For a reduction of nonlinear distortion produced by a dynamic loudspeaker, there exists a variety of approaches, one of them using the known principle of exact input-output linearization. In combination with a discrete state-space observer, this approach can successfully be applied as long as the amplifier driving the loudspeaker is dc-coupled. This paper discusses how ac amplifiers can be used in this context. It will be shown that an exact linearization is in general not possible with ac amplifiers due to insufficient stability of the feedback system. Two approaches will be presented that abandon the pursuit of an exact linearization in favor of stability and yield satisfactory results when implemented on a floating-point digital signal processor.

In this paper, an electrical circuit is proposed which is suitable for a parametric modeling of electrodynamic transducers, e.g., dynamic loudspeakers. Since the system modeled is known to be passive, the preservation of this property is a key issue of the approach presented here. This is of special importance when nonlinear effects are taken into account, as passivity always ensures different kinds of stability. The only nonlinear elements occurring in the circuit model are ideal transformers, the turns ratios of which are not constant but depend, in particular, on some signal quantity. As a consequence, the circuit can be digitally simulated by applying principles known from the theory of wave digital filters.

A wave digital filter (WDF) two-port has two input and two output terminals. By using these to appropriately connect a WDF to its transpose, the filtering effect is fully compensated except for an all-pass phase, and this independently of any selectivity requirement. This remains true if at the points of interconnection every second sample is reduced to zero, provided that a certain condition, easily to be satisfied exactly, is fulfilled. Very efficient solutions are then possible. The method is of interest e.g. for subband coding.