Double Duty
Most dual-channel compressors offer stereo linking, a feature that lets you run two channels—for example, stereo acoustic guitar or even an entire mix—through the compressor and have each channel be attenuated the same amount. That keeps one side’s level from dipping more than the other, which would throw the stereo image out of whack.

True stereo linking works by having the channel that exhibits the most gain reduction determine the gain reduction for the other channel. Another form of linking establishes a master/slave relationship between the two channels in which one side (typically the left) is the predetermined master and the other follows its attenuation pattern.

It is commonly said that compression becomes limiting at ratios of 10:1 and higher, but that is not the entire story. Actually, the detector circuits in compressors and true limiters differ by design. A compressor’s detector circuit is usually designed to detect RMS, or average, levels rather than transient peaks. Therefore, transient peaks almost always overshoot a compressor’s threshold level, no matter how high the ratio and how fast the attack time is set. A true peak limiter, on the other hand, employs a detector circuit that responds to peak energy levels and thus reacts faster.

Whereas all true compressors use RMS-sensing detector circuits, detectors for different models can differ substantially in their reaction times. That means two different compressors set to the same attack, release, threshold, and ratio values may nevertheless respond quite differently to the same signal. (That is one of the many reasons it is difficult to recommend specific control settings for compressing various instruments.)

Chain, Chain, Chain
Every compressor has a sidechain detector circuit that “sees” when the threshold has been exceeded and tells the compressor’s gain-control element or amplifier to attenuate the signal. The sidechain is not in the audio path; it’s merely a traffic cop that tells the compressor when to attenuate the signal. The circuits for threshold, ratio, attack, and release are also found in the sidechain.

Full-featured compressors typically provide sidechain inserts on their rear panels. Think of a sidechain insert as an effects loop that patches into a compressor directly before the detector; like the rest of the sidechain, it is not in the audio path, so its effect isn’t directly heard. Sidechain inserts therefore let you process the compressor’s input signal before it reaches the detector. That permits de-essing and other frequency-conscious applications. Here’s an example of how to perform de-essing.

To de-ess a vocal, first patch the send and receive from the compressor’s insert into an equalizer’s input and output, respectively. Next, boost the equalizer’s high frequencies and cut its lows and mids. That causes the compressor’s detector to hear the vocal as having excessive highs. Whenever the whistling sound of sibilance raises its ugly head, the sensitized detector circuit hears it much louder than it really is, causing the circuit to vigorously reduce gain in the audio path. With attack time set to around 50 µ and release time between 50 and 60 ms, the compressor can be made to quickly attenuate the sibilance and get out so the rest of the vocal is left unchanged. Of course, the compressor’s threshold must also be set properly—above the vocal’s average levels—for that to work.

You can also use a sidechain insert to make the detector react to a signal entirely unrelated to the audio-input signal. The classic example here is “ducking”: a sidechain application in which an announcer’s voice is set to trigger a music bed’s attenuation. To set up this type of ducker, play stereo music tracks through a dual-channel compressor and patch the voice-over track (or channel) into the sidechain insert’s receive jack. Next, set the compressor threshold low enough that it responds to every vocal utterance. When the announcer speaks, the detector hears the voice and instructs the compressor to lower the music bed. You can also use that technique to automatically lower, say, guitar levels whenever a lead vocal comes back in. To accomplish this, patch a mult of the vocal into the insert receive jack of the guitar’s compressor channel.

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Reprinted with permission from Magazine, February, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved