On Tap — DSP Manual

Carbonated Audio · v1.0.0 Tempo-synced sidechain ducker for modern mix engineers.

This document explains the signal processing inside On Tap from the top down: what each control does mathematically, why it's there, and how to reason about it on a mix. It's written for engineers and producers who want a real understanding of what the plug-in does, not just which knobs make things louder.

1. Overview

On Tap is a tempo-synced amplitude modulator — a sidechain ducker that doesn't need a sidechain. Instead of detecting a kick drum and reacting to it, On Tap runs a mathematically-defined ducking curve against the host's transport or an incoming MIDI trigger, pumping the signal on a rhythmic grid you choose.

Everything in On Tap reduces to three core ideas:

1. Pick a trigger — the host's PPQ clock, an incoming MIDI note-on, or (in the future) a sidechain audio trigger.
2. Sample a curve — a pre-computed envelope shape that describes how the gain should rise and fall across one cycle.
3. Apply the curve — multiply the audio by the resulting gain, optionally on a single band if you only want to duck the low end.

There's no detector, no threshold, no attack/release to tune. The "sidechain envelope" is whatever curve you pick, stretched and shifted by the Slope and Shift controls, looped at the tempo division you set.

2. Signal Flow

In ─► Bypass? ──► Trigger source (Sync / MIDI / Audio) ─► Phase counter
                                                            │
                                                            ▼
                                                    Curve lookup (16 presets)
                                                            │
                                                            ▼
                                                  Slope / Shift reshape
                                                            │
                                                            ▼
                                                    Duck gain = 1 − curve
                                                            │
                                                            ▼
                                                    Mix (0–100%) blend
                                                            │
                                                            ▼
                    ┌───────────────┬───────── Anti-click smoothing ──────────┐
                    │                                                         │
                    ▼ (Band Active OFF)                                       ▼ (Band Active ON)
              Apply gain to full                                       Split → LP │ HP
              buffer                                                   Apply gain to LP only
                                                                        Recombine LP + HP
                                                                             │
                                                                             ▼
                                                                           Out

Every stage runs per sample in the gain-application loop, so automation and knob tweaks are click-free when Anti-Click Smoothing is on.

3. Trigger Modes

On Tap supports three ways of knowing when to start a ducking cycle:

Sync (default)

The plug-in reads the host's ppqPosition (position in quarter notes since the start of the transport) and computes a phase value from 0.0 to 1.0:

phase = (ppqPosition mod lengthBeats) / lengthBeats

where lengthBeats is determined by the Length control (see Section 4). This means the ducking cycle is locked to your DAW's grid — start the transport and every cycle aligns exactly to a beat, bar, or bar-division.

When transport is stopped, phase is held at 0 (cycle does not advance).

MIDI

The plug-in listens for MIDI note-on messages on its input bus. Any note-on sets triggered = true and resets the phase to 0. The phase then advances at a fixed rate derived from a nominal BPM of 120 and the Length setting:

phaseIncrement = (120/60) / (sampleRate × lengthBeats)

When phase reaches 1.0, the cycle either loops (if Loop is on) or latches at 1.0 (silent, no ducking) and waits for the next note-on.

Use this when you want a ducking cycle gated to specific beats — e.g. feeding it from a kick MIDI clip to get chops on kick hits only.

Audio

Reserved for future revision. In v1.0 the mode is present but not active — phase advances based on the MIDI-style trigger state.

4. Length — Cycle Division

Options: 1/8, 1/4, 1/2, 1/1 — default 1/4.

Sets the length of one full ducking cycle in musical beats:

Shorter values give you per-beat chops; longer values give you slow, swelling fades over a whole bar. In Sync mode, phase resets exactly on the grid — the first duck starts exactly on beat 1.

5. Curve Presets

On Tap ships with 16 pre-computed envelope curves, lookup-table driven for zero-overhead per-sample access. Each is a 1024-point table stored as tables[preset][position], where position is the current cycle phase (0.0 to 1.0).

Each curve value means "how much signal to let through at this point in the cycle" — 1.0 is full volume, 0.0 is silent. The final duck gain is the inverse (1 − curve), so a curve value of 1.0 at phase 0 means no ducking at the top of the cycle, and dropping toward 0 means increasing ducking.

The preset bank is generated on construction and never recomputed, so changing presets is a pointer swap — no audio artefact.

6. Slope Offset

Range: −1.0 to +1.0, default 0.0.

Stretches or compresses the sloped portion of the curve by applying a power remapping to the position value before the lookup:

power = 2^(−slopeOffset × 2)
position' = position^power

At slopeOffset = 0, power = 1, position is unchanged (neutral). At slopeOffset = +1, power = 0.25 — the curve is "stretched" so the slope happens earlier. At slopeOffset = −1, power = 4 — the curve is "compressed" so the slope happens later.

Musically this gives you a single-knob attack/release shaper — push positive for a faster initial dip, pull negative for a lazier one, with the total cycle length unchanged.

