Parallel compression

Parallel compression, also known as New York compression, is a dynamic range compression technique used in sound recording and mixing. Parallel compression, a form of upward compression, is achieved by mixing an unprocessed 'dry', or lightly compressed signal with a heavily compressed version of the same signal. Rather than bringing down the highest peaks for the purpose of dynamic range reduction, it reduces the dynamic range by bringing up the softest sounds, adding audible detail. It is most often used on stereo percussion buses in recording and mixdown, on electric bass, and on vocals in recording mixes and live concert mixes.

The internal circuitry of Dolby A noise reduction, introduced in 1965, contained parallel buses with heavy compression on one of them, the two mixed in a flexible ratio. In October 1977, an article by Mike Bevelle was published in Studio Sound magazine, describing the technique as applied to studio recordings. Bevelle called it "side-chain" compression, though that term made for confusion with the side-chain compression technique which uses an external "key" or "side chain" signal to determine compression on a target signal. Bevelle's article, entitled "Compressors and Limiters", was reprinted in the same magazine in June 1988. A follow-up article by Richard Hulse in the April 1996 Studio Sound included application tips and a description of implementing the technique in a digital audio workstation.Bob Katz coined the term "parallel compression", and has described it as an implementation of "upward compression", the increase in audibility of softer passages. Studio engineers in New York City became known for reliance on the technique, and it picked up the name "New York compression".

The human ear is sensitive to loud sounds being suddenly reduced in volume, but less so to soft sounds being increased in volume—parallel compression takes advantage of this difference. Unlike normal limiting and downward compression, fast transients in music are retained in parallel compression, preserving the "feel" and immediacy of a live performance. Because the method is less audible to the human ear, the compressor can be set aggressively, with high ratios for strong effect.

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