Karplus-Strong physical modeling feeds an excitation signal into a tuned feedback delay to synthesize plucked or bowed strings

Surge XT’s String oscillator implements the Karplus-Strong algorithm: a short delay line whose length sets the pitch frequency, with feedback that sustains the tone. An excitation signal (noise, pink noise, ramp, or external audio) is loaded into the delay line at note-on to start vibration. Burst modes (plucked strings) load the delay once; Constant modes (bowed strings) continuously apply the exciter during note hold, modulated with Exciter Level as a ‘bow pressure’ analog. Two strings run in parallel with independent Decay (feedback amount) and Detune parameters. A Stiffness parameter adds low-pass or high-pass filtering in the feedback loop, creating inharmonic timbres at extremes.

Examples

Use Burst/Noise exciter for a plucked guitar. Use Constant/Noise with slow Exciter Level modulation for a bowed cello-like tone that varies in bow pressure.

Assessment

In Karplus-Strong, what determines the pitch of the resulting tone? What is the sonic difference between setting String 1 Decay to -100% vs. +100%? Why does the Stiffness control create inharmonicity?