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Expert mode is what you are going to have to switch yourself into to achieve something a little more ambitious than basic gating. In my previous simple explanation I assumed that you had a simple sound source such as a guitar with a noisy amplifier. It isnt a problem for the gate to decide whether it should be on or off as long as you have set the controls correctly, and you do have a little margin for error. But if you have a complex source then you may have more difficulty. By complex source, I mean when there are two or more sounds going on at the same time and you are trying to separate out one of them. This happens when you are gating a real drum kit, or when you have recorded a vocal at home and some extraneous noises have entered through the windows, walls and door of your un-soundproofed studio. The gate isnt musically aware and cant tell the difference between wanted and unwanted sounds; it opens whenever the incoming level exceeds the threshold. To help the gate decide when to open and close you can use the two filter controls. As you can see in Figure 2, they are set to the two extreme positions and no filtering takes place. Newcomers to gating sometimes tend to twiddle these controls at random expecting something miraculous to happen. Nothing miraculous, or even slightly awe-inspiring, will happen unless you first of all realise that you have a need to use these controls, identify what it is you have to do, and then carefully set the LF and HF filters to suit the signal you are gating while listening very closely to the output from the gate.
Suppose for instance you are gating a snare drum and want to stop the hihat, which is bleeding though into the mic and is just as loud, from opening the gate. You cant find a threshold level where this will happen (this is a classic example). The two sounds have very different frequency characteristics; the snare drum covers pretty well the full frequency range while the hihat is strong mainly at high frequencies. It seems sensible that if you can prevent the high frequencies from getting through to the triggering circuitry then only the lower frequencies of the snare will open the gate. In this case, all you need do is turn down the high frequency control to around 2kHz, or whatever gives the best result. You will have to tinker with the threshold again in all probability, but there is a very good chance that you will be able to gate out the hihat completely. In my other example, the background noise entering the mic is probably mostly low frequency in character since low frequencies have more penetrating power than high. The human voice is very strong between about 1kHz and 5kHz so set the LF and HF controls to these frequencies respectively. Achieving a precise setting may be difficult because you may now find that s and f sounds at the beginnings of lines dont have enough energy between 1kHz and 5kHz to trigger the gate. But if you take a little time all should be well (and you can budget for some sound insulation later).