Page "Karplus–Strong string synthesis" Paragraph 6

# The output of the delay line is fed through a filter.

The gain of the filter must be less than 1 at all frequencies, to maintain a stable positive feedback loop.

The filter is usually a first-order lowpass filter ( as pictured ).

In the original algorithm, the filter consisted of averaging two adjacent samples, a particularly cheap filter that can be done without a multiplier ( just shift and add operations ).