OVERVIEW

This is a reverb effect based on David Griesinger’s nested allpass filter topology, a groundbreaking algorithm that revolutionized digital reverb design in the 1970s-80s. Used in legendary hardware units like the Lexicon 224, 480L, and PCM series.

What makes this special:

• 4 Cascaded Allpass Stages: Each stage creates dense echo patterns that build exponentially – 4 stages can generate thousands of reflections
• Modulation: Each delay line has independent LFO control for chorus/shimmer effects, preventing metallic ringing
• Damping: Frequency-dependent absorption simulates air and surface materials
• Flexible Routing: 30+ routing patterns from classic Griesinger cross-feed to exotic matrix operations
• Stereo Decorrelation: Mathematical patterns (Golden Ratio, Fibonacci, Prime Numbers) create natural stereo width
• Biquad Allpass Cascade: Up to 256 additional first-order allpass filters for extreme diffusion

Signal Flow:

Input → Pre-diffusion (optional) → Stage 1 → Stage 2 → Stage 3 → Stage 4 → Post-diffusion (optional) → Dry/Wet Mix → Output

Between stages: modulation, damping, and routing patterns control how the signal flows between L/R channels.

Key Concepts:

• Allpass Filter: Delays signal while preserving magnitude, only affects phase – creates diffusion without coloring tone
• G Coefficient: Feedback amount (-0.99 to +0.99) – higher values = longer decay, can self-oscillate near ±1.0
• Nested Topology: Output of each stage feeds the next, creating exponential reflection density
• Cross-feed: Swapping L/R channels between stages creates stereo width and decorrelation

Typical Use Cases:

• Natural Reverb: Chamber, Hall, Plate presets with alternating cross-feed
• Creative Effects: Shimmer, Spectral Smear, Barber Pole for ambient/experimental music
• Sound Design: Destruction presets (Bitcrush, Chaos, Ring Mod) for lo-fi/glitch
• Mixing: Subtle diffusion on vocals, drums, or mix bus

Important Notes:

• High G values (>0.9) can cause runaway feedback – use DC Blocker if low-frequency buildup occurs
• Negative polarity routing (RED arrows) creates metallic tones – use sparingly
• Downsample creates intentional aliasing for lo-fi effects
• Always check mono compatibility for professional work

CPU Usage: Real-time processing via AudioWorklet. High cascade depth (>64 stages) or extreme modulation may cause performance issues on older devices.

TRANSPORT CONTROLS

PLAY: Start audio playback (loops continuously)

STOP: Stop playback

REC: Record the processed output

UPLOAD: Load your own audio file (AIFF not accepted sorry!)

DOWNLOAD: Save recorded audio as WAV file

SOURCE URL: Enter a direct URL to an audio file (must support CORS)

MASTER CONTROLS

PRESET: Load pre-configured reverb settings organized by style:

SIZE (DELAY MASTER): Scales all delay times simultaneously (1-1000%). Higher values = larger reverb space. Affects perceived room size without changing individual stage relationships.

MOD SCALE: Global modulation depth multiplier (0-200%). Controls how much pitch variation/chorus effect is applied to all stages. 0% = static delays, 100% = preset modulation, 200% = extreme warble.

DAMPING MASTER: High-frequency absorption across all stages (0-100%). Simulates air absorption and surface damping. 0% = bright/metallic, 50% = natural, 100% = dark/muffled.

DRY/WET MIX: Balance between original (dry) and processed (wet) signal (0-100%). 0% = dry only, 50% = equal mix, 100% = wet only.

OUTPUT GAIN: Final output level in decibels (-24dB to +12dB). Compensate for volume changes caused by different settings. 0dB = unity gain.

OUTPUT LEVEL METERS: Real-time PPM (Peak Programme Meter) showing L/R levels with color coding:

• Green: -60dB to -15dB (safe)
• Yellow: -15dB to -6dB (caution)
• Orange: -6dB to -3dB (hot)
• Red: -3dB to 0dB (clipping risk)

DOWNSAMPLE: Reduces sample rate for lo-fi/aliasing effects:

• OFF: Full 48kHz (clean)
• ÷2: 24kHz (slight degradation)
• ÷4: 12kHz (telephone quality)
• ÷8: 6kHz (extreme lo-fi, heavy aliasing)

DC BLOCKER: 48dB/oct high-pass filter at 100Hz to remove DC offset and rumble. Turn ON if you hear low-frequency buildup or “woofy” sound. OFF for full bass response.

