Processor LF Synth

Processor LF Synth is a monophonic stereo bass synthesizer built around one struck-and-sustained twelve-mode membrane. Each note excites the body at a surface coordinate, while a continuous modal-energy term lets that same body sustain under ADSR. The complete membrane tracks chromatically, glides between held notes, follows a common pitch transient, and restores the previous held note with last-note-priority allocation.

The public surface is deliberately direct. Sub, Modal Body, Mass Load, Coupling, Damping, and Excitation expose the membrane core. Upper Modes, Cutoff, Resonance, Membrane Drive, Filter Mix, and Tube shape the direct result. Stereo Shred adds eight upper-band distortion circuits, governed feedback with a separate return level, loop Color, and an explicit dry/wet mix without placing the bass fundamental inside its nonlinear loop.

Processor LF Synth interface showing output, membrane, tone, envelope, Stereo Shred, amplitude LFO, and MIDI controls — The complete bass path remains visible at once: membrane and pitch behavior feed tone and Tube shaping, followed by Stereo Shred, amplitude movement, and final output.

Workflow

Insert s3g Processor LF Synth on a stereo instrument track and send it MIDI notes.
Route the stereo output to speakers or headphones, play a low note, then set SUB for the centered first membrane mode and MODAL BODY for the sixteen-pickup surface fold.
Use MASS LOAD to move the circular-membrane ratios toward a pitch-legible harmonic family. COUPLING introduces a small split between symmetric modal pairs.
Set DAMPING for modal duration and EXCITATION for the balance between central low modes and asymmetric upper modes.
Use UPPER MODES to expose or restrain both the initial strike and sustained energy of the membrane's higher modes. A small physical strike floor remains at 0%, while the protected first mode is unchanged. MEMBRANE DRIVE saturates each physical pickup before stereo summation. CUTOFF, RESONANCE, and FILTER MIX shape the surface body in parallel with the first mode.
Raise TUBE for one coordinated change through valve gain, push-pull power transfer, supply compression, broad bass emphasis, high-frequency restraint, and cabinet contour.
Select a Stereo Shred CIRCUIT, raise SHRED for the post-amplifier nonlinear branch, and introduce FEEDBACK for pitch-related stereo regeneration. FB TONE LEVEL sets how much of that regenerated circuit tone reaches the wet path without changing the loop's decay setting. COLOR coordinates the circuit tone, feedback ratio, and loop bandwidth. Use MIX to retain as much of the unprocessed amplifier output as required; at 0%, Mix is exactly dry.
For rhythmic amplitude modulation, select TRANSPORT and a straight, dotted, or triplet DIVISION. Use FREE and FREE RATE for an autonomous sinusoidal LFO. AMOUNT sets the VCA modulation depth; POSITION places it immediately before or after Stereo Shred.
Finish with ADSR, GLIDE, PITCH TRANSIENT, and the topmost OUTPUT control.

Signal Flow

The most recently played held note sets one common frequency scale for twelve circular-membrane modes.
A short distributed force packet applies strike-position weights without a single-sample impulse. Each mode retains a naturally decaying strike-amplitude term.
A second, smoothly driven amplitude term sustains those same modes while the gate is held. ADSR controls the audible contour; Glide and Pitch Transient move the complete modal scale without restarting independent oscillators.
The modes are sampled at sixteen concentric virtual surface positions. Membrane Drive applies a bounded local transfer at every pickup before the pickups are equal-power folded to stereo.
The first membrane mode forms the protected centered foundation. The stereo surface body enters one oversampled topology-preserving state-variable low-pass.
Cutoff and resonance are smoothed directly. Note onset, pressure, and periodic motion do not move the filter.
FILTER MIX blends the processed surface body with its dry form. The first membrane mode bypasses this filter path, preserving low-frequency pitch and weight.
TUBE controls a single coordinated amplifier trajectory. The pedal section is permanently bypassed.
Stereo Shred follows the amplifier. A note-related crossover keeps a clean 180–500 Hz low branch outside the nonlinear path. The two upper branches use independent folded transfers and bounded delay loops tuned to nearby ratios of the played note. A final low-band replacement prevents nonlinear or crossover phase residue from subtracting the fundamental.
The Shred feedback gain is DC-blocked, low-pass contained, energy-governed, and source-envelope gated. Its audible return has a second energy governor and a smooth interruption envelope: downward control changes and note interruptions duck the return without clearing the delay memory, then recover gradually. It can regenerate after note release, but drains to silence rather than becoming a permanent no-input oscillator.
The sinusoidal amplitude LFO is a smoothed stereo VCA with selectable placement. PRE SHRED modulates the post-Tube signal entering Shred, changing how each cycle excites its nonlinear and feedback paths. POST SHRED modulates the complete Shred output, including regeneration. It never alters cutoff, resonance, pitch, modal ratios, or Membrane Drive.
Sub-92 Hz stereo difference is removed, and the remaining modal side component is restrained. The fundamental and body mid stay centered.
Smoothed onset/release gain, a windowed two-lane modal handoff for retriggers, drained nonlinear state, DC removal, and a linked stereo headroom guard prevent discontinuities and hard clipping before the plug-in sleeps.

