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Wideband Compressor section
Reduces the dynamic range of the audio and limits it.

 Volume compression (A.K.A. audio level compression) reduces the dynamic range of a sound. This means that loud sounds become softer, and soft sounds become louder.

 Limiting limits the maximum audio level below a certain threshold.

 For a lengthy discussion about compression, see Wikipedia: Audio level compression.

 The compressors and limiters in Stereo Tool are protected against causing distortion. So very aggressive settings and large amounts of limiting can safely be used.

 General panel
General compressor/limiter settings.
  • Wideband compressor
    Turns the compressor/limiter on.

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  • Compressor type
    Analog or Digital compressor type.

     The Analog compressor type is intended to replace the Digital one. It's behavior is generally more natural, so if you are starting on a new preset, it's probably a good idea to use Analog mode. On top of the better end result, it also uses far less processing power.

 Speeds panel

 Other settings panel

 Compatibility settings panel

 Quick adjust panel
Achieve several effects with a single slider.

 Most of these sliders impact the value of several other sliders that are described below, to achieve a certain effect.
  • Density
    Use more compression.

     Changes the input and output levels of the compressor such that the total output level is not affected but compressor starts to work at much lower input levels.

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  • Hotter
    Makes attack and decay faster or slower. Sound is more squashed.

     This slider adjusts both attack and decay to have more aggressive compression.

 Compatibility panel
Compatibility settings for older presets.
  • Heavy CPU (compatibility)
    Setting for old preset compatibility.

     Do not turn this on.

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  • Legacy relative release distance

 Levels panel
Settings that control the wideband compressor levels.

 More threshold settings panel
  • Drive
    Amplification of the input before the compressor/limiter.

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  • Output Level
    Amplification of the output level after the compressor and limiter.

     It is generally a good idea to make sure that the output level of each filter is set such that disabling the filter does not change the level. This makes it much easier to compare what each filter does (it can be turned on and off without having to adjust other settings).

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  • Threshold
    The input level above which the compressor becomes active.

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  • Threshold stop
    The compressor won't reduce the level further than this.

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  • Knee
    Makes the transition around the threshold more smooth.

     At the threshold the response to slightly different input levels changes abruptly. Knee smooths the transition.

 Limit panel
Wideband compressor limiter settings.
  • Limiter
    The maximum output level of the limiter.

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  • Limit speed
    Determines how fast the limiter goes up and down.

     Setting this faster reduces intermodulation distortion and pumping, but increases harmonics distortion. Also, for faster levels the output is louder.

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  • Relative limit time to the right
    How much slower the limiter releases than it attacks.

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  • Limiter distortion
    Allows the limiter attack to distort.

     Some people like this effect, especially on low frequency audio - bass kicks get a special type of 'edge'.

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  • Override limiter release
    Controls the release behavior of the limiter.

     The limiter attack is always as fast as possible without causing distortion. The same is true for release, but in some cases the release behavior can be too prudent. This slider overrides the standard limiter release behavior: If the release behavior that would be used based on the adaptive algorithm is slower than this, the configured release time is used instead. This does mean that very fast release times can cause some distortion.

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  • Limit before compress
    Perform limiting before compressing.

     This way very loud sounds won't affect the compressor as much.

 More limiter settings panel
  • Limit type
    Needed for old presets.

     Don't turn this on.

 Attack panel
Attack settings.
  • Attack speed
    The time a 86% volume reduction due to a higher input level takes.

     If the input level increases a bit, the volume goes down more slowly than if it increases a lot. This means that it's not possible to give a value in dB/ms.

    •
  • Max attack speed
    The maximum attack speed.

     This is for the "automatic/hydraulic door" behavior: Push harder against it and at some point (this speed) it won't move faster anymore.

 More attack settings panel
  • Start attack immediately

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  • Attack Gamma
    Lets the compressor respond faster to small differences and slower to bigger ones.

     Small differences in level are thus quickly compensated, with helps to reach the target level much faster. And the compressor attack responds less aggressively to big volume changes.

 Release panel
Wideband compressor release behavior.
  • Release speed
    The time it takes for the output level to climb by 10 dB if the input level falls silent.

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  • Max release speed
    The maximum release speed.

     Similar to Max attack speed, but for release.

 More release settings panel
  • Exceed Release Rise
    Determines how fast Exceed Max Release Speed responds to a very low input level.

     In the hydraulic door example, this determines how fast the oil heats up.

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  • Exponential Release
    Release faster if the input level drops more.

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  • Release hold
    Time for the 'brake' on the release to fade out.

