Dynamics 101 Vocabulary of Dynamics Processors

Introduction: Rick Naqvi is one of the best people I know to teach on the care and feeding of compressors and limiters, which makes sense, as he works for Presonus, which makes excellent compressors and limiters. He has given us permission to include it here on the ChurchSoundGuy blog. So as to avoid being overwhelming, it will be posted in several installments. This is part 3.

In scientific research, it is widely accepted that if one needs to master a subject quickly, learn the associated vocabulary and industry terms…

1. Compressors

Threshold

The Compressor threshold sets the level at which compression begins. When the signal is above the threshold setting, it becomes ‘eligible’ for compression. Basically, as you turn the threshold knob counterclockwise, more of the input signal becomes compressed. (If you have a ratio setting of greater than 1:1.)

Ratio

Ratio sets the compression slope. This is defined as the output level versus the input level. For example, if you have the ratio set to 2:1, any signal levels above the threshold setting will be compressed at a compression ratio of 2:1. This simply means that for every 1dB of level increase into the compressor, the output will only increase 1/2dB, thus producing a compression gain reduction of 0.5dB/dB. As you increase the ratio, the compressor gradually becomes a limiter. A limiter is defined as a processor that limits the level of a signal to the setting of the threshold. For example, if you have the threshold knob set at 0dB, and the ratio turned fully clockwise, the compressor becomes a limiter at 0dB. This means that the signal will be limited to an output of 0dB regardless of the level of the input signal.

Attack

Attack sets the speed at which the compressor ‘acts’ on the input signal. A slow attack time (fully clockwise) allows the beginning envelope of a signal (commonly referred to as the initial transient) to pass through the compressor uncompressed, whereas a fast attack time (fully counter-clockwise) immediately subjects the signal to the ratio and threshold settings of the compressor.

Release

Release sets the length of time the compressor takes to return the gain reduction back to zero (no gain reduction) after crossing below the compression threshold. Very short release times can produce a very choppy or ‘jittery’ sound, especially in low frequency instruments such as a bass guitar. Very long release times can result in an over compressed sound, sometimes referred to as ‘squashing’ the sound. All ranges of release can be useful at different times however and you should experiment to become familiar with the different sound possibilities.

Hard/Soft Knee

With hard knee compression, the gain reduction applied to the signal occurs as soon as the signal exceeds the level set by the threshold. With soft knee compression, the onset of gain reduction occurs gradually after the signal has exceeded the threshold, producing a more musical response (to some folks).

Auto

Places a compressor in automatic attack and release mode. The attack and release knobs become inoperative and a pre-programmed attack and release curve is used.

Gain

When compressing a signal, gain reduction usually results in an overall reduction of level. The gain control allows you to restore the loss in level due to compression. (Like readjusting the volume.)

Link

When the Link button is pushed in, this channel becomes the slave of the channel to it’s left. All of this channels controls become disabled and metering should be referred to the channel to the left. Essentially, the left channel is the master and the channel with link pushed in is the slave channel in a stereo linked pair.

Compressor Sidechain

The sidechain jack interrupts the signal that the compressor is using to determine the amount of gain reduction to apply. When no connector is inserted into this jack, the input signal goes directly to the compressor’s control circuitry. When a connector is inserted into this jack, the signal path is broken. This signal can then be processed by an equalizer for example to reduce sibilance (de-essing) in a vocal track. The signal is then returned to the unit via the connector. The signal returned to the sidechain could be that of a narrator or vocalist. In this application, the audio that is passing through the compressor will automatically ‘duck’ when the narrator speaks or vocalist sings.

2. Expanders

Downward Expansion

Downward expansion is the most common expansion used in pro audio and recording. This type of expansion applies noise reduction to all signals below a set threshold level.

Ratio

The expansion ratio sets the amount of noise reduction applied to a signal once the signal has dropped below the expansion threshold. For example, a 2:1 expansion ratio attenuates a signal 2dB for every 1dB it drops below the threshold. Ratio’s 4:1 and higher act much like a noise gate without the ability to tailor the attack, hold and release times.

3. Noise Gates

Threshold

The gate threshold sets the level at which the gate opens. Essentially, all signals above the threshold setting are passed through unaffected, whereas signals below the threshold setting are reduced in level by the amount set by the range control. If the threshold is set fully counter-clockwise, the gate is turned off (always open), allowing all signals to pass through unaffected.

Attack

The gate attack time sets the rate at which the gate opens. A fast attack rate is crucial for percussive instruments, whereas signals such as vocals and bass guitar require a slower attack. Too fast of an attack can, on these slow rising signals, cause an artifact in the signal producing an audible ‘click’. All gates have the ability to ‘click’ when opening, however a properly set gate will never click. Below is an example of a gate opening very fast on a pure sine wave. Notice the almost vertical edge of the waveform when the gate opens. This is what produces the high frequency artifact known as ‘click’.

Figure 2.1 shows a gated sine wave. Notice that when the gate opens, an artifact occurs in the waveform sometimes referred to as the gate ‘click’. To avoid the click, add some attack time such that the gate opens at approximately the same frequency as the signal.

Hold

Hold time is used to keep the gate open for a fixed period of time following the signal going below the gate threshold. This can be really useful for effects such as ‘gated snare’ where the gate remains open after the snare hit for the duration of the hold time then abruptly closes.

Release

The gate release time determines the rate at which the gate closes. Release times should typically be set so that the natural decay of the instrument or vocal being gated is not affected. Shorter release times help to clean up the noise in a signal but my cause ‘chattering’ in percussive instruments. Longer release times usually eliminate ‘chattering’ and should be set by listening carefully for the most natural release of the signal.

Range

The gate range is the amount of gain reduction that the gate closes down to. Therefore, if the range is set at 0dB, there will be no change in the signal as it crosses the threshold. If the range is set to -60dB, the signal will be gated (reduced) by 60dB, etc.

Spectral or Frequency Selective Gating

There are many situations, especially with drums, when other instruments open up the gate instead of the instrument being gated. For example, tom-tom mics are generally placed very close to the cymbals on a drum kit, creating the possibility of the cymbals opening up the tom-tom gates. A frequency selective gate will allow the user to specify which frequencies are ‘eligible’ for gating. Therefore the cymbals may be removed from ‘eligibility’ by using a low pass filter to filter out the high frequencies of the cymbals. The gate will no longer open up during cymbal crashes, leaving the low frequencies of the tom-tom to properly open the gates.

Gate Keying

It is sometimes impossible to set a noise gate to open precisely when an instrument sounds in a noisy environment. Most gates feature an external gate key that will allow the gate to be opened using an external signal, in effect ‘triggering’ the gate to open. For example a bass guitar gate might be ‘triggered’ using the kick drum to tighten up the low end of a mix. Or a snare track might be ‘triggered’ using a click track or sequencer to place the snare right in time with the music.

Courtesy Presonus. Used by permission.

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David McLain | Technical Geek| CCI SOLUTIONS
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