Modal Body

The physical foundation combines circular-membrane modal identities, strike-position weighting, sixteen virtual surface samples, and a stereo pickup fold. Sustained modal energy, harmonicizing mass load, pair splitting, ADSR, glide, a shared pitch transient, and a protected first-mode path make that structure behave as a playable bass instrument.

Control	Model translation	Audible effect
Modal Body	Scales the stereo fold of sixteen locally sampled membrane positions independently of the first mode.	Moves from the fundamental membrane mode toward the complete vibrating surface.
Mass Load	Radial density loading moves circular-membrane ratios toward an approximately harmonic family.	Increases pitch-legible thickness rather than unrelated metallic inharmonicity.
Coupling	Applies a small opposite tuning displacement to symmetric modal pairs.	Widens paired resonances without creating a second body.
Excitation	The forcing point moves from the membrane center toward its rim.	Center favors symmetric low modes; the edge admits more asymmetric upper modes.
Damping	Upper modes lose energy faster as damping rises.	Moves from an extended, heavy membrane toward a compact, dark response.
Upper Modes	Scales modes two through twelve inside the struck and sustained body.	Moves from a dark fundamental-focused membrane toward its full surface spectrum.
Membrane Drive	A bounded local transfer is applied independently to all sixteen surface pickups before stereo folding.	Adds dense projected harmonics without placing saturation inside the resonant filter loop.

Background on air-loaded, composite, coupled, non-uniform, and nonlinear membrane and plate behavior appears under Modal Synthesis and Physical Modeling.

Tube Stage

TUBE is intentionally one audible control rather than a collection of internal circuit trims. At zero, the amplifier stages are neutral. Raising it coordinates cascaded asymmetric preamplification, push-pull power saturation, signal-dependent supply compression, broad low-frequency emphasis, a restrained upper response, and a bass-cabinet bandwidth contour. A controlled low-band restoration around the nonlinear stages maintains pitch evidence at higher settings. Entering or leaving the neutral endpoint crossfades the complete amplifier path so the stored transformer and cabinet response changes smoothly.

The trajectory is a functional reduced-order bass-amplifier model rather than a component-for-component simulation. It combines cascaded asymmetric preamplification, a push-pull power stage, broad tone shaping, supply compression, and a cabinet contour.

Stereo Shred

The stereo upper-band processor combines selectable nonlinear transfers with bounded delay feedback tuned for the instrument's lower register. CIRCUIT selects SHRED, WOOL, RAT, ZONE A, ZONE B, FUZZ I, FUZZ II, or DIODE. Circuit changes crossfade over 20 ms. SHRED sets the selected transfer's intensity in two independent channels, FEEDBACK opens their governed regeneration loops, FB TONE LEVEL controls only the audible regenerated contribution, COLOR coordinates circuit voicing with note-related loop frequency and bandwidth, and MIX crossfades from the exact post-Tube dry signal to the bass-specific processed path.

The bass retuning follows a common functional pattern: a protected low branch is blended with a separately distorted upper branch. This preserves pitch and weight while allowing the upper spectrum to become much more aggressive.

This implementation translates those principles rather than reproducing any one device. Every selectable circuit operates only on the separated upper branch. The crossover follows the current note within a bounded bass range; Color moves the feedback resonances through higher note-related ratios and opens or closes the feedback and post-circuit bandwidths. The two loops retain a small fixed left/right tuning offset. Regenerated energy is filtered out of the protected low branch before recombination, so higher settings add stereo motion and abrasion while the centered membrane foundation remains available.

