Converting Series Biquad Filters into Delayed Parallel Form: Application to Graphic Equalizers

Digital filter transfer functions can be converted between the direct form and parallel connections of elementary sections, typically second-order ('biquad') sections. The conversion from direct to parallel form is performed using a partial fraction expansion, which usually requires long division of polynomials when expanding proper and improper transfer functions. This paper focuses on the conversion of a series of biquad sections to the parallel form, and proposes a novel way to implement the partial-fraction expansion without the use of long division. Additionally, the resulting structure is the delayed parallel form in which the section gains remain small. The new design and previous methods are compared in a case study on graphic equalizer design. The delayed parallel filter is shown to use the same number of operations as the series form during filtering. The conversion of a recently proposed series graphic equalizer into the delayed parallel form leads to an improved parallel graphic equalizer design relative to all known prior approaches. The proposed conversion technique is widely applicable to the design of parallel infinite impulse response filters, which are becoming popular as they are well suited to implementation using parallel computers.