Swirl Chorus manual
Swirl provides Chorus, Dimension, Ensemble and Vibrato, with a separate drive section. This manual describes version 1.
Start with a preset
Choose a factory preset from the display at the top of the panel: click it for the preset menu, or step through presets with the arrow buttons beside it. Or press one of the four mode keys, then Mode Init. Pressing a mode key on its own keeps the other controls unchanged. The six Dimension Buttons keys load one of four button settings or two combinations.
For chorus, start with a partial dry/wet mix. For vibrato without doubling, set Mix to 100%. Try Bass Widener on bass or Vocal Double on a lead vocal.
Voice and motion
| Control | What it does |
|---|---|
| Mode | Chorus uses one voice per channel. Dimension uses opposite stereo sweeps and a filtered wet path. Ensemble uses three voices per channel. Vibrato provides pitch movement and is usually used fully wet. |
| Rate | Main sweep speed, 0.02–8 Hz. With the Sync switch at Lock, select a division from eight bars to a quarter note. |
| Sync | Follows host tempo and song position. Bar divisions follow the time signature. Without host tempo, the fallback is 120 BPM. A transport seek resets the sweep immediately and can cause a pitch discontinuity. |
| Depth | The size of the delay-time sweep. |
| Depth in Cents | Changes Depth to nominal upward pitch excursion, 1–100 cents. Rate changes then adjust delay excursion to maintain a similar pitch range. Very slow rates and small Size settings can limit the available excursion. Downward deviation is slightly larger because cents are logarithmic. |
| Width | Changes the phase offset between the left and right modulation. Zero aligns the sweeps; it does not prevent cancellation between dry and delayed audio. Use Mono to check the result. |
| Size | Scales delay times from 25–160%. Outside Dimension mode, longer settings also darken the wet path. Small Size with high Depth can produce flanging as the sweep reaches its minimum delay. |
| Mix | Equal-power dry/wet blend. Intermediate settings can increase level or cause cancellation, depending on the source and delay settings. |
| Flutter | Speed of the second sweep, 0.2–25 Hz, independent of Rate and Sync. Its speed includes a small random drift. |
| Shimmer | Blends the second sweep into the modulation. Ensemble includes some flutter even at zero. Shimmer changes the combined pitch movement, so a Depth value in cents describes only the nominal slow sweep. |
Dimension and the five named drive voices were developed using captures from reference plugins. They are not complete circuit models of the original hardware.
Tone and mix controls
Tone tilts the wet signal around 720 Hz. Age adds progressively more low-pass filtering. It controls darkening, not worn-chip dynamics.
Bass Protect retains the dry low end while applying chorus above a gradual crossover. Select Off or 40–500 Hz. Start at 300 Hz for bass; Bass Widener, Warm Bass and Mono Thick already use that setting. Some change remains near the crossover frequency. Drive can still affect bass according to its routing.
Duck reduces the chorus contribution when the input is loud, then restores it as the level falls. The detector is linked across stereo channels, with a 3 ms attack and 180 ms release. The amount of ducking depends on input level. Vocal Double provides a starting point at 40% Duck.
In and Out provide ±12 dB gain trims before and after processing. The two VU meters in the header show the input and output levels, reading 0 at −18 dBFS. Input is measured after In trim; output includes bypass and mono audition. Each meter's lamp lights when a peak reaches 0 dBFS. Hover over a meter and the display shows its peak and RMS levels in dBFS. These are sample-level meters, not true-peak or loudness meters.
Bypass compares the original input at the same latency, excluding both trims. Mono sends the summed output to both channels. Both controls survive preset changes and are saved as session parameters.
Drive
The Drive switch turns the drive section on and off independently of chorus. The Voice selector chooses Soft Clip, Tape Pre, Console, Germanium, Triode or Pentode.
Drive runs from 0–100%. Zero is clean. The first quarter blends in the selected character; higher settings increase saturation. At maximum, Soft Clip receives 12 dB of gain and the five other voices receive 24 dB. Level compensation is calibrated around −15 dBFS RMS input. Source level and dynamics still affect output loudness, so use Out trim when comparing settings.
The drive section has its own Mix for parallel saturation. The Order switch sets where it sits: Pre processes the signal before chorus, including its dry component, and Post processes the chorus output.
Voice, routing and quality changes use short fades and allow the new processing state to settle. Mode changes crossfade between chorus engines. These transitions reduce switching clicks, but the change in sound remains audible.
Drive quality and latency
Only the drive section is oversampled; chorus runs at the host sample rate. The Quality switch chooses Std or HQ.
| Host sample rate | Std | HQ |
|---|---|---|
| 44.1 / 48 kHz | 8× | 32× |
| 88.2 / 96 kHz | 4× | 16× |
| 176.4 / 192 kHz | 2× | 8× |
HQ reduces drive aliasing at a higher CPU cost. Inactive drive skips oversampled processing.
Latency is 136 samples at either quality and in bypass, approximately 2.83 ms at 48 kHz. The plugin reports this to the host for compensation. Bypass keeps the effect running for a smooth return; disable Drive to avoid its oversampling cost.
Presets, A/B and locks
Swirl includes 53 factory presets. Click the display for the preset menu, where you can save a user preset or delete one. An asterisk beside the name indicates unsaved edits. User presets are XML files in the platform application-data folder under SwirlChorus/Presets.
Lock Mix and Lock Out keep those controls unchanged while browsing presets. The locks last for the plugin instance and are not saved with the project. HQ, Bypass and Mono are always preserved across preset loads.
A/B switches between two temporary settings; its lamp is lit while B is in use, and the display shows B before the preset's name. The first press copies A into B. Edit B, then switch back to compare; each slot retains its edits and preset identity. HQ, Bypass, Mono and window size stay with the session. The A/B slots are not saved with the project. Save a user preset if you need to keep an alternative setting.
Editing controls
Click a knob's value window to type a value. For a percentage control, 50 and 50 % both mean 50%. Double-click a knob to reset it. Undo and Redo step through edits made in the window. The display shows the control you last touched. The window opens at 1008 by 664 pixels and resizes from 75% to 200% of that; hover over a control for its tooltip.
The screens
Three screens show what Swirl is doing as it runs.
Bucket Brigade draws each voice's delay time as it sweeps, in milliseconds up the screen, with the newest moment at the right edge. The left channel is the bright line and the right channel the dim one, so the gap between them is Width. Depth sets how far the lines swing, Size where they swing about, Rate how fast, and Shimmer and Flutter the faster wobble on top; Ensemble shows its three voices a side. The shaded strip under 1 ms is where the sweep reaches into flanging. The screen follows the engine's own sweep, so it locks to tempo when Sync is on, and it stands still while no audio runs.
Stereo Image is a goniometer of the output: sound in the middle stands up the screen, sound to one side leans toward its L or R diagonal, and the chorus's width spreads it across. Its height follows the level. The scale underneath is the correlation of the two channels: +1 is mono, 0 is wide, and readings toward −1 cancel when summed to mono.
Drive draws the drive's transfer curve as an X-Y scope would: a full-scale tone along the bottom, what the drive makes of it up the side, for the Voice, Drive and Mix settings. A straight diagonal is clean; the models with memory draw a loop. At Pre, the dot marks where the input's peaks reach.