Distortion with a memory

Every distortion plugin can bend a transfer curve. tap.overdrive~ is built on the observation that the pedals people actually love — the Tube Screamer lineage, and specifically the Mad Professor Little Green Wonder that served as this object's listening reference — don't apply one curve to the whole spectrum. Their clipper lives inside an op-amp's feedback loop with frequency-dependent parts around it, and that loop is most of the sound: bass sees less gain and stays tight, mids break up first, and the knee never quite flattens because the clean signal always rides through. A memoryless waveshaper — including both modes of the Jamoma-era tap.overdrive~ this object succeeds — structurally cannot do any of that. This one can, because the shaper sits inside a lowpass feedback loop: distortion with a memory.

Companion material: the reference page and help patcher in the TapTools-Max package, and the verification notebook, where every number below is an executed, plotted measurement of the shipping kernel. The figures in this chapter are measurements too — regenerated from the same kernel through the C ABI by book/figures/overdrive.py, never drawn by hand.

What the loop buys

The claim worth leading with, because no static curve can make it: the object's small-signal gain tilts with frequency, and the tilt grows with drive. Measured between 80 Hz and 4 kHz, the tilt is +5 dB at drive 0 (just the voicing EQ), +16.3 dB at drive 0.5, +17.2 dB at drive 0.9. Low frequencies are pinned near-clean by the feedback while mids and highs take the full drive gain — so a low E stays articulate under the same setting that saturates the pick attack. That is the Tube Screamer "tightness" in one plot:

The measured headline. A memoryless shaper's version of this figure is three horizontal lines.

The second structural trait: the transfer never flattens. A unity clean path is summed around the clipper — the non-inverting op-amp topology — so however hard the shaped part saturates, output keeps rising with input (measured strictly monotonic at every drive setting). The old sine-shaper mode's hard ±1 plateau, a large part of what read as "digital," is gone by construction.

Compression without a ceiling: the slope falls as drive rises, but never to zero.

The knobs, one by one

drive — 0 to 1, edge-of-breakup to saturated

Normalized, like every musical parameter on this object, with the perceptual mapping done inside (the knob sweeps the clipper's gain from +6 to +46 dB, with a level compensation tracking it). drive 0 is a pedal's gain knob at full counterclockwise — still warm, not bit-clean; bypass is the clean switch. The normalized range maps directly onto MIDI/OSC controllers, and onto Q15/Q31 fixed-point for the embedded ports this kernel is written to survive.

body — the signature voicing control

The LGW's defining knob, reproduced as linear pre/post EQ around the clipper (that's what it is in the pedal — voicing, not nonlinearity). Toward −1, fuller lows reach the clipper and the top gets a slight shelf lift; toward +1, the lows thin and tighten and an upper-mid bell pushes forward — centered at 1150 Hz, deliberately above the classic TS hump. Measured at the extremes: 100 Hz moves by 10 dB, the 1150 Hz push adds 4 dB, the counterclockwise treble lift is +2.5 dB at 8 kHz. The exact centers and gains are by-ear placeholders pending the in-Max voicing pass against LGW demos — the shape of the control is final, the seasoning isn't.

The knob's whole range. Note the crossover around 500 Hz: body trades lows against upper mids around a stable center, like the pedal.

asymmetry — the even harmonics the old object couldn't make

Both Jamoma modes were odd functions: odd harmonics only, the entire "warmth" vocabulary absent. asymmetry biases the clipper: at 0 the path is exactly symmetric (measured H2 at −151 dB — the numerical floor), and raising it brings the even series up smoothly (H2 at −26 dB by asymmetry 0.6). The default sits at 0.15, a small nonzero warmth chosen by ear. Asymmetric clipping generates DC, so a DC blocker sits permanently after the clipper — measured output mean under full drive, full asymmetry: 10⁻¹⁰. (The original TTOverdrive contained a DC blocker whose output was computed and then discarded; this one is load-bearing.)

The same tone, the same drive — the only change is asymmetry, and the even series (H2, H4, …) appears between the odd lines.

oversample — 1, 2, 4, or 8; default 4

Clipping makes harmonics; harmonics past Nyquist fold back as inharmonic junk. At 1× a hard-driven 5 kHz tone puts its folded seventh harmonic at −22 dB relative to the fundamental — clearly audible garbage at 12993 Hz. At the default 4× the same component measures −36 dB, with the true harmonics unchanged. Turn it down to 1× only when CPU matters more than the top octave, or when you want the fizz.

preamp, output, smooth, bypass, mute

Input and makeup gain in dB (±24) — the only unit-bearing parameters, because gains are the one place real units belong. Everything ramps click-free over smooth milliseconds (default 20).

Where it sits in a patch

Mono by design; wrap it in mc. for multichannel like the rest of the package. It takes line-level signals as happily as guitar DI — the drive mapping is normalized to full-scale digital, not to pickup output. For the LGW move, start at drive 0.4, body -0.3, asymmetry 0.15 and ride body against the source's low end. For a clean boost that just thickens, drive 0 with asymmetry 0.3. For fuzz territory this is the wrong object on purpose — the loop keeps pulling it back toward articulation.

Every claim above is pinned twice: as an executed measurement in the notebook, and as a hard assertion in the kernel's Catch2 suite (tests/overdrive_test.cpp), which CI runs on every push. The math behind the loop — including why it had to be solved zero-delay, and what happens if you don't — is in the machine chapter: The clipper in the loop.