On 02-Nov-18 03:06, Andrew Simper wrote:
If you prize symmetry then you can use a cascade with 2 x one pole HP and 2 x one pole LP to make a 4 pole BP (band pass) then you can use the same old FIR based output tap mixing to generate all the different responses. It may not be so easy to do in a real circuit, but in software we're not bound by what is easy to build :)


Symmetry is one of the things. The other is the shape of the amplitude response. I'm personally not convinced by the -4dB dip prior to the resonance, although YMMV. At any rate it doesn't qualify as a "bread and butter" LP IMHO ;) With BP8 it's getting way worse.

Incidentally, another way to come at more or less the same structure is raising the orders of LP and HP filters (by stacking identical 1-poles in series) in the transposed Sallen-Key (Fig.5.23 of the book). Since TSK is essentially a bandpass ladder with a special output mode, it's actually the same. Further ways (originating at lowpass ladder) to look at this can be found here:
and here


Vadim Zavalishin
Reaktor Application Architect
Native Instruments GmbH

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