For good generality, at least one of the excitation signals should be a filtered noise signal . An example implementation is shown in Fig. 9.57, in which there is a free-running bandlimited noise generator filtered by a finite impulse response (FIR) digital filter. As noted in §9.4.4, such a filtered-noise signal can synthesize the perceptual equivalent of the impulse response of many high-frequency modes that have been separated from the lower frequency modes in commuted synthesis (§8.7.1). It can also handle pluck models in which successive plucking variations are imposed by the FIR filter coefficients.
In a simple implementation, only two gains might be used, allowing simple interpolation from one filter to the next, and providing an overall amplitude control for the noise component of the excitation signal.
Literature Relevant to Brasses
More General Quasi-Periodic Synthesis