ALLPASS CASCADE DIFFUSION

Additional allpass filters that can be inserted at various points in the signal path for extra diffusion and coloration.

ROUTING: Where the allpass cascade is inserted:

• Off: Bypass cascade entirely
• Pre-diffusion: Before reverb stages (smooths input)
• Inside Loop: In the feedback path (dense, metallic)
• Mid-cascade: Between stages 2 & 3 (hybrid)
• Post-diffusion: After all stages (polishes output)
• Parallel Blend: Mixed with main signal
• Isolate: Only allpass, no reverb stages

FREQUENCY: Center frequency of the allpass filters (100-1000Hz). Lower = warmer/darker, Higher = brighter/airier.

FREQUENCY SPREAD: Stereo detuning between L/R allpass filters (0-300%). Creates wider stereo image. 0% = mono, 100% = typical spread, 300% = extreme width.

G COEFFICIENT: Allpass feedback amount (0.5-0.9). Higher values = more resonance/coloration. 0.7 = neutral, 0.9 = highly resonant.

MOD DEPTH: Frequency modulation range in Hz (0-300Hz). Adds vibrato/chorus to the allpass filters.

MOD RATE: Modulation speed in Hz (0.01-10Hz). Slow = gentle shimmer, Fast = Leslie/rotary effect.

CASCADE DEPTH: Number of cascaded allpass stages (1-256). More stages = denser diffusion but more CPU load. 2-4 = typical, 16+ = extreme smearing.

ROUTING PATTERN

Controls how the four reverb stages feed into each other and between L/R channels.

STANDARD PATTERNS:

• Independent L/R: No cross-feed (narrow stereo)
• Alternating Cross-feed: Stages 2 & 4 cross L↔R (Griesinger original)
• All Stages Cross-feed: Every stage crosses (wide stereo)
• Odd/Even Cross: Only odd or even stages cross
• Ping-Pong: L→R→L→R sequential bouncing

MATRIX PATTERNS:

• Hadamard: Orthogonal mixing matrix (even diffusion)
• Householder: Reflection-based mixing (smooth)
• Feedback Matrix: Dattorro-style figure-8 topology

EXOTIC PATTERNS: Experimental routings with polarity inversions, rotations, and mathematical sequences (Golden Ratio, Fibonacci, Möbius, Tesseract, etc.). Use for creative sound design.

ARROW LEGEND:

• Gray: Independent routing (L→L, R→R)
• Green: Positive cross-feed (L→R, R→L)
• Red: Negative polarity (phase inverted)
• Blue: Rotation/spiral effects
• Yellow: Matrix operations
• Magenta: Mixed polarity combinations

STEREO OFFSET (L/R DECORRELATION)

Creates stereo width by using different delay times for L and R channels.

OFFSET PATTERN: Mathematical relationship between L and R delays:

• None: L = R (mono)
• Lexicon: Prime number pairs (professional standard)
• Golden Ratio: R = L × 1.618 (natural-sounding)
• Haas Delay: 3-30ms offset (psychoacoustic widening)
• Inverse: Swaps L↔R between stages
• Random/Chaos: Unpredictable scatter

SPREAD AMOUNT: How much offset to apply (0-200%):

• 0-100%: Blend from mono to full offset
• 100-200%: Full offset + side signal boost (ultra-wide)

CURRENT DELAYS: Shows actual L | R delay times for each stage. Updates in real-time as you adjust offset pattern and spread.

STAGE PRESETS

Quick-load buttons for changing delay times, modulation profiles, and G coefficients across all 4 stages.

