Karplus-Strong string synthesis

Karplus-Strong string synthesis refers to a method of physical modelling synthesis that loops an impulse through a filtered delay line to simulate the sound of a hammered or plucked string or some types of percussion. This is a subtractive synthesis technique based on feedback.

How it works

1. Feed an impulse into a delay line.
2. Filter the output of the delay line. The gain of the filter must be less than 1 at all frequencies.
3. Send the filtered output to the output device and back to the delay line, adding it to any incoming impulses.

Tuning the string

The period of the resulting signal is the period of the delay line plus the average group delay of the filter; the frequency, as usual, is the reciprocal of the period. Often, in a digital system, delay lines are available only in whole-sample periods. To fix this, create the filter with a time-varying group delay. Also, if the group delay is too nonlinear, harmonic frequencies may be sharpened or flattened relative to the fundamental frequency.

Holding the period constant produces vibrations similar to those of a string or bell; Increasing the period sharply after an impulse will produce a drum-like sound.