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compressor. Any dynamics processor consists of the "Dynamics Duo"" or two signal paths:

an audio path through the VCA that needs to be as clean as possible--low noise, low

distortion--and a detector path that contains the smarts to figure out what to tell the VCA to

do.

The detector path is often referred to as the "sidechain." (Sounds like a wallet protection

device).The sidechain consists of an audio level detector that converts the audio signal from

which the compressor is "keying" to a control voltage.This control voltage is then modified

by the threshold, ratio, attack, release, (etc.) controls which follow the detector. This

modified voltage controls the gain of the VCA.

If it sounds like Mr. Sidechain is the real brains of the Dynamics Duo, while Mr. VCA is

just the brawn, you're absolutely right. The particular sound of any dynamics processor is

due more to the way Mr. Sidechain handles his affairs than how well Mr.VCA carries out his

orders. Common problems such as the proverbial "pumping" and "breathing" in some

compressors results from sidechain circuitry design, not from VCA deficiencies. But, lest we

fail to give credit where credit is due, never underestimate the essential role that Mr. VCA

plays in providing a clean signal free of noise and distortion.

3. Detection: RMS —True or False?

The principal component of the sidechain is the detector, and its ability to accurately

detect audio levels in the proper way considerably affects the dynamics of the processed

signal. Common sense would tell you that, given a particular audio signal, the detector must

compute a result that corresponds to the result computed by the human ear, if that detector

is to truly function as an audio detector.

There are many ways to do audio level detection, and as you may surmise, different

detection methods yield different results. While some compressor/limiters use peak

detection and fast response characteristics, their action is frequently displeasing to the ear,

even at modest compression ratios, as rapid gain changes result in unnatural signal

dynamics.This is due to the psychoacoustic phenomenon that the human ear responds, not

to the peaks, but to the average power of the signal. In other words, the ear responds to the

sum of the energies of all frequency components present.

Detection and the Dynamics Duo - November 1998

Detection and the Dynamics Duo

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