'. This can usually be remedied by the inclusion of one or more 'overhead' microphone(s), which can act as a general 'audio glue' for all the other gated sources. ==Recording usages== A good example of time-controlled noise gating is the well-known "[[gated reverb]]" effect heard on the drums on the [[Phil Collins]] hit single "[[In the Air Tonight]]", created by engineer-producer [[Hugh Padgham]], in which the powerful reverberation added to the drums is cut off by the noise gate after a few [[milliseconds]], rather than being allowed to decay naturally. This can also be achieved by: sending the 'dry' snare signal to the reverb (of other process) unit, inserting a noise gate on the path of the reverb signal and connecting the snare sound to the [[Side chain (sound)|side chain]] of the gate unit. With the gate unit set to 'external sidechain' (or 'external key') the gate will respond to the snare signal level and 'cut off' when that has decayed below the threshold, not the reverberated sound. It is a common production trick to use spurious combinations of side chain inputs to control longer, more sustained sounds. For example, a [[hi-hat]] signal can be used to control a sustained [[synthesized]] sound to produce a [[rhythmic]] melodic (or harmonic) signal which is perfectly in time with the [[hi-hat]] signal. A good example of this use of the device can be found on the [[Godley & Creme]] [[concept album]] ''Consequences''. The album's story required the creation of a number of special [[sound effects]] that would convey the impression of [[natural disasters]]. For the "Fire" sequence, Godley and Creme used a noise gate, triggered by the sound of multitracked voices, that created the 'voice' of a raging [[bushfire]]. During the recording of this segment, each time the voice signal began, it triggered the noise gate to open up another channel, which carried a pre-recorded loop of a crackling sound (created by overdubbing the sound of [[Bubble Wrap]] being popped in front of a microphone). The combined voices and crackling created an eerie and quite convincing 'talking fire' effect. A noise gate can sometimes (and to varying success) be used to 'tighten up' the playing of [[musicians]]. If, for example, in a recording the kick drum is quite 'in time' but the bass player is slightly ahead, controlling the bass guitar via a gate whose sidechain is fed by the kick drum signal can pull the two sounds together. ==References== <references /> [[Category:Audio effects]] [[Category:Effects units]] [[Category:Audio engineering]] [[Category:Sound recording]] [[Category:Noise reduction]] [[Category:Electronic circuits]] [[es:Compuerta de Ruido]] [[fr:Noise gate]] [[it:Noise gate]] [[pl:Bramka szumów]]