Auto-Swell / Stereo Amplitude Modulator - User Guide

Stereo amplitude modulation for mono or stereo Sounds, with periodic, gated, stochastic, and single-arc envelopes.

Author: Shai Cohen Version: 1.3 (2026) Output: Stereo Sound

Contents:

What this does Quick start Presets Swell shapes Stereo modes Parameters Processing details Visualization Limits and cautions

What this does

Auto-Swell is a stereo amplitude modulator. It creates one modulation envelope for the left channel and one for the right channel, scales each envelope by the requested depth, multiplies the audio by those envelopes, and combines the two processed channels into a stereo Sound.

The processor accepts mono or stereo input only. Mono input is duplicated to stereo before processing. Stereo input is processed channel by channel. Sounds with more than two channels are rejected instead of silently discarding additional channels.

Core gain law: if the raw envelope is m(t) in the range 0...1 and depth is d in the range 0...1, the applied gain is g(t) = (1-d) + d*m(t). Therefore 0% depth gives a constant gain of 1, while 100% depth uses the raw envelope directly.

Quick start

Select exactly one mono or stereo Sound object.
Run Auto-Swell_v1.3.praat.
Choose a preset or use Custom.
Set the envelope shape, rate, depth, and stereo relationship.
Leave Normalize output on if you want the final stereo Sound peak-scaled to 0.95; turn it off if you want the modulation-induced level change preserved.
The result is named <source>_swell_<ShapeName>.

Important: Normalize_output is not a limiter and is not attenuation-only. Praat's Scale peak: 0.95 scales the entire stereo result to a peak of 0.95, so it can either attenuate or amplify.

Presets

Preset selections overwrite the listed parameters. Parameters not mentioned by a preset remain at their form values, but may be irrelevant to that preset's shape or stereo mode.

Preset	Shape	Rate	Depth	Stereo behavior
Custom	Form setting	Form setting	Form setting	Form setting
Gentle Tremolo	Sine	0.3 Hz	50%	Identical L/R envelope
Fast Tremolo	Sine	6 Hz	70%	Identical L/R envelope
Rhythmic Chop	Pulse gate	4 Hz	100%	Identical L/R; pulse width 50%
Stereo Ping-Pong	Sine	0.5 Hz	90%	Phase offset 180°
Polyrhythmic Drift	Sine	0.4 Hz	70%	Right rate = left rate × 1.618
Textural Contrast	Left: Triangle	0.5 Hz	80%	Right: Exponential rise
Single Swell	Rise-fall	Rate ignored	95%	Identical L/R envelope

Swell shapes

Shapes 1-9 and 11 are periodic. Shape 10 is stochastic. Shape 12 is one non-periodic arc across the entire file.

#	Shape	Implemented envelope	Special behavior
1	Sine	`0.5 + 0.5sin(2piratet + phase)`	`Phase_start` applies.
2	Triangle	Linear rise 0→1 over half a cycle, then fall 1→0.	`Phase_start` applies.
3	Sawtooth	`(rate*t + phaseCycles) mod 1`	Natural rising phase; `Phase_start` is ignored.
4	Reverse sawtooth	`1 - ((rate*t + phaseCycles) mod 1)`	`Phase_start` is ignored.
5	Square	1 for first half-cycle, then 0.	Fixed 50% duty cycle; `Phase_start` is ignored.
6	Exponential rise	`1 - exp(-5*p)`, where `p` is cycle position.	Resets each cycle.
7	Exponential fall	`exp(-5*p)`.	Resets each cycle.
8	S-curve	Quadratic ease-in/ease-out: `2p^2` then `1-2(1-p)^2`.	Periodic; not a cosine curve.
9	Double peak	`abs(sin(2pirate*t + phase))`	Absolute value creates two peaks per nominal cycle.
10	Smoothed random	Gaussian random values around 0.5, three smoothing passes, then full-envelope normalization to 0...1.	Stochastic; no phase-addressable cycle.
11	Pulse gate	1 while cycle position is below `Pulse_width`, otherwise 0.	Duty cycle is controlled by `Pulse_width_percent`.
12	Rise-fall envelope	One exponential rise over the first half of the file and one exponential fall over the second half.	Ignores rate, phase offset, and `Phase_start`.

Smoothed random is not a mathematical random walk. The implementation generates Gaussian random samples, smooths them three times, and normalizes the resulting envelope. No random seed is set by the script.

Stereo modes

1. Mono (same both channels)

The right modulation envelope is an exact copy of the left modulation envelope. The output is still stereo. On mono input, both audio channels are identical and remain centered. On stereo input, the source's existing L/R differences remain; they simply receive the same gain envelope.

