Gating and Expansion

 

Last month we covered compression and its uses. This month we’re going to look at a close cousin to it that is often used in tandem with it. Gating and expansion use almost the exact opposite approach as compression but have similar controls. Compressors and gates/expanders share design features as well.

       The settings for a gate are very similar to those on a compressor. Controls include threshold (the lowest level that will pass through), attack (how quickly the gate responds once the level falls below the threshold), release (how long the gate stays closed once the level rises above the threshold) and ratio (the rate at which the gate goes from completely open to completely closed). Typical settings would be about –40db for the threshold, 50ms for the attack, 100ms for the release and a ratio of about 4:1. Other controls may include soft or hard knee (determines whether the change from open to closed is smooth or abrupt), input level, output level and side-chain operation

       Gating is used in both tracking and mixing to get rid of unwanted noise or microphone bleed. More often than not, when recording drums, you will get the other instruments in the kit bleeding through the snare or kick drum microphone. In order to properly process those tracks, its generally desirable to get rid of the other drums sounds on that track. That’s where the gate comes in.  What it does is gets rid of any sounds on a track that are below the level set on the gate It is very useful in getting rid of room noise on a track such as air conditioning noise or unintended outside noises. It is not a panacea, however, for correcting inadequately insulated recording spaces. A gate can also be used during tracking to influence a performance by forcing the performers to play no softer than a determined level.

       The side-chain operation of a gate applies the result of gating one track to another track while leaving the original track unaffected. One of the most common uses is to clean up a sloppy bass track where the bassist plays on top or before the beat. The gate is placed on the kick drum track and the side-chain is set to the bass track. With the proper settings, the kick track  will signal the gate to open, allowing the bass track to follow the kick exactly. Another use is to let the kick open and close a pad track to give it a breathing kind of quality.

       Expansion is exactly the opposite of compression. It will take tracks that have a narrow dynamic range and expand them. It is most commonly used when mixing tracks that have been overly compressed during tracking in order to give them a bit of life. The controls are exactly the same as a compressor but function in the opposite fashion. Sounds below the threshold will be pushed lower or gated entirely and sounds above the threshold will be raised. Make no mistake—it cannot completely uncompress a track so be careful when applying effects to your tracks.

       As technology has progressed, software designers have incorporated features such as  look-ahead processing that hardware compressors just can’t do. They also have negative attack times and automatic level compensation.

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