Practical Considerations for Multichannel Equalization

Abstract

Given a multichannel loudspeaker system, the selection of the crossover frequency between the subwoofer and the satellite speakers is important for accurate (i.e., distortion-free), reproduction of playback sound. Presently, many home theater systems have selectable crossover frequencies, which are part of the bass management filter capabilities, which are set by the consumer through listening tests. Alternatively, if the loudspeakers are industry certified, the crossover frequency is set at 80 Hz. Adesirable feature is that, besides distortion-free sound output from the individual subwoofer and the satellite speakers, the combined subwoofer and satellite room acoustical response should exhibit negligible variations around the selected crossover frequency. In this chapter, we present an automatic crossover frequency selection algorithm based on an objective measure (viz., the spectral deviation measure) for multichannel home theater applications that allows better control of the combined subwoofer and satellite response, thereby significantly improving audio quality. Initially, some results are presented that show the effect of crossover frequency on low frequency performance. Additional parameter optimization of the bass management filters is shown to yield improved performance. Comparison between the results from crossover and all-parameter optimization, of the bass management filters, for multiposition equalization is presented. As also shown, cascading an all-pass filter, or adding in time-delays in loudspeaker channels, can provide further improvements to the equalization result in the crossover region. Alternative techniques for fixing the crossover blend, using a cascade of all-pass filters, are also presented.