XO Wave: Compressor

Note: The term "level" is used extensively in this document, and in XO Wave. In other contexts, it might also be called "volume" or "loudness".

The Compressor in XO Wave is a powerful tool for mixing, mastering, and recording; and can even be useful when editing. If you have never used a compressor, you may want to start with the How to Use Compression tutorial. If you are experienced with compression already, you may be able to just jump right in and use the XO Wave Compressor without any difficulty. Either way, this page explains XO Wave's Compressor, a powerful and flexible tool for controlling dynamics in your recording projects.

Signal Path

The diagram at the right shows the signal path for XO Wave's Compressor. The main signal first goes through a gain reduction step, then a make-up gain step, and finally a Limiter. There is also an additional signal path called the Side Chain, which is an alternate signal that controls the behavior of the compressor, but is not heard directly in the Compressor's output.

The amount of gain reduction at any time depends on the ratio and threshold settings, and on the strength of the signal in the Side Chain (which by default is the same as the input signal). To make up for the gain lost during gain reduction, make-up gain is available to boost the level of the entire signal. Because the gain reduction step tends to reduce the loudest parts more than the softest parts and the make-up gain increases all sounds equally, the net effect of these two steps is usually to make the whole track sound "louder" or "hotter", where the quieter segments are closer to peak volume.

The first two stages (gain reduction and make-up gain) both modify the signal in a smooth way, without jarring changes in gain or amplitude. As the signal gets stronger, the gain reduction steps in little-by-little to reduce the signal. For example, with the ratio set to 1:2, a 2 dB increase in signal strength results in a 1 dB increase in the output. The smoothness of the gain-reduction step over time is controlled by the attack, and release settings.

The third stage, the Limiter, modifies the signal according to a strict rule: never let the signal go above its threshold. When the signal goes above the threshold, it engages the Limiter which reduces it by the amount needed to keep it in range. This is useful for digital recordings, which are often constrained to specific amplitudes, but it can result in a very harsh sound because the sudden changes in gain can distort the audio significantly. Because of this, the limiter in XO Wave uses lookahead and release controls to smooth the transition from normal to limiting behavior. In the case of signals that are above the threshold for long periods of time, however, audible distortion may still occur.

Common Interface Features

There are 3 parts of the Compressor's graphical interface panel. The top provides common meters and the Meter Hold control. The middle provides a tabbed area which can switch between Basic, Compressor, Limiter, and Side Chain controls. The bottom provides additional common controls. The top area includes the following:

• Compression State indicator: The three simulated LEDs in a traffic-light configuration show the state of the Compressor. Green means that the signal is below the threshold and therefore no compression is occurring. Yellow means that the signal is below the main threshold but above the "knee" of the Compressor, meaning that partial compression is taking place. Red means that the signal is loud enough that the Compressor is fully active.
• Limit indicator: The limit indicator activates when the Limiter has detected audio signal which exceeds its threshold, and the reduced the gain to keep the signal in range. Note that because of the Limiter's advanced "lookahead" feature, an active limiter does not necessarily mean that hard clipping has occurred.
• Gain Reduction Meter: This meter (a long strip of squares) indicates how much the signal's gain has been reduced by the Compressor.
• Meter Hold: Here you can adjust the "ballistics" of the gain reduction meter, meaning how fast the meter changes value. Like the main XO Wave meter, the gain reduction meter is a peak meter, meaning it measures the peak value recently experienced. The Meter Hold setting controls how long (in milliseconds) the meter "remembers" a peak before it "forgets", so a new peak can register.

The bottom of the Compressor panel offers three buttons.

• Bypass Compressor disables the Compressor (including makeup gain). This is useful for comparing the compressed and uncompressed signals, or if you just want to use the Limiter.
• Bypass Limiter disables the Limiter. Many people only use a limiter towards the end of their signal chain.
• Side Chain Listen allows you to listen to the side chain, rather than the main output of your Compressor/Limiter. This is most useful while you are setting up the Side Chain EQ.

Basic Use

When you open the Compressor, it initially shows the most commonly used controls. The center of the window has a tabbed panel. By selecting tabs other than the initial Basic tab, you can expose the more advanced controls, described in detail below.

The Basic tab contains a threshold control, which works just like most compressors' threshold control; and a gain control, which allows you to set the gain applied after compression. This is sometimes called "make-up gain" since it is typically used to compensate for the reduction in signal caused by the Compressor.

Just as importantly, the Basic tab contains a Preset pop-up menu, which lets you select from a list of Compressor presets for controls such as Ratio, Attack, Release, and Side Chain settings. Although we cannot provide presets for every situation, they offer a good starting point, and can be a real time saver. It is important to note that when you apply a preset, all the parameters will be reset -- including any automation you may have. Fortunately, like most things in XO Wave, changing presets is undoable, so if you apply a preset by accident or change your mind, you can just Undo.

Usually, the first thing you want to do when using the Compressor is select the most appropriate preset from the list (Note: we at XO Audio are still tweaking the presets, so please let us know if you have any suggestions!) After that, lower the threshold until you get the amount of gain reduction you want. Finally, add some gain to make up for the signal you lost during compression (The gain reduction meter can give you an idea of how much the gain is being reduced at any given time.)

When adjusting gain, you should realize that this occurs after the compression stage, and before the Limiter stage. This means that if you add too much gain, the Limiter will step in and reduce the gain further. When the Limiter is reducing the signal level, the red "limit" indicator at the top of the Compressor becomes active. The Limiter is a "hard" or "brickwall" limiter, meaning that it will do whatever it has to do to keep the signal level in range. For tiny sections of audio, this is usually not a problem, but if you hear audible distortion or see the limit indicator stay on for more than just a flash, you should probably either increase the amount of compression, or decrease the gain setting.