MIDI and Performance

The instrument uses last-note-priority monophonic allocation. Playing another note while one is held transfers the existing voice legato and follows GLIDE; releasing the current note restores the most recent still-held note. Repeating the active key retriggers its envelope and distributed membrane force packet. ATTACK shapes both the output VCA and that physical force packet: sub-millisecond settings retain the struck membrane, while progressively slower values replace the impulse with the continuous modal drive instead of merely revealing stored impact energy later. Strike force also follows velocity without a fixed hard floor. When a non-legato note interrupts an audible release, the outgoing membrane overlaps the new one briefly while retaining its pitch, velocity, envelope, and filter response. The amplifier and Shred feedback remain one shared post circuit, so this handoff removes clicks without becoming audible polyphony.

CLAP note pressure, MIDI polyphonic aftertouch, and MIDI channel pressure increase the sustained membrane drive. MIDI RECEIVE selects OMNI or channels 1–16. Velocity controls the initial strike force and sustained modal energy rather than merely scaling the final output.

CLOCK defaults to TRANSPORT. Available periods are 4/1, 2/1, 1/1, and straight, dotted, or triplet values through 1/16, plus 1/32. A synchronized amplitude cycle follows host beat position sample by sample. Free mode uses FREE RATE in Hz. AMOUNT is normalized from no modulation to full VCA closure. POSITION defaults to POST SHRED for compatibility. Moving it to PRE SHRED is internally interpolated over 20 ms to prevent topology-switch clicks.

Application Ranges

For a pure foundation, maximize SUB, lower MODAL BODY and UPPER MODES, and keep Filter Mix and Tube conservative.
For thick sustained bass, raise Modal Body and Mass Load, use moderate Coupling, lower Damping, and add Tube after setting Membrane Drive. Keep Shred Mix low when only a trace of edge is required.
For acid-like lines, use a low static Cutoff, moderate Resonance and Membrane Drive, pronounced Glide, and transport-synchronized amplitude modulation.
For distorted drum-and-bass, jungle, or breakcore articulation, compare RAT, ZONE A/B, and SHRED, use Color to place the regenerative band, raise Feedback deliberately, and retain the dry punch with Mix. Use PRE SHRED for cyclic changes in distortion and feedback excitation, or POST SHRED for a cleaner VCA contour over the complete result.

Controls

Panel	Controls	Role
Output	Out	Final stereo level; always the first row at the top of the first column.
Modal Core	Sub / Modal Body / Mass Load / Coupling / Damping / Excitation	Directly sets the first mode, surface body, ratios, pair split, decay law, and strike radius.
Pitch	Transpose / Glide / Pitch Transient	Sets chromatic range and the two essential pitch gestures.
Envelope	Attack / Decay / Sustain / Release	Defines held-note articulation.
Tone	Upper Modes / Cutoff / Resonance / Membrane Drive / Filter Mix / Tube	Shapes modal balance, local pickup saturation, the stable parallel filter path, and coordinated amplification.
Stereo Shred	Circuit / Shred / Feedback / FB Tone Level / Color / Mix	Selects eight nonlinear transfers and independently levels smooth, bass-separated stereo regeneration after Tube while retaining an exact dry endpoint.
Amplitude LFO	Clock / Division / Free Rate / Amount / Position	Controls sinusoidal host-synchronized or free amplitude modulation and places its VCA before or after Stereo Shred.
Routing	MIDI Receive	Selects OMNI or a specific MIDI channel.

Factory Presets and State

The sixteen factory presets are authored entirely from the exposed controls: Reference, Pure Sub, Mass Loaded, Coupled Body, Dark Membrane, Long Membrane, Short Pluck, Glide Line, Pitch Impulse, Slow AM, Quarter AM, Eighth AM, Free AM, Parallel Shred, Tube Weight, and Regenerated Bass. Parallel Shred now demonstrates RAT, while Regenerated Bass uses ZONE B. MIDI Receive remains unchanged when a preset is selected. Randomization also uses only the exposed synthesis surface and keeps feedback, Color, and wet level inside ranges chosen to preserve low-frequency weight.

Saved projects retain the complete membrane, amplifier, Stereo Shred, amplitude-LFO, MIDI, and output surface. Earlier project states remain compatible: missing newer controls receive settings chosen to preserve the prior signal flow and wet balance, while older motion depth is mapped proportionally to the current amplitude-LFO Amount.

Limits

The implementation is a reduced modal synthesizer rather than a finite-element membrane solver. Its controls do not represent calibrated dimensions, materials, tension, pressure, or mass. The first mode remains phase-continuous and centered, and the continuous-energy term is a musical bass-synth adaptation rather than a literal bowed or externally driven drumhead. Filter, amplifier, transformer, and cabinet stages remain musical approximations.

The amplifier and distortion stages are original reduced-order DSP designs, not component-level models of specific products.