Compressors and limiters are specialized amplifiers used to reduce dynamic
range--the span between the softest and loudest sounds. They pump up the volume when things get quiet and slams down the peaks when they get loud.
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gate is a switch that turns off the audio when it goes below a defined threshold. It's great for removing room noise, and makes the recording dead silent when it kicks in.
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sidechain is an output and input back into a compressor. You can connect other signal processors to it. The connected processor only gets the signal when the compressor is working. Sidechains are often used with eqs to make a de-esser, which will soften the loud "SSS" and "PP" sounds from a vocalist when they exceed the compressors threshold.