     When attack has been active, release is not immediately activated to avoid excessive movement. Instead, the release is held back for a while. This slider determines how long.

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  • Release Gamma
    Lets the compressor respond faster to small differences and slower to large ones.

     Small differences in level are quickly compensated, with helps to reach the target level much faster as long as differences in level are small. This gives a much more sparkling, 'alive', sound. But... Big differences are less quickly compensated. See Release Inertia for a solution for that.

     Another explanation to further clarify things: In the compressors, if there's a volume change, it takes quite long for the level to 'stabilize'. That's because the closer the actual level gets to the 'target' level, the slower it moves (the shape is asymptotic). Something similar happens in release. This seems to be a good thing, and traditionally this is what compressors do.

     What Flatness does is:
    • If the difference in level is 6 dB, nothing changes
    • If the difference in level is less than 6 dB, for Flatness values > 1 the change speed is increased.
    • If the difference in level is greater than 6 dB, for Flatness values > 1 the change speed is decreased.
    More technical: The Flatness'th root of the difference in level is used - so for 2 that's the square root etc.

     What this means: The higher the Flatness value is, the more the movement to the new level will look like a straight line instead of an asymptote.

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  • Release Inertia
    Adjusts release behavior to match human hearing for more natural results.

     Without Inertia and Release Gamma, after a very big volume spike the speed at which the audio returned was always the same - but determined by how much it had to move up. So, if the volume dropped by 6 dB and after 100 ms the volume went up 3 dB, then for a volume drop of 12 dB that would be 6 dB. Sounds perfect.

     But it's not. Say you have a huge drop, for example after a very loud 'S' in the high frequency band, where normal volume differences are at most a few dB and this S suddenly sticks out 20 dB. For a difference of 4 dB, after 100 ms the difference in level is 1 dB - 75% of the difference is reduced. Now, this last 1 dB is really nearly unnoticeable, so for your ears the release kinda stops after 100 ms. But, for a difference of 20 dB, after 100 ms the difference is still 5 dB! And you need more than another 100 ms before you reach this 1 dB point.

     So, after a loud sound you hear a gap at settings that sound good for small volume differences.

     Release Gamma helps a lot for the final part of release: Small differences get compensated faster. But at the same time, bigger differences take longer to recover, which causes the same effect for really big differences as before.

     Inertia fixes this. With inertia combined with Release Gamma you can make the release happen in a nearly constant time, without the slowdown at the end that you would have without Release Gamma, but also without the slower recovery for very big volume differences. Basically, the release happens in a nearly straight line, but the slope of the release depends on how much level must be compensated. With high Inertia values, release can even be faster for very big differences than for smaller ones, which can be good to quickly fill up the gap after a loud sound.

     For bigger Gamma values you need bigger Inertia values.

     In case things are not yet clear now, here's another explanation: For release, especially large differences must be compensated very fast - for 2 reasons:
    • Big differences mean very dynamic input, and for more dynamic input it's good that more compression occurs.
    • If you have a loud sound, and it takes multiple seconds for the level to get back, that sounds really bad.
    Example:
    • Sound drops by 4 dB. When 3 dB has been restored, you really won't hear much difference anymore in level.
    • Sound drops by 40 dB. Now, when 39 dB has been restored you really don't hear much difference anymore.
    So in one case when 75% restoration is there we're good, in the other we need 97.5%. And since - without Release Gamma - the behavior is asymptotic, reaching 97.5% takes multiple times as long as reaching 75%. Higher Release Gamma values only make things worse.

     Why is this bad? Well, it makes it nearly impossible to find a good Release speed, what works well for small differences will be far too slow for big differences, and what works well for big differences will sound very aggressive on small differences.

     So, the time it takes for the level to be restored to a level where human hearing stops to notice a difference - say 1 dB below the target level - must be nearly constant.

     Inertia ('heavyness') makes sure that once release is moving up, the speed won't slow down until the target is reached. For big drops the effect is much bigger than for small drops, which is exactly what is needed.

     Release Inertia and Release Gamma must be configured to work properly together. The best way to do this is to record a sample with different level tones (Loud - soft, loud - less soft, loud - just a little less loud), and check if all take approximate the same time to reach a level slightly below the target level.

     Analogy

     If you have to drive 10 meters, you just barely hit the gass and drive very slowly.
    If you have to drive 1 km, you hit the gass and speed up (Release hold), then release the gass and let the car roll slowing down towards the end.
    With Inertia, you would not release the gas until you're very close to the end and then hit the brakes to stop.

  • Continuous Release
    Increases release speed if the level is reduced more.

     This makes the release behavior non-linear.

