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Dynamics Processors -- Technology

Chapter 3 -- Side-Chain Controls

A number of parameters govern side-chain activity, but the four primarily ones are Threshold, Ratio, Attack and Release. Some dynamics controllers offer front-panel adjustment of these parameters, or software control, while others auto-set them at optimum values.

Threshold and Ratio have unambiguous definitions:

Threshold

Like crossing through a doorway, this is the beginning point of gain adjustment. When the input signal is below the threshold for compressors, or above the threshold for expanders, a dynamics processor acts like a piece of wire. Above the threshold, the side-chain asserts itself and reduces the volume (or the other way around for an expander). A workable range for compressors is -40 dBu to +20 dBu. A good expander extends the range to -60 dBu for low-level signals.

Ratio

Once the signal exceeds the threshold setting, just how much the volume changes depends on the ratio setting. A straight wire has a ratio of 1:1 -- the output tracks the input -- a 2 dB change at the input produces a 2 dB change at the output.

A severe ratio is 10:1. For a 10:1 ratio, a 10 dB blast at the input changes only 1 dB at the output -- this represents heavy processing. Kinder, gentler ratios are in the 2:1 to 3:1 range.

Figure 4 shows the normal ranges for ratio controls of 1:1 to 10:1. If provided, the lower limit of 1:1 is for bypassing.

Figure 4. Input vs. output for various relationships.

Gain

Sometimes referred to as "make-up gain" in compressors, this controls the desired output level with compression active. The preferred range for professional applications is ±12 dB with a center-detent 0-dB unity gain position. Gain can be done in the main signal path, or in the side-chain as control offset.

Hard Times

Unfortunately precise definitions for the terms attack and release do not exist due to a lack of industry standards. Moreover, manufacturers make this worst by not explaining how they define the terms. Most don't, they just list a range of settings, leaving the user to guess if the time shown represents how long it takes to get to the end of the gain change, or to the middle, or the 3-dB point, or what -- caveat emptor.

Attack

Defines how quickly the function responds to an increase in side-chain input level above the threshold. For compressors and AGC, this defines how quickly the gain is turned down. For gates and expanders, this defines how quickly the gain is turned up.

Because increasing time has a diminishing effect on gain for compressors, it is practical to specify attack as the time required for gain to settle to a defined percent of final value. Typical are 86% or 95% of the final value.

Attack times for compressors generally range between 25 ms and 500 ms.

For expanders (with ducking & gate features) this range changes to 0 ms ("instantaneous" attack, relative to any DSP or look-ahead delay for digital processors) to 250 ms (since longer attack times are not necessary).

In expand mode, attack time determines the rate of gain increase as the control signal moves toward or above the set threshold.

In gate mode, attack time determines how quickly the gate opens once the control signal exceeds the threshold setting.

In ducker mode, attack time determines how quickly the signal is reduced as the control signal exceeds the threshold setting.

Release

Defines how quickly the function responds to a decrease in side-chain input level below the threshold. For compressors and AGC, this defines how quickly the gain is turned back up once these processes have stopped. For gates and expanders, this defines how quickly the gain is turned down. Release is typically defined by an RC (resistor-capacitor) time constant in the log domain, resulting in a constant dB per second gain change at the output. Because the dB per second is constant, release can be specified directly in dB/second or as the time required for a 10 dB change (typically a 10 dB step).

It is important to understand the difference between release rate -- as determined by this control -- and release time. There is no industry standard and different manufacturers define this control differently.

Rane defines this control, in a compressor for example, as how long it takes for the gain to change by 10 dB, not how long it takes to return to unity gain (no gain reduction).

To calculate the actual release time requires a little math: Release Time = (Gain Reduction x Release Setting) / 10 dB

Example: with the release control set to 1 sec, when a signal with 5 dB of gain reduction presently applied suddenly drops below threshold, the release time is:

(5 dB x 1 sec) / 10 dB = 0.5 sec.

Typical compressor and expander release settings are between 25 ms and 2 seconds.

In gate mode, the release time determines how quickly the gate closes as the control signal drops below the threshold setting.

In expand mode, the release time determines how quickly the signal is turned down as the control signal moves below the set threshold.

In duck mode, the release time determines how quickly the signal is ramped up when the control signal drops below the threshold setting.

Knee

Unique to compressors, this function controls the action at the threshold point. Hard knee does nothing until the signal exceeds the threshold point, and then applies full compression.

Use of a soft knee significantly reduces distortion caused by abrupt transitions from unity gain to a compressed signal. An accurate soft knee response is difficult to achieve using analog methods. Digital implementations allow locating the center of the knee exactly at the threshold with a mathematical function defining a smooth transitioning from unity gain to the specified ratio. Note in Figure 5 that a proper soft knee response does not alter the ultimate gain reduction achieved above the knee, which commonly occurs in analog designs. A soft knee is defined by the "span." The span defines how many dB below the threshold compression begins and how many dB above the threshold compression reaches the specified ratio.

Soft knee begins applying a small amount of compression just before the threshold point is reached, continues increasing compression through the threshold point and beyond, finally applying full compression to the highest level signals. Depending on the application and source material, soft knee settings sound more natural. However for maximum loudness before compression (equipment protection for instance) use hard knee settings.