Most of the time, that's all you'll need to do. For finer control of the Compressor, Limiter, or Side Chain, a variety of other controls are available by selecting the appropriate tabs, documented in detail below. In addition, the later sections of this document describe use of the Compressor for special purposes, such as de-essing.

Basic Controls

The functions of the Basic controls are:

• Preset: The Preset pop-up menu allows you to select a preset for the settings. This is especially useful when first setting up the Compressor. Note that if you select a preset, existing Compressor settings (including automation) will be cleared. If you accidentally select a preset or you change your mind, you can Undo the change.
• Threshold: This controls the Compressor's trigger threshold. Signal which is below the threshold is not compressed. Lowering the threshold results in compressing more of the signal, while raising it exempts more of the signal from compression. For more info, see our tutorial: How to Use Compression.
• Gain: The gain control allows you to add gain (volume) to make up for the gain removed by the Compressor. The Compressor reduces the signal level only when the signal is above the threshold, but the gain control adds the same amount of gain to the entire signal (otherwise it would just negate the Compressor).

Advanced Controls

In addition to the common and basic controls above, the Compressor offers additional options grouped under 3 tabs: Compressor, Limiter, and Side Chain.

Compressor Tab

The Compressor tab allows you to fine-tune compression settings. The Threshold & Gain settings are shared between tabs, so a change here will also be reflected in the Basic tab, and vice-versa.

• Attack: You can control both the attack time and the algorithm used when the signal goes above the threshold. These control how long it takes for the compression to take effect after the signal goes over the threshold.
• Release: You can control both the release time and the algorithm used when the signal falls below the threshold. This controls how long it takes for the Compressor to stop compressing after the signal drops below the threshold.
• Threshold: This threshold control is the same control as is seen in the Basic tab. It controls the volume level above which the Compressor activates.
• Knee: This control allows the Compressor to start working before the signal reaches the threshold. This allows slightly "softer" or "gentler" compression, and can make the effect considerably more subtle.
• Ratio: This control works the same as the ratio control found on most compressors. When the signal goes above the threshold, the gain of the (virtual) circuit is reduced by this amount. For example, with the ratio set at 2:1, an increase in input of 2 dB above the threshold will result in an increase in the output of only 1 dB. Higher ratios yield more compression, but only signals that reach the threshold (knee) are affected.

Limiter Tab

The Limiter prevents the signal from ever going above a certain level. The simplest limiter design would simply "clip" the signal if it went above a certain level. This can create a very harsh sound, so XO Wave's Compressor has a "lookahead" feature which, by delaying the signal slightly, is much less noticeable. When driven to an extreme, however, no limiter can avoid clipping. Therefore it is important to use the Limiter carefully.

• Lookahead: This control allows the Limiter to start reducing the signal before the peak occurs. This can make it considerably less obtrusive; it corresponds to the Attack & Release settings of the Compressor proper.
• Release: This control allows the Limiter to continue reducing the signal even after the signal is below the threshold, again to avoid jarring volume changes.
• Threshold: Controls the threshold at which the Limiter will cap the signal level.

Side Chain Tab

The Compressor's Side Chain allows you to modify the control signal for the Compressor (which it compares to the specified threshold) without modifying the 'real' audio signal which the Compressor adjusts, based on those comparisons. This is useful for precise control of the Compressor's behavior, such making the compressor more or less sensitive to certain frequencies.

The Side Chain Listen option plays back the modified side chain signal instead of "real" input signal; toggle it on to confirm that your Side Chain EQ is having the desired effect; toggle it on and off to compare the side chain to the unmodified signal.

The two most common uses of side chain are:

• Preventing bass from "punching out" the rest of a mix. Compared with our ears, compressors tend to be overly sensitive to bass frequencies. When compressing a stereo mix, the bass can cause the compressor to over-react, resulting in an overly compressed sound, often with other instruments sounding like they've been "punched out". The solution to this is to reduce the bass going through the Side Chain.
• "De-essing" a vocal track. Often, recorded vocals have very strong "sibilant" sounds (consonants such as "s" and "th") which can be unpleasant, especially when other effects such as reverb are added. Generally, it is best to use proper mike selection and mike technique to avoid sibilants from the start, but there are many situations where sibilance was not avoided during tracking, or where the effects added after tracking enhanced the sibilance more than desired, so a good de-esser is always good to have. Sibilant sounds occupy a frequency range from around 3kHz to 10kHz, and sometimes even higher. Your first instinct might be to use EQ to get rid of it, but that tends to dull the sound of the whole track. Instead, you can use a compressor with a Side Chain. The Side Chain should be set to emphasize the undesired frequencies, so the compressor will take care of them. In XO Wave, we just go to the Side Chain tab and use the EQ settings to turn up the frequencies that are most objectionable. The Side Chain Listen feature is helpful for finding those pesky frequencies. Turning up the frequencies we want to get rid of may seem counter-intuitive at first, but the compressor does a much better job at smoothly reducing sibilance than manual EQ.

Many people also wish to use the Compressor's Side Chain for "ducking". In XO Wave, the Envelope Generator effect provides more convenient control over ducking than the Compressor.

The controls available in the Side Chain are the same as those available in the Basic Butterworth Filter. Please see our EQ Tutorial for more detail on this filter.

Conclusion

The XO Wave Compressor is a powerful and versatile tool that can act as a compressor for individual instruments or a whole stereo mix, or provide special functions such as a de-essing. By carefully selecting a preset, you can usually do most of what you need with very little effort. If the presets don't suit you, XO Wave offers a wealth of controls for fine-tuning the Compressor to meet your needs.

-- Bjorn Roche