DELAY TIME PRESET: Pre-calculated delay patterns:

• Classic Reverb: Griesinger Primes (11.7, 19.3, 31.1, 43.7ms), Lexicon, EMT, Bricasti standards
• Mathematical: Fibonacci, Golden Ratio, Powers of 2, Prime Numbers
• Tonal/Musical: A440 Chord, Octaves, Harmonic Series (creates pitched resonances)
• Density: Low/Med/High/Ultra (controls reflection density)
• Spatial: Tiny Room to Impossibly Large (room size emulation)
• Experimental: Micro Timing, Extreme Contrast, Logarithmic, Anti-Pattern

MOD PROFILE: Modulation depth patterns:

• Static: No modulation (clean, precise)
• Subtle: Minimal shimmer (0, 0.1, 0.3, 0.5ms)
• Classic: Traditional reverb chorus (0.2, 0.5, 1.0, 2.0ms)
• Lush: Rich modulation (0.5, 1.2, 2.0, 3.5ms)
• Extreme: Heavy warble (1.0, 2.0, 3.5, 5.0ms)
• Special: Chorus (equal all stages), Vibrato (deep/slow), Detune (micro), Shimmer Wash, Barber Pole

G COEFFICIENT PRESET: Feedback patterns:

• Griesinger Classic: 0.7, -0.7, 0.7, -0.7 (alternating polarity, natural)
• High Density: ±0.8 (longer decay, denser)
• Low Density: ±0.5 (shorter decay, cleaner)
• All Positive: 0.8, 0.75, 0.7, 0.65 (resonant, can self-oscillate)
• Golden Ratio: ±0.618 (mathematically balanced)

INDIVIDUAL STAGE CONTROLS (STAGES 1-4)

Each of the 4 reverb stages has independent controls:

DELAY L/R (ms): Delay time for left and right channels (0.1-500ms). Longer = larger space. Can be different for stereo width.

G COEFF: Allpass feedback coefficient (-0.99 to +0.99):

• Positive: Smooth, natural reverb tail
• Negative: Phase inversion, can create metallic tones
• 0: Bypass this stage entirely
• ±0.7: Standard (Griesinger original)
• ±0.9+: High feedback (long decay, can ring)

MOD DEPTH (ms): How much the delay time fluctuates (0-10ms). Creates chorus/shimmer effect. 0 = static, 2+ = noticeable pitch variation.

MOD RATE (Hz): Speed of modulation (0.01-20Hz). Slow (0.1-1Hz) = gentle movement, Fast (5-20Hz) = vibrato/Leslie effect.

DAMPING: High-frequency rolloff per stage (0-1). Simulates air absorption. 0 = bright, 0.5 = neutral, 1 = heavily damped/dark.

PRESET SAVE/LOAD/EXPORT/IMPORT

SAVE PRESET: Saves current settings to browser’s localStorage with a custom name. Persists between sessions.

LOAD PRESET: Recalls previously saved presets from localStorage. Shows list of available preset names.

EXPORT JSON: Downloads current settings as a .json file. Use for backup, sharing, or version control.

IMPORT JSON: Loads a .json preset file. Compatible with exported presets. Useful for preset libraries.

TECHNICAL NOTES

• Algorithm: 4-stage nested allpass cascade with modulation, damping, and flexible routing based on David Griesinger’s research
• Sample Rate: 48kHz (Web Audio API default)
• Processing: Uses AudioWorklet for low-latency, glitch-free performance
• Input Attenuation: -15dB internal padding to prevent clipping with high feedback
• Modulation: Sinusoidal LFOs per stage with independent phase
• Damping: First-order lowpass filter in feedback path
• Biquad Allpass: First-order allpass filters with frequency-dependent phase shift

TIPS & TRICKS

• Classic Reverb: Start with “Chamber” or “Hall” preset, adjust SIZE to taste
• Wide Stereo: Use “Ultra-Wide” preset or enable “All Cross” routing + high Stereo Spread
• Shimmer Effect: “Shimmer” preset + high MOD SCALE + low damping
• Lo-Fi: Enable DOWNSAMPLE ÷4 or ÷8 + “Bitcrush” preset
• Infinite Sustain: “Infinite Freeze” preset (G values at 0.99) – careful, can self-oscillate!
• Prevent Feedback Runaway: Keep G values below 0.95, use DC BLOCKER if rumble builds up
• Cleaner Sound: Lower MOD SCALE, use Static or Subtle mod profile
• Experimental: Try Exotic routing patterns + Random stereo offset + high cascade depth

🔘 GRAY (#666) – Independent Routing

What it means: No cross-feeding between L and R channels. Each channel processes independently in parallel.

Signal flow: Stage N Left → Stage N+1 Left | Stage N Right → Stage N+1 Right

Polarity: N/A (no interaction)

Sound character: Narrow stereo image, cleaner and more focused, less diffusion, good for mono-compatible reverb

Used in: Independent L/R, Serial, Parallel

🟢 GREEN (#0f0) – Positive Cross-feed

What it means: Channels swap with positive polarity (no phase inversion). Signal crosses L→R and R→L with phase intact.