7. Shift

Range: −1.0 to +1.0, default 0.0.

Phase-shifts the curve inside its cycle. Applied before the slope remap:

position' = (position + shift + 10) mod 1.0

The +10 is there to guarantee a positive value before mod for negative shift values. The net effect: the curve starts at a different point of the cycle, letting you dial in where the duck lands relative to the beat. Shift = +0.25 rotates the curve a quarter of a cycle to the right; Shift = −0.5 flips it to the opposite half.

Use Shift to "delay" or "pre-duck" against the beat without touching Length.

8. Mix

Range: 0% to 100%, default 100%.

Parallel dry/wet blend on the ducked gain. Computed per sample:

targetGain = 1 − (Mix × (1 − duckGain))

At Mix = 100%, the full duck is applied (targetGain = duckGain). At Mix = 50%, the duck amount is halved — a 20 dB dip becomes ~10 dB. At Mix = 0%, the plug-in is effectively inactive (targetGain = 1.0 constant).

This lets you dial in subtle pumping without picking a different curve preset, and is the right control to automate if you want the ducking to "build" into a chorus.

9. Loop

Boolean, default ON.

Only relevant in MIDI (and future Audio) trigger modes.

• Loop ON — after each cycle completes, phase wraps back to 0 and the curve repeats until a new trigger arrives.
• Loop OFF — the cycle runs once per trigger, then phase latches at 1.0 (full volume, no ducking) until the next MIDI note-on.

In Sync mode, Loop is effectively always on — the host's PPQ doesn't stop advancing until you stop the transport.

10. Anti-Click Smoothing

Boolean, default ON.

Controls whether the gain-change smoother runs across sample changes.

• Smoothing ON — a one-pole lowpass smoother sits on smoothedGain, with a time constant of ~6 ms (computed as 6/1000 × sampleRate samples). This guarantees any abrupt curve transition (e.g. the stepped preset crossing a boundary, or a shift-automation movement) fades smoothly across the change.
• Smoothing OFF — gain is applied as-is, sample-accurate. You get sharper transients on stepped curves, at the cost of possible clicks.

Default to ON unless you specifically want the stepped behaviour (preset 10) to click on every boundary.

11. Band Split (Low-Band Only Ducking)

Band Active: Boolean, default OFF. Band Frequency: 20 Hz to 5,120 Hz, default 320 Hz, logarithmic skew.

When Band Active is on, On Tap splits the incoming audio into two bands using a Linkwitz-Riley crossover, ducks only the low band, and recombines:

1. Split — input runs through both a 2nd-order Linkwitz-Riley lowpass and a matched highpass at the Band Frequency. (JUCE's juce::dsp::LinkwitzRileyFilter — the split is phase-coherent, so recombining at unity is flat.)
2. Duck the low band — the per-sample gain curve is applied to the low band only.
3. Recombine — low + high summed back into the output.

Why? Full-range ducking is great for creating space for a kick on a bus, but on a full mix it kills your cymbals, vocal sibilance, and reverb tails. Band Split lets you chop just the sub-kick energy without touching the top end — modern "side-chain only the bass" approach used on modern EDM and pop mixes.

When to use: - Active OFF — full-range ducking. Good for buses, drum loops, pads sitting behind kicks. - Active ON, ~100 Hz — just the sub rumble ducks out of the way of the kick fundamental. - Active ON, ~300 Hz — ducks kick + bass body, leaves guitars, vocals, cymbals, snare all unaffected. - Active ON, ~800 Hz — aggressive, affects most of the low midrange. Less musical; creative tool.

12. Bypass

Global bypass, checked first in the process loop. When true, the audio passes through untouched at unity gain and none of the DSP is executed.

13. Latency and CPU

On Tap is zero-latency. No FFT, no lookahead, no oversampling.

CPU cost is dominated by: - A single LUT lookup + linear interpolation per sample (always cheap) - One multiply per sample for the gain application - Two 2nd-order biquads per channel when Band Split is active (slight increase)

Expect roughly the cost of a gate or a tempo-synced LFO. Safe to stack dozens of instances in a session.

14. Channel Configuration

• Main I/O: stereo in, stereo out. Mono supported.
• MIDI input: accepts note-on messages for MIDI trigger mode (the plug-in declares NEEDS_MIDI_INPUT = TRUE in its CMake manifest).
• Sample rates: all sample rates supported. Crossover coefficients recompute when sample rate changes.
• Block sizes: arbitrary.

15. Credits and References

• Linkwitz-Riley crossover — S. Linkwitz, Active Crossover Networks for Noncoincident Drivers, JAES 1976. The reason the two-band split recombines flat.
• Robert Bristow-Johnson, Cookbook formulae for audio EQ biquad filter coefficients — source for the filter math JUCE's dsp::IIR::Coefficients uses internally.

Questions, bug reports, or sound-design notes: mixedbysoda@gmail.com.