Digital audio precompensation

#The present invention generally concerns digital audio precompensation, and more particularly the design of a digital precompensation filter that generates one or several input signals to a sound generating system, with the aim of modifying the dynamic response of the compensated system.

#A system for generating or reproducing sound, including amplifiers, cables and loudspeakers, will always affect the spectral properties of the sound, often in unwanted ways. The reverberation of the room where the equipment is placed adds further modifications. Sound reproduction with very high quality can be attained by using matched sets of cables, amplifiers and loudspeakers of the highest quality, but this is cumbersome and very expensive. The increasing computational power of PCs and digital signal processors has introduced new possibilities for modifying the characteristics of a sound generating or sound reproducing system. The dynamic properties of the sound generating system may be measured and modeled by recording its response to known test signals, as well known from the literature. A precompensation filter, R in FIG. 1, is then placed between the original sound source and the audio equipment. The filter is calculated and implemented to compensate for the measured properties of the sound generating system, symbolized by H in FIG. 1. In particular, it is desirable that the phase and amplitude response of the compensated system is close to a prespecified ideal response, symbolized by D in FIG. 1. In other words, it is thus required that the compensated sound reproduction y(t) matches the ideal yref(t) to some given degree of accuracy. The pre-distortion generated by the precompensator R cancels the distortion due to the system H, such that the resulting sound reproduction has the sound characteristic of D. Up to the physical limits of the system, it is thus, at least in theory, possible to attain a superior sound quality, without the high cost of using extreme high-end audio equipment. The aim of the design could, for example, be to cancel acoustic resonances caused by imperfectly built loudspeaker cabinets. Another application could be to minimize low-frequency resonances due to the room acoustics, in different places of the listening room.

#Digital precompensation filters can be applied not only to a single loudspeaker but also to multichannel sound generating systems. They can be important elements of designs aimed not only to generate better sound, but also to produce specific effects. The generation of virtual sound sources, rendering of sound, is of interest in, for example, the audio effects of computer games.

#The invention, together with further objects and advantages thereof, will be best understood by reference to the following description taken together with the accompanying drawings, in which:

#FIG. 1 is a general description of a compensated sound generating system;

#FIG. 2A is a graph illustrating the amplitude response of an uncompensated loudspeaker model;

#Sections 1-3 describe linear cases, section 4 generalizes the structure and design principle to problems with non-linear and possibly time-varying system models as well as non-linear and possibly time-varying compensators, and section 5 finally describes some implementational aspects.

#1. Design for Linear Models and Filters

#For a better understanding of the invention, it may be useful to begin by describing the general approach for designing audio precompensation filters.