Karplus-Strong (“plucked string”) generator/filter and initialization
hpluck() is an implementation of the Karplus-Strong plucked string algorithm described in the Computer Music Journal, vol. 7 no. 3, pp. 43-55. Basically it fills a table array with random numbers on initialization and applies a low-pass filter to the table during performance. The signal thus begins with a burst of noise and dies away to a sine wave. This sounds remarkably liked a plucked string. This unit must first be initialized with a call to hplset(). which (groan) is still in Fortran, so the name must be followed by the underscore and all arguments must be passed by address. The value xlp is the loop time (1/hz), dur the expected duration of the note, dynam, specified in hz is a brightness factor, plamp is the overall amplitude of the result, seed, is a random seed value for the initial table, sr is the sampling rate and array which must be dimensioned at least at (9+xlp*sr). The arguments for hpluck() itself are the loaded array and an arbitrary input signal, which will be effected with a comb-like output. The relation between the input signal and the pluck can be manipulated by tinkering with the amplitude of the input signal and plamp. If plamp is set to 0, the whole effect of the pluck will be on the input signal, and if the input signal is 0, the result will be normal plucked string synthesis. We find that since the plucked signal dies away to dcbias that it is usually necessary to add an envelope to avoid a thump at the end of the note. The strength of the initial pluck can then be tinkered with by manipulating the dur argument, which calculates coefficients to bring the signal to dcbias in that amount of time.
The source code for the STRUM instrument contains much better plucked-string algorithms. This ugen is pretty ancient (note the reference to Fortran above).