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GroupDelay - All-Pass Disperser

Note: This manual is AI-generated and will be updated progressively.

Group delay effect using cascaded all-pass filters for transient shaping and phase dispersion.

Overview

GroupDelay uses cascaded all-pass filters to create frequency-dependent phase delay (group delay). This allows for rich transient enhancement, stereo width adjustment, and subtle to dramatic phase-based effects.

Quick Start

  1. Connect audio sources to L/R AUDIO INPUTS
  2. Adjust FREQ knob to set center frequency (20Hz-20kHz)
  3. Use AMOUNT knob to control number of all-pass stages (0-32)
  4. 0: Bypass (no phase delay)
  5. Increasing values: More group delay
  6. Adjust PINCH to control resonance Q (0.5-16)

Parameters

Main Controls

FREQ (20Hz~20kHz, default 1kHz) - Center frequency for phase manipulation - Suitable for pitch-shifting and envelope control - CV input supported (±10V range) - CV attenuation trim included

AMOUNT (0~32, default 4) - Number of all-pass filter stages - 0: Bypass (no phase delay) - Higher values: More pronounced group delay - Fractional values allow smooth adjustment - CV input supported (±10V range) - CV attenuation trim included

PINCH (0.5~16, default 0.3) - Filter Q value - 0.5: Wide peak, subtle resonance - Higher values: Narrower peak, emphasized resonance - CV input supported (±10V range) - CV attenuation trim included

Inputs/Outputs

  • L IN: Left channel input
  • R IN: Right channel input
  • L OUT: Left channel output
  • R OUT: Right channel output

Signal Processing Flow

  1. Input signal reading
  2. CV Pre-processing (2025 enhancement)
  3. IIR low-pass filter applied to frequency CV input (~2Hz cutoff)
  4. Low-frequency LFO modulation passes through, high-frequency jitter removed
  5. Oscillation and artifact prevention
  6. All-pass filter chain processing
  7. Specified number of stages in cascade
  8. Fractional values use equal-power blending
  9. Gradual Parameter Updates (2025 enhancement)
    • 2 filters updated per frame with frequency/Q
    • All 32 filters updated over 16 frames
    • Distributed parameter change impact reduces oscillation
  10. Soft clipping (±5V range)
  11. Output level LED for visual feedback

Stability Measures (2025 Enhancement)

Multiple safeguards ensure stable group delay operation:

Numerical Safety

  • Double-precision filter computation: All filter coefficients managed in 64-bit precision
  • Quantization noise reduction: Minimized numerical errors during 32-stage cascade

Oscillation Prevention

  1. IIR LFO Filter (~2Hz cutoff)
  2. Removes high-frequency jitter from CV inputs

  3. Passes LFO signals (0.1-10Hz), removes noise

  4. Gradual Parameter Updates

  5. 2 filters updated per frame
  6. Parameter changes distributed over 16 frames

  7. Reduces coefficient update shock

  8. Fast Coefficient Smoothing

  9. Update rate: 0.5% per sample
  10. ~180ms @ 44.1kHz to reach target value

  11. Prevents clicks and artifacts

  12. Auto-reset on anomalies

  13. Detects NaN/Inf output and auto-resets
  14. Prevents error propagation
  • Low risk: AMOUNT = 8~16 stages (maintains effect, improved stability)
  • Medium risk: AMOUNT = 16~24 stages (stronger effect, moderate stability)
  • High risk: AMOUNT = 24~32 stages (maximum effect, caution with LFO)

⚠️ Frequency Modulation Warning

Excessive modulation of the FREQ parameter can cause the filter to self-oscillate. Follow these guidelines:

Safe Frequency Modulation: - LFO frequency: Keep within 0.1~5Hz range (IIR filter removes high-frequency jitter) - Modulation speed: Use gradual, smooth changes - CV attenuation: Avoid excessive modulation depth (use CV attenuator)

If Oscillation Occurs: 1. Fix FREQ: Disconnect CV input, set to fixed value via knob 2. Reduce AMOUNT: Decrease to 16 stages or lower 3. Decrease CV amplitude: Attenuate LFO output level 4. Slow LFO rate: Use 0.1~1Hz for slow, gentle modulation

Critical: Using AMOUNT = 24~32 stages with fast CV modulation (>5Hz) significantly increases oscillation risk.

Applications

  • Transient thickness and width control
  • Stereo image adjustment
  • Effect fine-tuning
  • Creative spatial effects
  • Phase manipulation
  • Dynamic spatial effects with LFO modulation