Signal flow: Stage N Left → Stage N+1 Right (+1.0) | Stage N Right → Stage N+1 Left (+1.0)

Polarity: +1 (0° phase shift)

Sound character: Wide stereo image, smooth natural diffusion, phase-coherent (mono-compatible), classic Griesinger sound

Used in: Alternating Cross-feed, All Stages Cross-feed, Odd/Even Cross, Golden Ratio

🔴 RED (#f00) – Negative Polarity Cross-feed

What it means: Channels swap with negative polarity (180° phase inversion). Signal crosses L→R and R→L but flipped.

Signal flow: Stage N Left → Stage N+1 Right (-1.0) | Stage N Right → Stage N+1 Left (-1.0)

Polarity: -1 (180° phase shift)

Sound character: Can create metallic or hollow tones, phase cancellation effects, less mono-compatible, comb filtering artifacts, resonant “colored” reverb

Used in: Polarity Chaos, Antiphase, Checkerboard, Möbius, Phase Flip

Warning: Extreme negative feedback can cause instability!

🔵 BLUE (#0af) – Rotational/Ping-pong

What it means: Complex rotation using trigonometric functions. Treats stereo field as 2D vector space with continuously variable polarity.

Signal flow (Spiral): L’ = L × cos(θ) – R × sin(θ) | R’ = L × sin(θ) + R × cos(θ)

Where θ rotates: Stage 1 = 0°, Stage 2 = 90°, Stage 3 = 180°, Stage 4 = 270°

Polarity: Variable (smooth rotation through complex plane)

Sound character: Swirling/spinning stereo image, Leslie/rotary speaker effect, smooth phase transitions, immersive spatial movement, Doppler-like modulation

Used in: Spiral, Double Helix, Ping-pong

Math note: This is a 2D rotation matrix – the same math used in graphics to rotate points. It treats L/R as X/Y coordinates and spins them!

🟡 YELLOW (#ff0) – Matrix Operations

What it means: Orthogonal diffusion matrices. Each output receives weighted sum of BOTH inputs using positive AND negative mixing simultaneously.

Signal flow (Hadamard): L’ = 0.707 × (L + R) | R’ = 0.707 × (L – R)

0.707 = 1/√2 (maintains energy)

Polarity: Mixed (+1 and -1 simultaneously)

Sound character: Dense, even diffusion with no preferred spatial direction, mathematically optimal energy distribution, smooth “professional” reverb tail. Used in Lexicon and EMT algorithms.

Used in: Hadamard Matrix, Householder Matrix, Golden Ratio, Fibonacci Cross

Why yellow? It’s neither green (positive only) nor red (negative only) – it’s both at once.

🟣 MAGENTA (#f0f) – Mixed Polarity Combinations

What it means: Exotic topologies with multiple simultaneous polarities. Combines positive, negative, and weighted routing in non-standard mathematical operations.

Examples:

• Double Helix: Two intertwined spirals with opposite polarities (like DNA strands)
• Quantum: Simultaneous parallel AND serial processing with phase interference
• Tesseract: 4D hypercube with alternating polarity matrix

Polarity: Highly complex (variable, non-linear, multi-dimensional)

Sound character: Unpredictable/chaotic diffusion, psychoacoustic effects, non-natural spatial imaging, creative/experimental sounds. Can be unstable with high feedback.

Used in: Double Helix, Quantum, Tesseract, Möbius Strip, Polarity Chaos

Why magenta? Mix of red (negative) and blue (rotation) = exotic combinations that don’t fit simple categories.

VISUAL SUMMARY

PRACTICAL USAGE GUIDE

For natural reverb: Use GREEN (positive cross-feed) or YELLOW (Hadamard/Householder). Avoid RED except in small amounts.

For wide stereo: Use GREEN on all stages or BLUE (spiral). Combine with high Stereo Spread.

For experimental sounds: Try RED (negative polarity) or MAGENTA (exotic patterns). Use with caution – can be unstable.

For mono compatibility: Stick to GRAY, GREEN, or YELLOW. Avoid RED entirely. Test with mono fold-down.