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A noise gate or gate is an electronic device or software that is used to control the volume of an audio signal. Comparable to a compressor, which attenuates signals ''above'' a threshold, noise gates attenuate signals that register ''below'' the threshold.〔Hodgson, Jay (2010). ''Understanding Records'', p.86. ISBN 978-1-4411-5607-5.〕 However, noise gates attenuate signals by a fixed amount, known as the range. 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'.〔.〕 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. Gates typically feature 'attack', 'release', and 'hold' settings and may feature a 'look-ahead' function.〔Hodgson (2010), p.87.〕 ==Roles== Noise gates 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. 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. 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 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.〔(【引用サイトリンク】title=Drawmer DS201 Dual Noise Gate Operator's Manual )〕 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 an external ''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 ''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 also helps avoid chattering, as described above. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Noise gate」の詳細全文を読む スポンサード リンク
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