Air-G Volt
User Manual
Version 1.0
Bus compressor based on Logical 4 by Airwindows.
Original source code released under an open license by Chris Johnson.
User Manual
Version 1.0
Bus compressor based on Logical 4 by Airwindows.
Original source code released under an open license by Chris Johnson.
Air-G Volt is a bus compressor based on the Logical 4 algorithm by Airwindows, described by its author as the best compressor he has made to date. Internally it is a cascade of up to three butterfly-style compressors (ButterComp style) chained together, with Power Sag stages between them — a simulation of the power-supply droop of analog gear.
Its character changes a great deal depending on how hard it is pushed. At low ratios it behaves like a smooth, musical bus compressor, in SSL territory. As the ratio rises it leaves that ground and enters a zone of creative effects and envelope distortion: with high RATIO and fast reaction, transients can invert and the plugin “explodes” instead of compressing. Chris Johnson presents this as a desirable feature, not a flaw.
It is part of the same family as Velvet, Pulse, Pop! and Taped: reimplementations of Airwindows algorithms, with their own interface and additions aimed at mixing and mastering work.
Logical 4 uses Power Sag and ButterComp technology, among other internal Airwindows sources. At low ratios (1–2:1) it sounds clean and transparent, like a console bus compressor. At high ratios (8–16:1), especially with fast reaction, it leaves classic compression and enters envelope distortion: a creative tool for drums, electronic music and special effects.
Air-G Volt interface.
Sets the level at which the compressor starts to act. Fully right it sits at 0 dB (no compression) and lowers to the left down to −20 dB. The lower it is, the earlier the compressor engages and the more of the signal gets compressed.
With a low THRESHOLD and a high RATIO the signal is completely crushed; raising the THRESHOLD frees the dynamics and lets more transients through. It is the control that decides how much signal the compressor works on.
This is the most important control of the plugin. It goes from gentle ratios up to 16:1, with a staged behavior: as you raise it, more compressors come into play in the chain.
With a high RATIO — especially if REACTION is fast — the cascade stops compressing and starts inflating the signal: attacks invert and the plugin “explodes” instead of squeezing. It is an intended effect, not an error. The central bar signals it: two pulses are born at the centre and open out to the sides (see Meters and indicators). Switching very fast between very high and very low ratios may produce an occasional artifact.
The reaction speed of the compressor (attack and, in part, release). It ranges roughly from 1 to 100 ms and defines much of the plugin’s character.
Selects the shape of the compression curve — how abrupt the transition into the compressed state is. It has three positions that snap as you turn the knob: SOFT, MEDIUM and HARD.
The active option lights up in amber and the other two dim down, so you can read it at a glance. Switching positions produces no level jumps.
Input gain into the whole plugin. The neutral point (0 dB) is at the centre of the knob; it adds up to +20 dB to the right and attenuates down to −20 dB to the left. Because it raises the level reaching the detector and the cascade, it works as drive: more INPUT means more compression for the same THRESHOLD.
Output level (makeup gain). The neutral point (0 dB) is at the centre; ±20 dB. It compensates for the level loss introduced by compression, to match volumes when comparing against the bypassed plugin.
Mirrors INPUT and OUTPUT. With LINK on, if you raise INPUT by a certain amount of dB, OUTPUT drops by exactly the same amount — and vice versa, because the link is bidirectional. This way you change how much compression you add (via drive) without changing the perceived volume. Since both knobs are already symmetric (±20 dB, centre at 0 dB), each move of one has its exact mirror in the other. Turning LINK on or off causes no level jump.
Leave INPUT and OUTPUT at the centre (0 dB), pick a moderate RATIO and lower THRESHOLD until the VU shows the desired reduction. Compensate the level with OUTPUT — or turn on LINK to A/B compare fairly, adding drive without the volume fooling you.
Blend between the compressed (wet) and the original (dry) signal. At 100% you hear only the processed signal; as you lower it, more dry signal is mixed in.
Lowering MIX enables parallel compression: the transients and body of the original signal are preserved, with the density and control of the compression underneath. Very useful on the drum bus and the 2-bus, when you want punch and cohesion at once.
High-pass filter applied only to the detection signal (the sidechain): the audio you hear is not filtered, only what the compressor uses to decide how much to compress. It goes from 20 Hz (no effect) up to 300 Hz.
Raising it stops the lows from triggering compression, so the plugin pumps less with kicks and powerful bass, and the mix breathes better. At minimum the filter is in bypass, so the default behavior matches the original algorithm bit for bit.
A little SC HPF (between 60 and 120 Hz) keeps the kick and bass from driving all the bus compression. The result is a more even glue, with less pumping on every low-end hit.
The meter shows in real time how much gain reduction the compressor is applying, in decibels. The scale is calibrated so the needle matches the real reduction dB across its whole travel, making it a precise reference of how hard the plugin is working. Logical 4 compresses both channels in a linked way, which is why a single meter reflects the activity of both.
A simple activity indicator: it glows while the plugin is processing audio and goes off when the DAW bypasses it or stops passing signal. It does not pulse or blink — it just shows the plugin is active.
It has two behaviors: when the output approaches the limit (above −0.8 dB), the LED starts to light up gradually, following the signal. If the output exceeds 0 dBFS, it latches at maximum and stays lit until you click on it to reset it.
The blue background bar reacts to the audio. Its glow grows subtly with the amount of compression, as a live indicator that the plugin is working. And when the cascade stops compressing and starts inflating the signal (high RATIO + fast REACTION on transients), two pulses are born at the centre of the bar and open out to both ends, signalling at a glance that the regime has changed: it no longer compresses, it expands. The sweep is detected from the real audio — it appears when the signal actually inflates, not from the RATIO position.
Low RATIO (1–2:1), medium REACTION, THRESHOLD set for a few dB of reduction. Adds cohesion and glue to the 2-bus or a group, in console-compressor territory, without over-coloring.
RATIO 2:1, REACTION to taste: slower lets the attack through, faster clamps it. KNEE on MEDIUM or HARD so it grabs decisively. Ideal to give body and consistency to percussion.
High RATIO (8–16:1) and fast REACTION: intentional envelope distortion. Transients invert and the signal “explodes”. The bar signals when you enter this zone. Excellent for electronic drums and sound design.
High RATIO with very slow REACTION: removes grain and grit, and everything sounds more even and sustained. A different way to use high ratios, without the inversion effects.
The plugin lives between two worlds: to the left of RATIO it is a clean bus compressor and to the right it is an effects machine. The best way to get to know it is to set a source with clear transients (a drum kit), raise RATIO and REACTION together and listen to the exact moment when compression turns into inversion — the central bar marks it. From there, back off to the right amount for each source.
Air-G Volt was developed by Air-G Audio as part of the line of plugins based on the work of Airwindows.
The Logical 4 algorithm is the work of Chris Johnson (Airwindows), whose original source code is published under an open license at github.com/airwindows. Air-G Volt is a reimplementation of the original algorithm with its own additions: INPUT control, a three-position KNEE selector, an SC HPF filter on the detector, a gain-reduction meter, an expansion indicator and clip detection.
To Chris Johnson for his tireless work at Airwindows and for sharing decades of audio-processing research completely openly.
Air-G Volt v1.0 — Air-G Audio. The Logical 4 algorithm is the work of Chris Johnson / Airwindows and is used under the terms of its open license.