Scratch

A stereo turntable model: it records your incoming audio onto a virtual platter and lets you drag the needle across it. Hold the Pos knob still and it's clean passthrough; move it — by hand or with CV — and you get the whole vinyl gesture, platter lag and overshoot, stylus drag noise, and the high-end smear of a record being shoved under the needle.

How it works

Audio streams into a rolling two-second buffer — the grooves behind the live edge. Pos points the read head somewhere in that window, but it doesn't snap there: an α–β tracker (the same math used to model a spinning platter's inertia) chases the target, so the head accelerates, lags, and coasts past like real vinyl. When the tracker is running at normal speed nothing is being scratched, so the effect auto-fades back to the dry signal; the grit only appears while you're actually moving. Faster scratches roll off more treble and kick up more surface hiss, decorrelated between left and right for genuine stereo width.

Controls

  • Pos — the needle's position on the record, from 2 s behind the live edge up to the live edge itself (1× passthrough). Holding it still is silent; sweeping it is the scratch.
  • Inertia — how heavy the platter feels. Low values track your moves almost instantly; high values give a slow, draggy platter that lurches and overshoots.
  • Mix — wet/dry blend over the dry input. Fully up is the full effect.
  • Grit — how much stylus and surface noise the scratch kicks up. At 0% the scratch is clean and digital, 100% is the classic record-under-the-needle hiss, and pushing to 200% gives a gritty, dust-and-static platter.
  • Smear — how dark a fast scratch gets. It sets the treble floor the high end rolls down to as you speed up: low (1.5 kHz) warbles and tape-smears the dives, high (12 kHz) keeps them bright and clicky. The default 4 kHz is the classic record voice.

Inputs & outputs

  • inL / inR — the stereo source to scratch; mono works fine patched to one side.
  • Pos / Inertia / Mix / Grit / Smear CV — voltage control over each knob. Patch a fast envelope or LFO into the Pos CV to play hands-free scratches, baby-scratch stutters, and tape-stop dives.
  • outL / outR — the scratched stereo result.

Tips

  • The scratch lives in the gesture, not the position — automate poscv with a sharp shape (ramp, S&H, a plucked envelope) and the rhythm of your moves becomes the rhythm of the scratch.
  • Crank Inertia and nudge Pos for slow turntable spin-downs and warped pitch dives.
  • Back Pos off the live edge, then sweep poscv across its full range to haul the needle from two seconds behind up to now — a rewind-and-catch-up over everything you just played. At the default Pos of 1 the needle is already pinned to the edge, so turn the knob down first or positive CV does nothing.