I will briefly review types of dynamics processors (covered extensively in my previous book) for reference in the following discussion regarding dynamics tactics.
Compressors/Limiters and Expanders/Gates
Compressors limit the dynamic range of program material by reducing the vol- ume of the louder sounds. What is reduced is determined by the user-controlled threshold setting such that any sound that goes over the threshold is compressed and those under the threshold pass through unchanged. The degree the louder sounds are compressed is controlled by the ratio—the higher the ratio, the greater the compression. Limiting is simply compression with a high ratio (typi- cally 20:1 or higher).
Building a Mix
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Expanders are the opposite of compressors—they expand the dynamic range of program material by decreasing the volume of the quieter sounds. What is reduced is determined by the user-controlled threshold setting, such that any sound that fails to goes over the threshold is reduced in volume and those above the threshold pass through unchanged. The degree the quieter sounds are re- duced is controlled by the ratio—the higher the ratio, the greater the expansion. Gating is simply compression with a high ratio (typically 20:1 or higher). What follows focuses on compressors/limiters because of their much more widespread use in mixing.
Tube and Optical Compressors verses VCA and FET Compressors
There are a variety of technologies used to detect level and thereby trigger com- pression or expansion based on the threshold setting. Tube and optical detectors are both older technologies (e.g., the Fairchild 660 and the UA LA-2A), though modern versions are also manufactured (e.g., Manley Variable Mu and the Tube Tech CL-1B). VCA (Voltage Control Amplifier) and FET (Field Effect Transis- tor) detection designs use newer technologies (e.g., dbx 160 and the Empirical Labs Distressor). Software compressors emulate the characteristics of one or more of these kinds of detection circuits.
While various technologies create different characteristics in regards to attack, release, and ratio functions, software design can model any characteris- tics and create any kind of hybrid compressor function. As a rule, the tube and opto type compressors have slower attack-time capabilities and a softer knee. The slower attack makes the compressors suitable when there is no need to clamp down quickly on transients. The softer knee (variable ratio) means that the louder sounds above the threshold will be compressed more than the qui- eter ones that are also above the threshold.
The VCA and FET compressors are capable of very fast attack times and greater control over ratio characteristics so they are more flexible in use. Each compressor design is capable of more or less transparency—that is the extent to which they do not color the sound—but the VCA and FET designs are capable of producing less coloration while the tube and opto designs can provide the classic “valve distortion” (a thickening or “warmth” created primarily by even- order harmonic distortion), which is highly valued in some circumstances but may not be desired in others (it may read as muddiness or blurring of the sound). (There’s more on this in the sections on dynamics strategies and in chapter 6 on processing individual elements in your mix.)
RMS versus Peak Level Reading
Compressors can also vary in how they read audio level. RMS level detection (Root of the Mean value Squared) looks at average level over time, whereas peak level detection reacts to the momentary audio peaks in level. Some compressors offer a choice between the two and some offer control over the “window” size of
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the RMS readings. That is, as the RMS detection looks at a smaller and smaller window of sound for its average, it becomes more and more like a peak detect- ing compressor. In general, RMS detection is better at general “leveling” com- pressor function and is going to produce gentler results while peak compressors do better at taming sounds with a lot of quick dynamic changes (like snare drum tracks).
Variations in attack time function similarly to peak versus RMS detection, with slower attack times producing more gentle, leveling type results and fast attack times better at taming sharp dynamic transitions (and consequently dull- ing the sound in some circumstances).
Brickwall Limiters
Brickwall limiters are similar to traditional limiters in that they reduce the dynamic range of program material, but they employ a ratio of infinity:1. That means that program material is never allowed to exceed the threshold (this is the “brick wall”). They employ “look-ahead” technology to do this, meaning they analyze the signal and then process it, causing significant delay in the pro- cessing and requiring delay compensation if used any place other than the final stage (stereo buss) of the mix.
Brickwall limiters represent a kind of hyper-compression that has found significant application in both mixing and mastering, with some questionable results. The most frequent application of brickwall limiting is on the overall program material—the stereo buss. Many argue that the brickwall limiting is significantly overused and a detriment to both the sound of the music and the experience of the consumer (search “loudness wars” on the Internet for more information/opinions). Because of its prevalence and importance, I have cov- ered this critical issue in chapter 3 and also discussed the way it has changed the relationship between mixing and mastering in appendix B.
Brickwall limiters are sometimes used on individual tracks or on subgroups to provide maximum leveling. Tracks (such as lead vocals or raps) and subgroups (such as drums) are sometimes both compressed and brickwall limited to make them as present as possible. At what point the “presence” afforded by brickwall limiting becomes overbearing or unmusical is an aesthetic decision—it has its place, but should be applied with a clear understanding of its effect.