What it does
4-band (octave) equaliser implemented using polyphase IIR all-pass second-order up and down samplers plus extra notch filters for additional control.
This is an elaboration of "MultirateEqualiserBuiltFromScratch.dwb" whereby additional notch filters are included in the sub-bands to provide even more control of the equalisation curves.
This is a good example of multi-rate signal processing whereby extremely efficient filtering can be achieved by successively downsampling the signal such that each "band" is running at a lower sample rate. In this example, each band is an octave lower than the previous (due to the factor of 2 downsampling). The downsamplers are implemented using 2-channel recursive filter banks (consisting of complementary hi - lo allpass filters implemented in polyphase form). This is the most efficient method of implementing the filter banks. (However, since these IIR filters do not exhibit linear-phase characteristics they may be unacceptable in some applications.) This DrawingBoard clearly illustrates the powerful multi-rate functionality of WaveWarp, a key feature of the software.