Decay Stretching

To stretch the decay, and reduce the lowpass effect at high frequencies,the simple lowpass can be replaced with a two-point weigthed average

$$y(n) = (1-S)x(n) + Sx(n-1),$$

where $S$, the stretching factor, is between 0 and 1.

For stability, $S$ can't be greater than 1.

When $S = 1/2$, the filter reduces the the previous two-point averager.

When $S$ = 0 or 1, the frequency-dependent term (delay) disappears, and the gain response is unity for all $f$.

At intermediate values, $0 < S < 1$, the note duration is finite, with a minimum for $S = 1/2$.

The resulting decay time is then a function of loss factor $\rho$ and stretch factor $S$.