The noise gate This is where you 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. In this case, it isn't a problem for the gate to decide whether it should be on or off, as long as you've set the controls correctly, and you do have a little margin for error. But if you have a complex source, you may have more difficulty. By 'complex source', I mean two or more sounds going on at the same time, one of which you're trying to separate out. This happens when 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 isn't musically aware and can't 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, these are initially set to the two extreme positions, where no filtering takes place. If you don't need them, leave them in this position, as extreme settings can slow the gate down a little. Newcomers to gating sometimes twiddle these controls at random, expecting something miraculous to happen. Nothing miraculous, (or even mildly awe-inspiring), will happen unless you have twigged what the Filters are for and how they affect the gate's performance. The Filters only affect the Key signal driving the gate, not the sound being processed, and Drawmer have thoughtfully added a Key Listen feature, which allows the effect of the Filters to be heard when setting up. If the external Key input isn't being used, the main input also feeds the side-chain, so you'll hear a filtered version of the input in Key Listen mode. If the unit is set for External Keying, you'll hear the external signal processed via the Filters.

To illustrate how the Filters might be used is best achieved by example: suppose, for instance, that you're gating a snare drum and want to stop the hi-hat (which is bleeding though into the snare mic at quite a high level) from opening the gate. Without the Filters, you can't find a Threshold level where the snare drum opens the gate and the hi-hat doesn't. However, the two sounds have very different frequency characteristics; the snare drum covers pretty well the full frequency range, while the hi-hat is strong mainly at high frequencies. It seems obvious that if you can prevent the high frequencies from getting through to the triggering circuitry, 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 2kHz or so. You can hear the effect of the filter in Key Listen mode, and the object is to retain as much of the snare sound as possible, while removing or reducing the contribution from the hi-hat. You'll have to tinker with the Threshold again, in all probability, but there's now a good chance that you'll be able to gate out the hi-hat completely, with no unwanted false triggering.

My next example concerns vocals: much of the background noise entering a vocal mic is probably low frequency in character, since low frequencies tend to leak more readily than high. The human voice, on the other hand, has very strong harmonics between about 1Hz 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 don't have enough energy between 1kHz and 5kHz to trigger the gate. In this case, lower the frequency of the LF Filter until the gate triggers reliably but the low-frequency spill is still excluded. For very low-frequency spill, a lower Filter setting of around 250Hz might be more appropriate and is less likely to cause the gate to 'miss' wanted sections of vocal. - more