Cascaded to parallel conversion for IIR filters

Started by August 15, 2006
Hi All,
I am new to this group, I am working on Graphic Equalizer for octave 1 with biquad IIR filters. I had implemented this one in cascade form and now trying to implement same in parallel form. First of all I calculate the single equivalent Transfer Funtion from several cascaded biquad filter and then using residuez of matlab (which is based on partial fraction expansion) try to realize the filter in parallel, but found the different frequency response from cascade and parallel. One more thing, the frequency response will be same when I skiped some initial center frequencies (like upto 250 Hz) and then apply the same thing and it does work well. I didn't understand why the filters with all center frequencies(with lower frequency) not give the same response as cascaded structure. Does it the problem with frequency resolution ? Also is there any other method to solve this problem as i want to implement the same thing for real-time ?