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The next time you're at the supermarket pay attention to how the automatic doors open and close. Walk towards the door, the motion detector senses your presence and opens up to let you in. Once you walk through, it senses you're no longer there and closes automatically to keep the heating and cooling costs down. Simple, right? Well, you've just experienced a real-life version of the dynamics processor we call a gate. A gate and its close sibling the expander are extremely common dynamic processing tools. They're generally used to allow the desired clean signal to pass through while removing any unwanted low-level noise, like hum from a guitar amp or bleed from a track, like the sound of a headphone mix that bleeds into a vocal mic during a vocal recording session.
Take a listen to this drum track and notice the bleed of other pieces in this kit. (music playing) Now listen with the gate in effect. Notice how the kick sound is much cleaner and the bleed is eliminated. (music playing) We use gates and expanders to remove or reduce the bleed between notes or phrases, which allows us to process a signal in isolation without processing the bleed.
Here is another example. Listen to the snare drum with compression and reverb applied to it, but without a gate. Notice how the compression and reverb are drawing out the bleed of the other drums in the snare track, especially the kick in the hi-hat. (music playing) Now let's listen with the gate engaged.
The compression and reverb are allowed to work cleanly on the snare signal in isolation, without all that muddy kick bleed or hi-hat exciting the reverb. (music playing) Gates were extremely popular in the past because of the noise floor issues with tape.
Using extreme compression or limiting would tend to bring up the tape hiss in silent passages significantly, so gates and expanders were used to treat signals before heavy processing, maintaining a reasonable signal-to-noise ratio. A gate works a little bit like a reverse compressor, in that instead of attenuating signals over the threshold, it actually allows them to pass through, just like those doors at the supermarket. Gates feature the same threshold, attack, and release controls as compressors. Think of the attack time as the amount of time it takes the door to open up when it senses motion, and the release time is how fast it closes after you walk through.
Some gates feature a hold parameter that allow the gate to remain open a pre-specified amount of time, regardless of whether or not the signal has fallen back below the threshold. The hold control is handy for keeping the gate from overreacting or chattering as it attempts to ride the wide dynamic variations of complex signals. Again, think about that automatic door. It generally stays open for a little bit just to make sure you get through, before slowly closing shut. If we look at a gate's transfer curve, we can see that the output of anything under the threshold is infinitely attenuated, or gated, while the signals above the threshold are passed through in a linear one-to-one fashion.
Now, just because I can, will I gate every track that has bleed? Not necessarily. Sometimes a little bit of bleed or air in the signal can help glue or mix together, and sometimes getting the gate to trigger correctly just causes more problems than it solves. On certain tracks I'll opt to edit out the bleed by hand, removing the portions of the waveform I don't want using my DAW's editing tools. This could be a much cleaner way of dealing with instruments that aren't always playing, like toms. Be extra careful not to over-gate things like vocals.
You never want to cut off the beginning and ends of your words unless you're using it as an effect. You don't always want to kill all the breaths, as these give the track a human quality that I like to retain more times than not. In the next movie, let's look at a variation of a gate called an expander.
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