Release Gate panel
Gating settings for release.

 If the current output is much softer than the target level, this stops release from acting. This helps for example to avoid releasing when there's only noise present, which would boost the noise. Also, for example during speech, if there's a moment of silence, this stops the compressor from releasing completely, which would cause the attack to get a very big spike that it needs to handle.

 Hold panel

 More gate settings panel
  • Auto gate speed

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  • Gate speed
    Determines how fast the gate closes.

     This value is relative to Release speed.

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  • Gate freeze
    Level below which all release action stops.

     If the output audio is below this level, release stops completely. Be careful with this, if it is set too high, it can cause the compressor to not come back after a big spike!

     See also Gate slowdown.

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  • Gate slowdown
    If the input level is lower than this, release is slowed down.

 Dynamic attack/release panel

 More dynamic attack/release settings panel

 When moving slower panel

 When moving faster panel
  • Max dynamic adjustment

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  • Reduce fast movements

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  • Reduce slow movements

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  • Max speedup

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  • Max slowdown

 Detection panel
Audio level detection settings.

 More detection settings panel

 More behavior settings panel
  • Detection type
    Chooses between RMS or Peak level measurement.

     Peak mode can cause quite large reactions to a single small spike in the sound. RMS mode responds more like human hearing does, but low frequencies seem to be counted a lot stronger than in peak mode, which easily causes pumping.

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  • ITU-BS.1770 Bass
    Respond less strong to bass because to human ears it seem to sound less loud.

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  • ITU-BS.1770 Head
    Respond more to high frequencies because they sound louder to humans.

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  • Feedback
    Chooses between feed forward and feedback mode.

     In feed forward mode, the input is used directly for the measurement. In feedback mode, the output level is measured instead of the input level.

     Feedback mode is known to sound more natural, but the level control is far less accurate. For example, say the input level is 6 dB too loud and the ratio is 1:1000. Then in feed forward mode, the level will be reduced by about 6 dB. But in feedback mode, once the level is reduced by about 3 dB, the compressor will 'see' that it needs about 3 dB of reduction and not reduce the level further.

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  • Lookahead
    Lookahead time.

     This determines how much the compressor looks ahead. For fast attack speeds (under 10 ms), this helps a lot to remove the short spikes that remain after compressing. However, we have also noticed that in some cases it can cause the sound to become more restless, so don't overuse this.

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  • Look-ahead
    Lets the compresor respond to the sound a bit in the future.

     This means that the initial spike of a loud sound gets reduced better, which can give a more natural sound.

     The attack of the limiter is already protected, and if you don't use very short attack times for the compressor this probably has little effect.

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  • RMS size
    Size of the RMS detection area, when using RMS detection mode.

     Using an RMS detection area slightly improves the standard RMS behavior. More importantly, it protects the compressor against overreacting. If a loud sound stops, without using an RMS detection area, the compressor can keep moving down for a short while. This is particularly noticeable on loud S sounds in higher bands - even after the S stops, the level drops down further, and the audio after the S sounds dull. It takes a while for release to take over and increase the level again.

     When using an RMS detection window, attack stops immediately after the loud sound and release takes over directly. In some cases this can reduce unwanted drops of the highs after a loud S by more than 3 dB.

 Ratio panel

 Behavior panel
Controls the compressor behavior.

 Channels panel
  • Ratio
    Determines how strongly the compressor responds to changing input levels.

     Say, at one moment a sound comes in at the threshold level, so nothing happens to it. If another sound comes in at 6 dB above the theshold level, the input should be reduced by half. The ratio indicates how much of the increase in input level is not removed. At a the lowest ratio (1:1), the compressor is basically disabled. At the maximum ratio, 1000:1, 1/1000th of the increase is kept.

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  • Progressive ratio

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  • Progressive ratio __UPDATED__

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  • Relative release distance
    If the level drops dramatically, this controls how fast the release can get.

     In Analog Compressor Symmetric modes, the attack and release behavior are symmetrical. This is generally a good thing: Handling attack and release the same (albeit with different speeds) is the most "natural" design. But there is one problem with this. A very big attack should be handled quickly, but if attack and release are handled the same and the level briefly drops to zero, that would also cause release to act very fast. Which may cause over-reactions.

     This setting controls what - in case of silence - the release will use as its input level.

 Window panel
  • Channel separation
    Process channels separately, combined, or in between.

     At 0%, the two channels will always behave the same. At 100%, they move completely separate of each other.

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  • Channel linking
    Sets the maximum difference in change between the audio channels.

