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The theory of wave digital filters, which are well suited to model analog networks including those containing transmission lines, is extended to the time-varying case. This allows simulation of the moving vocal tract or parametric amplifiers. A representation consistent with the physics of time-varying reactances and transmission lines requires not only time-varying filter coefficients, but also structural modifications in the n-port adaptors and reactance one-ports of the filter. The number of coefficients in the adaptors is increased, and stability and pseudopassivity are not assured.