P33

📖 What Pearson does

Chapter 10 builds a family of circuits that modulate other circuits. Modulation means: take a control signal (often from an LFO, envelope, or other source) and use it to vary a parameter of a sound-generating circuit in real time. Projects:

  • The Vantastic Vactrol. A light-controlled variable resistor built from an LED and a light-dependent resistor. The brighter the LED, the more current through the LDR.
  • Pulse-width modulation. Vary the duty cycle of a square wave. The “PWM” knob on a Moog synth does this.
  • Pulse-width modulation with vactrols. Combine the two above.
  • A button-controlled VCA. A voltage-controlled amplifier whose gain is set by a button press.
  • An LFO-pingable VCA. Like the above, but the gain is set by a slow LFO.
  • The piezo drum trigger. A piezo element that, when struck, produces a short trigger pulse that fires an envelope.
  • Ring modulation. Multiply two audio signals. The result has sum-and-difference frequencies. The “Dalek voice” effect.
  • Stereo tremolo. A stereo modulator: the left and right channels are modulated by LFOs that are out of phase.

🎓 Background: modulation as a software concept

In code, modulation is a parameter that changes over time. There are three main kinds:

  • LFO modulation. A slow oscillator (sub-audio frequency) that drives a parameter. Example: an LFO at 4 Hz modulates filter cutoff. The filter “wobbles” at 4 Hz.
  • Envelope modulation. A one-shot rise-and-fall shape. Example: an envelope generator produces a quick attack and a slow decay. The amplitude of a note follows that envelope.
  • Step modulation. A pre-set sequence of values. Example: a pattern of pitches (this is the sequencer!).

AMY supports all three. LFOs are exposed via amy_event.mod_source and mod_target fields. Envelopes are exposed via bp0_times and bp0_values. Step modulation is what our sequencer does internally — every step is a discrete jump to a new value.

🔧 Try it on the device

Exercise 1: ring modulation via FX.

The PO-33 has a punch-in effect called RING_MOD (when fully implemented). Ring modulation multiplies the slot with a slow sine wave, producing sum-and-difference frequencies. Try:

> fx RING_MOD
> note 9 60

You should hear a metallic, bell-like tone with a slightly detuned quality. That is the ring modulator at work.

(As of v0.6, the RING_MOD case in apply_fx() is a no-op. The infrastructure is in place; the wiring is the missing piece.)

Exercise 2: pulse-width modulation via FX.

> fx PULSE_WIDTH
> note 9 60

When wired, this will change the duty cycle of the underlying square wave oscillator. A 50% duty cycle sounds bright; a 10% duty cycle sounds thin and “talkbox-y”.

Exercise 3: tremolo via FX.

> fx STUTTER_4
> note 9 60

The note rapidly retriggers 4 times. This is the gating version of tremolo. For a smoother tremolo, we’d need an LFO driving the amplitude of the note — that is supported in AMY but not wired to a verb.

Exercise 4: an envelope on every note.

Every note event has a built-in envelope. AMY’s default envelope is “fast attack, medium decay, sustain at half”. It’s what makes a note sound like a note rather than a drone. Try:

> note 9 60
> note 9 60
> note 9 60

You hear the same note three times, each with the default envelope. That envelope is the closest analog of Pearson’s envelope generator — implemented in software.

🛠 Code reference

  • LFO modulation — AMY’s amy_event struct has mod_source and mod_target. We have not yet exposed this via UART.
  • Envelope — AMY’s bp0_times[] and bp0_values[]. Default envelope is set in amy_default_event().
  • Ring mod FX — main/audio/amy_bridge.c, apply_fx(), case PO33_FX_RING_MOD:. Currently a no-op; the AMY code is there (awaiting_factor and similar).

🚫 What we can’t simulate

  • The vactrol. Pearson’s light-controlled resistor is a beautiful analog of a knob that turns itself. We have no light sensor on our device. The closest is the I²S mic, which is a “sound-pressure controlled gain” — not the same.
  • The piezo drum trigger. A piezo’s impulse response is fast (sub-millisecond) and short. Our INMP441 mic has its own AGC (automatic gain control) that smooths impulses. They are not the same.