Gates and Expanders

By

Kevin Doyle

     Gates and Expanders are primarily; though not only, noise management devices. Unwanted noise is the enemy of all parts of the recording process. Unwanted noise can be caused by the ambient environment or can be generated by, or in, the recording/processing circuitry or storage medium id it is tape. This problem is magnified when you are dealing with multi-track recording as the noise on each track can be added to the noise on all the tracks. This means that the volume of the noise can go well passed audible and into the really troublesome region. Not really the way you want to produce your recordings Once noise is part of a signal, it is almost impossible to remove it, and if you do you will probably change the tone of the sound. There are new software programs such as Waves Noise Eliminator that work very well, but it is expensive and does colour the sound a little. It makes sense then to manage the noise that can be present on the signal before it gets there and to reduce the amount of noise contributed by each track to the mix down, as much as possible. In particular, if there is no signal content during a quiet section, for example the quiet sections between vocal phrases, then the track is only contributing unnecessary noise to the final mix. For example; hi-hat leakage on a snare track, gated reverb, keying additional sounds… An ex/gate appears to make the louder parts of the signal louder but doesn’t, it actually lowers the level of the softer parts of the signal making the louder parts more audible. If you are using any equalization or compression, you'll normally want to use the gate first in the signal chain. The reason for this is that successful gating usually requires adjustment of the gate's response to the exact levels and timbres of the wanted and unwanted parts of your audio signal. Tweaking a pre-gate equalizer could easily mean that you have to also re-tweak your gate's threshold and filtering controls. Compressing a signal before gating it can make reliable triggering even more difficult to achieve — the compressor will make the unwanted signal a 'moving target' and require a higher threshold that might start to eliminate part of the desired signal. When mixing a multitrack recording, it is common practice to employ several gates, even if individual tracks don't seem too noisy on their own. This is because noise is cumulative, with every playback track of a multitrack recording contributing to the general level of noise arriving at the mix buss, so tracks are always best muted when not in use. If you are working with a digital system you can edit out any regions containing only noise, or you can simply mute tracks whenever they're unused with mixer automation, if you have it. However, the fact that gates can be set up to perform this function automatically often means that gating proves a more elegant solution

How it works

When a signal falls below a preset threshold, the signal is eliminated or lowered in amplitude. When the signal moves above the threshold, the gate then opens and doesn’t affect the signal anymore.

Attack Time

Once a threshold has been set, the signal must pass that threshold for the gate to open. How fast it will open is determined by the attack time. In order for drums to work you will require a very fast attack time in order to allow the transient to fully pass without any deterioration of the initial part of the waveform. With instruments that do not contain initial transients, a medium attack time will be fine. However, signals that contain a lot of low frequency information will require a med to slow attack time because of half cycle distortion. For background ambient noise problems, a slow attack will be needed so it doesn’t sound like an on/off switch.

 

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