Stereo and Mono Sidechain Compressors for Tracktion
level, we’ve made the vocal track louder in the mix without reaching a volume at any point that will cause distortion. To do this requires a very fast compressor or limiter, but it’s a good example of the principle.
Since to most listeners, louder equals better, another use for compression is to make a mix sound louder. Often there’s a single sound - a snare drum for example – that’s noticeably louder than anything else in the mix. A drummer hits the snare louder on some beats, and the loudest hit determines the maximum level that can be recorded. By compressing the overall mix and sitting on the loudest hits, the average level of the song can be raised.
Stereo or “Program” Compressors
A compressor is basically a single channel device, but stereo compressors, often called “program compressors,” are most often used to compress a full stereo mix.
The thing that differentiates a stereo compressor from simply patching one compressor into each channel is that the signal that controls the amount of gain reduction is shared by both channels.
Reducing the level of one channel of a stereo pair will cause the balance to shift to the louder side. We don’t want the stereo image to wander around when one channel goes over threshold and the other one doesn’t, so we connect the level detectors of the two compressors together. Now when either channel requires some gain reduction, that same amount of gain reduction is applied to both channels.
Compressor Artifacts
Two uncomplimentary terms often used to describe the sound of a compressor are breathing and pumping.
Breathing
Breathing is most noticeable on a solo voice and is often, in fact, the sound of the vocalist breathing. If release time is short, the gain rises quickly during pauses between words, just as the singer takes a breath. The increased gain makes the breath more audible.
Hearing a singer taking a breath may not always be desirable or dignified, but at least it’s organic. Few recordings are made in an absolutely silent environment, however. Any ambient noise in the room will be boosted when the gain rises, creating an artificial “breathing” sound, perhaps even bringing leakage from the singer’s headphones along with it.
All compressors exhibit some breathing, but careful adjustment (which includes controlling room acoustics and mic positioning) can minimize it.
Pumping
Pumping is another compressor artifact. It’s more apparent when compressing an overall mix than a single track.
One instrument in the mix that’s louder than the others will trigger the compressor into action. If that instrument stops playing, even for an instant, the level of the mix will increase notice- ably. Each time the dominant instrument starts or stops, it “pumps” the average level of the mix up and down.
Compressors that work best on full program material generally have very smooth attack and release curves and a slow release time to minimize the pumping effect.
User’s Guide
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