     For example if the left channel would be attenuated by 10 dB and the right channel by 5 dB, but channel linking is set to 2 dB, then the right channel will be attenuated by 8 dB instead. So the loudest channel pulls the other channels down to at most this many dB above its own level.

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  • Constant sum

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  • Lowpass below
    Lowpasses the compressor control signal.

     By lowpassing the control signal from the compressor, distortion from fast compressor action is removed. It also helps smoothe out remaining spikes when the Attack is a bit too slow, since the filtering works symetrically (not only forward in time).

 Non-standard attack panel
Protection against spikes for slow compressor settings.

 Some presets use very slow attack and release times. This can sound great, but the level control for sudden volume increases is less good.

 This section contains the settings for an extra compressor that takes over in such cases. It does not affect normal audio.
  • Loud burst protection
    Enables loud burst protection.

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  • Level difference
    Increases the level of the 2nd compressor with faster attack.

     Because the attack is so much faster, the audio level of the 2nd compressor is generally a bit lower. If we would take the minimum of the two, we would always look at the 2nd compressor, but that should only happen in extreme cases. By increasing the output level and then taking the maximum of the two, the 2nd compressor only has an effect on the sound if its output level is quite a bit lower. For example, if this value is set to 2.00, the 2nd compressor will not kick in if the level difference is less than 6 dB.

    •
  • Minimum drop
    Disables the 2nd compressor if the attenuation didn't suddenly drop a lot.

     The 2nd compressor should only be active if there's a huge difference between the volume when using a normal and very fast attack, but that's not all - if you play very dynamic music it should not kill the punch. This slider controls how much the attenuation must have suddenly dropped (in the fast attack 2nd compressor) for it to be taken into account.

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  • Fast Attack
    The fast attack time.

     To be useful, this must be a lot smaller than Attack speed - typical values are around 1-5 ms.

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  • Release speedup
    Controls how much faster the 2nd compressor release is.

     Beside a faster attack, the release for the 2nd compressor can also be made faster. This helps to prevent long-term volume drops after a short loud spike in the sound. This value controls how much faster the release is than Release speed.

 Non-standard release panel
Experimental settings. Should probably not be used.
  • Dynamic release
    Dynamically increase the release speed if the volume drops more.

     If this is set to 0 the release always runs at exactly the same speed. A similar effect can be reached with Release Gamma.

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  • Dynamic release to 0 dB
    Release acts as if the input level is always at 0 dB.

     So the release speed depends only on how deep the level has dropped. See also Continuous Release.

 Non-standard tweaks panel
Settings that control compressor/limiter envelope detection.

 In a compressor we have 2 things: An 'envelope', basically a line that follows the audio level, and the compressor behavior itself. If the level drops a lot, release is faster - and this is based on the envelope. Now, if the envelope just follows sample levels, then there will be a lot of near-0 values (just when a waveform crosses 0) which would cause infinitely fast release behavior. The envelope line needs to be made such that this doesn't happen.

 So, around a peak in the waveform, for the surrounding samples we should not allow the envelope to reach much lower values than the value of that peak.

 That works fine for high frequencies. But if you take a bass, the sample values are dropping slowly and in the valleys the level will still approach 0. Which still causes issues with release behavior. Because of that, there's some code that measures DC offset and increases the Base smoothing to something close to infinity when there's more DC offset present. Base smoothing controls how big the area is that's considered for DC measurement (lower frequency = bigger area). What we are actually measuring here is DC offset in a specific direction divided by total (absolute) power.
  • RMS block size
    The size of the area around the current sample used to calculate the RMS level.

     Bigger values means less precise timing of attack/release behavior, but also less effect from low frequencies (less pumping). Generally, the RMS block size should be set just high enough to not cause distortion when using the limiters (Threshold) a lot.

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  • Peak smoothing
    Controls envelope smoothing around peaks in the waveform.

     Lower values may cause distortion, but too high values reduce the precision of the limiters and (to a much lesser extent) the compressor release behavior.

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  • Bass detection
    Controls upto which frequency bass should be detected for Smooth Base power.

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  • Smooth Bass power
    If bass is present (Base detection), increases the Base smoothing temporarily.

     This slider controls how strongly the bass affects the release behavior. Setting it higher means more release slowdown when we see bass.

     If you use this for a single multiband band, then you need less of this because there are less other frequencies that hinder bass detection.
Side chain section
Lets the compressor respond more or less to certain frequencies.

 For example, you can boost the bass in the side chain to increase pumping caused by bass - or you can reduce it to reduce bass pumping.

 Side chain panel
Old wideband compressor section
The deprecated Classic Singleband Compressor is only available for compatibility reasons.