Noise gate
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A noise gate is an electronic device or software logic that is used to control the volume of an audio signal. They are commonly used in the recording studio and sound reinforcement. Small portable units are also used by rock musicians 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.
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: the gate is closed. 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.
More advanced forms of noise gates have more features. Most common is the ability to control how quickly the gate will close once the level has dropped below the threshold. This is known as the release, and allows for a smooth decay rather than an abrupt cutting of the sound when the gate closes. Likewise the ability to set the time it takes for the gate to open is available on some gates. This is known as the attack control. If the attack time is too short a click can be heard when the gate opens. Sometimes there is a hold control. After the gate opens, the hold time determines the minimum time the gate will stay open. When used with a speech signal, the hold time can prevent the gate from closing during short pauses between words or sentences.
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 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 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.
[edit] Audio noise reduction
In audio post-processing, noise gating reduces steady noise sources such as rumble from LP records, 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 is broken up into many frequency bands by a collection of overlapping band-pass filters, 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 passbands survive.
The technique was implemented in real-time electronics in some audiophile record players 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 track on the Beatles album Abbey Road) or is weak compared to the noise (as in very faint tape recordings), the noise gating can add 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.
[edit] 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 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. In fact 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 synthesised 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 slighly ahead, controlling the bass guitar via a gate whose sidechain is fed by the kick drum signal can pull the two sounds together.
