Fast Spectral Swirl — Multi‑Channel

An STFT‑based effect that circularly rotates and/or permutes spectral bins over time to create shimmering, time‑varying timbres. Designed for speed and for coherent multi‑channel processing.

Author: Shai Cohen Affiliation: Department of Music, Bar-Ilan University, Israel Version: 0.1 (2025) Praat

Contents:

What this does Quick start Concepts Swirl Modes Parameters Presets Multi‑Channel Strategies Tips & Pitfalls Math

What this does

Per frame, the spectrum is taken via STFT. Bins are circularly shifted and/or phase‑advanced with a rate that can be constant or modulated. The result is a swirling timbre that preserves overall energy while redistributing partials. Multi‑channel handling keeps imaging stable when desired—or purposely decorrelated for wide, spacious effects.

Real‑time friendly: small‑hop STFT with in‑place bin rotation.
Deterministic or stochastic: constant, LFO‑modulated, or random‑walk swirl.
Coherent surround: link channels to preserve localization; offset to design motion.

Output: Channel count matches input; use with stereo/5.1/7.1 objects. For downmixing, see the dedicated guide.

Quick start

In Praat, select a mono or multi‑channel Sound.
Run script… → fast_spectral_swirl_multi_channel.praat.
Pick a preset (e.g., “Subtle shimmer”).
Adjust rate, depth, and banding as needed.
Choose link_channels = yes for stable imaging, no for wide/decorrelated swirl.
Set mix (dry/wet) and normalize if needed. Press OK.

Concepts

STFT & Bin Rotation

STFT: X(k,n) = Σ_t x(t) w(t-nH) e^{-j2πkt/N}
Swirl via circular shift S bins: X'(k,n) = X((k - S(n)) mod K, n)
Alternative phase advance: X'(k,n) = X(k,n) · e^{j φ(k,n)}

Rotation re‑orders magnitudes across frequency; phase advance simulates spectral shearing. Both reconstruct via overlap–add.

Swirl Modes

🎛️ Constant Shift

Fixed S bins per frame; gentle chorus‑like shimmer.

🌊 LFO‑Modulated

S(n) = S0 + Sd · sin(2π f_LFO nH/Fs + φ) for periodic swirl.

🎲 Random Walk

Per frame, S jitters within bounds; seedable for repeatability.

🧩 Banded

Apply different shifts per band (low/mid/high) or per Bark/ERB groups for tone‑aware results.

Parameters

Parameter	Type	Default	Description
fft_size	option	2048	Power of two; resolution vs latency
hop	integer (samples)	N/4	Smaller hop → smoother, higher CPU
window	option	Hann	Hann/Blackman/Hamming
mode	option	LFO	Constant, LFO, Random, Banded
shift_bins	integer	2	Base circular shift per frame
rate_hz	real	0.6	LFO rate for modulated mode
depth_bins	integer	6	LFO/random amplitude (bins)
phase_shear	real	0.0	Additional linear phase advance per bin
bands	text	""	CSV edges in Hz (e.g., 0,300,3000,Nyq)
band_shifts	text	""	CSV shifts per band (bins)
link_channels	boolean	yes	Use same shifts for all channels
stereo_offset	integer	0	Extra shift for R (or even channels) to widen
random_seed	integer	0	0=auto; otherwise reproducible
dry_wet	real	0.5	0=dry, 1=wet
normalize	boolean	yes	Normalize peak to norm_target
norm_target	real	0.99	Peak amplitude target
formant_keep	boolean	no	Reduce shifts near vocal formants (experimental)
report	boolean	no	Print effective shifts per frame/band

Presets

✨ Subtle shimmer

fft=2048, hop=N/4, mode=LFO, shift=1, depth=3, rate=0.4, link_channels=yes, dry_wet=0.3

🌪️ Fast swirl

fft=1024, hop=N/4, mode=LFO, shift=2, depth=8, rate=2.0, stereo_offset=+1

🧊 Frozen highs

bands=0,300,4000,Nyq; band_shifts=0,0,2; formant_keep=yes

🎲 Grainy random

mode=Random, depth=10, random_seed=42, link_channels=no, dry_wet=0.6

Multi‑Channel Strategies

Linked (coherent): Same S(n) across channels maintains localization; use small stereo_offset for gentle width.
Decorrelated (wide): Offset or randomize per channel; keep low‑frequency bands linked to protect bass image.
Surround beds: Link L/R and Ls/Rs pairs; add ±1 bin offsets to rears for movement without smearing center.

Tips & Pitfalls

Latency vs detail: Larger FFT improves selectivity but increases latency.
Clicks: Use Hann window and overlap ≥ 4 for smooth OLA.
Vocal integrity: Enable formant_keep or reduce high‑band shifting.
Gain staging: Spectral re‑arrangement can cause peaks; normalize after processing.
Mono check: Excessive inter‑channel offsets may comb‑filter in mono.

Watch out: Very large shifts at small FFT sizes can sound noisy; increase FFT or reduce depth.

Math

Bin Rotation & Phase Shear

Circular shift by S(n): X'(k,n) = X((k - S(n)) mod K, n)
Phase shear: φ'(k,n) = φ(k,n) + α·k ⇒ X'(k,n) = |X| e^{j φ'(k,n)}
Inverse STFT via OLA reconstructs y(t).

LFO Mapping

Let u = nH/Fs (seconds). S(n) = S0 + Sd · sin(2π f_LFO u + φ).
For banded swirl, use S_b(n) per band b and apply within band edges.