2026-02-22

Undertone - Pitch-tracking Ring Mod

Summary Architecture

Guitar
  |
TL072 buffer
  |
Split

Path A (Audio to ring)
  -> Transformer -> Diode Ring

Path B (Carrier generation)
  -> LM393 (square)
  -> CD4013 (/2)
  -> Buffer/driver
  -> Transformer -> Diode Ring

Output of ring
  -> Mix with dry
  -> Output stage

TL072 - Audio Buffer and Driver

• Guitar input buffer (high impedance)
• Splits signal to dry path
• Splits signal to ring mod audio input transformer
• Splits signal to comparator path (for pitch/octave)

Why it is used:

• Stable, low-impedance drive
• Good for single 9V supply
• Can bias around Vref

Important:

The transformer input side needs enough level. If TL072 gain is too low, the ring mod will sound weak.

LM393 - Comparator (Zero Crossing Detector)

Role:

• Converts biased guitar signal into square wave
• Feeds CD4013 clock input

Important:

• Open collector output
• Requires pull-up resistor
• Benefits from hysteresis to prevent chatter

Without proper hysteresis:

• Clicking
• Double-triggering
• Unstable octave tracking

CD4013 - Divide-by-2 Octave Generator

Role:

• Takes comparator output
• Toggles on each clock edge
• Outputs square wave one octave below input

Used as:

• Carrier source for ring mod

Limitations:

• Produces square wave
• Hard edges can make ring mod harsh unless filtered

CD4046 (Discussed as Option)

Discussed when accurate pitch tracking came up.

What it is:

• Phase-Locked Loop (PLL)

What it could do:

• Lock to input frequency
• Produce stable fundamental
• Provide cleaner carrier than divide-by-2 approach

Why not used yet:

• More complex
• Needs loop filter tuning
• More design time

CD40106 (Schmitt Trigger Inverter) - Mentioned Option

What it is:

• Hex Schmitt trigger inverter

Potential role:

• Cleaner squaring than LM393
• Built-in hysteresis
• Simpler implementation

Why it is attractive:

• Less finicky than LM393
• Often better for guitar logic extraction

Transformer (Triad Mentioned)

Classic passive ring mod approach discussed:

• One transformer for audio input
• One transformer for carrier input
• Diode ring in between

Why transformer coupling matters:

• Proper diode switching requires symmetrical drive
• Proper diode switching requires enough voltage swing
• Transformers help isolate and balance signals

Concern raised:

LO drive strength.

Correct concern: diode ring needs sufficient carrier amplitude to switch diodes hard.

Diodes (1N5817 Schottky)

Why Schottky:

• Lower forward voltage (~0.2-0.3V)
• Easier to switch at 9V
• Softer transition than silicon

Classic alternative:

• 1N4148 (silicon switching diode)

Effect of diode choice:

• Lower Vf -> easier switching, potentially softer tone
• Higher Vf -> requires stronger carrier, sharper switching