Figure 9.34 shows a complete piano synthesis system along the lines discussed. At a fixed dynamic level, we have the critical feature that the model is linear and time invariant. Therefore we may commute the ``Soundboard & Enclosure'' filter with not only the string, but with the hammer filter-bank as well. The result is shown in Fig.9.35.
As in the case of commuted guitar synthesis (§8.7), we replace a high-order digital filter (at least thousands of poles and zeros ) with a simple excitation table containing that filter's impulse response. Thus, the large digital filter required to implement the soundboard, piano enclosure, and surrounding acoustic space, has been eliminated. At the same time, we still have explicit models for the hammer and string, so physical variations can be implemented, such as harder or softer hammers, more or less string stiffness, and so on.
Even some resonator modifications remain possible, however, such as changing the virtual mic positions (§2.2.7). However, if we now want to ``open the top'' of our virtual piano, we have to measure the impulse response that case separately and have a separate table for it, which is not a problem, but it means we're doing ``sampling'' instead of ``modeling'' of the various resonators.
An approximation made in the commuted technique is that detailed reflections generated on the string during hammer-string contact are neglected; that is, the hammer force-pulse depends only on the hammer-string velocity at the time of their initial contact, and the string velocity is treated as remaining constant throughout the contact duration.
Multiple Force-Pulse Synthesis