Time-varying wave digital filters and vocal-tract models

For the digital simulation of time-varying analog systems (especially such containing transmission lines) by wave digital filters it is insufficient to use time-varying coefficients in a filter structure simulating a time-invariant system. Rather, the structure itself has to be modified. The modifications in the basic building blocks of such a filter, namely, n-port adaptors and reactance one-ports, are derived from the physics of time-varying inductances, capacitances and transmission lines. The number of coefficients in the adaptors is increased, and pseudopassivity is not preserved. An application is the simulation of the human vocal tract as a time-varying transmission-line system with terminating and shunt impedances. First experiments concerning the difference between exact and quasi-time-invariant treatments are reported.