2. Phase offset

Both channels use the same shape and rate, but the right envelope receives Phase_offset_degrees. This can create level-based left/right movement, especially when the source began as mono. Smoothed random and Rise-fall are rejected in this mode because they do not have a meaningful phase-addressable cycle.

3. Different rates

The left channel uses Swell_rate_Hz. The right channel uses Swell_rate_Hz * Right_rate_multiplier. The same shape is used on both sides. Smoothed random and Rise-fall are rejected because the mode requires a shape whose temporal behavior is meaningfully controlled by rate.

4. Different shapes

The left channel uses Swell_shape and the right channel uses Right_swell_shape, both at the left/base rate. This mode may use periodic, stochastic, or single-arc shapes. If a side uses Rise-fall, that side still ignores the rate parameter.

Parameters

Parameter	Default	Actual behavior
Preset	Custom	Selects one of seven predefined configurations or leaves the form settings unchanged.
Swell_shape	Sine	Left/base modulation shape.
Swell_rate_Hz	0.5	Positive. Period is `1/rate`. Ignored by Rise-fall.
Depth_percent	80	Validated to 0...100. Sets the modulation gain range from `1-depth` to 1.
Stereo_mode	Phase offset	Selects the relationship between left and right modulation envelopes.
Phase_offset_degrees	90	Positive; converted to radians. Used by Phase-offset mode on periodic shapes.
Right_rate_multiplier	1.5	Positive multiplier used only by Different-rates mode.
Right_swell_shape	Triangle	Used only by Different-shapes mode.
Phase_start	Start at minimum	Applies only to Sine and Triangle. Other shapes use their natural cycle start.
Pulse_width_percent	50	Validated to 0...100; used by Pulse gate. 0% is always off before depth scaling; 100% is always on.
Random_smoothness	10	Positive. Controls the sample offset used by the three smoothing passes: `round(sr/(rate*smoothness))`, clamped to 2...1000 samples.
Normalize_output	On	If on, scales the completed stereo Sound to peak 0.95. This may amplify or attenuate.
Draw_visualization	On	Draws the source, L/R envelopes, result, and summary strip.
Play_result	On	Plays the final Sound after cleanup.

Processing details

INPUT exactly one mono or stereo Sound 1. PREPARE CHANNELS copy source mono input -> duplicate to stereo extract channels 1 and 2 2. GENERATE RAW ENVELOPES create left envelope from Swell_shape create/copy right envelope according to Stereo_mode 3. APPLY DEPTH g(t) = (1-depth) + depth * m(t) 4. APPLY AMPLITUDE MODULATION left_out(t) = left_in(t) * gL(t) right_out(t) = right_in(t) * gR(t) 5. COMBINE combine processed L/R channels to stereo name: <source>_swell_<ShapeName> 6. OPTIONAL PEAK SCALING if Normalize_output = yes: Scale peak to 0.95 7. CLEANUP / PLAY remove temporary envelope/channel objects select final Sound optionally play

Depth and normalization are separate operations. With Depth = 0%, the modulation stage itself is transparent, but if Normalize output is enabled the final Sound can still be rescaled to a 0.95 peak.

Visualization

The Picture window uses the Praat AudioTools standardized layout while retaining the processor-specific modulation views:

Input waveform: the original Sound.
L mod: the depth-scaled left modulation envelope, plotted from 0 to 1.
R mod: the depth-scaled right modulation envelope, plotted from 0 to 1.
Output waveform: the final stereo result after optional peak scaling.
Summary strip: input duration/sample rate/channel count, shape/rate/depth/stereo mode, normalization state, output peak, pulse width, and random smoothness.

The waveform panels use Praat's own drawing scales; they are not a shared-amplitude before/after comparison. The modulation panels use the explicit 0...1 range.

Limits and cautions

Input channel count: mono and stereo only. Wider multichannel files are rejected.
Hard transitions: Square, Pulse gate, Sawtooth, and Reverse sawtooth contain discontinuities. Multiplying audio by abrupt gain changes can create broad-band sidebands or click-like transients.
No envelope-edge smoothing: the script does not add crossfades around gate transitions.
Smoothed random: stochastic and normalized per generated envelope; it is not phase-addressable and is therefore unavailable in Phase-offset and Different-rates modes.
Rise-fall: always spans one arc across the entire Sound and ignores rate and phase controls.
Phase_start scope: only Sine and Triangle use the three start-position options.
Normalization: optional peak scaling changes overall gain after modulation; disable it when absolute level changes are part of the intended result.