[sdiy] What would be THE best, most versatile VCF available on DIY community ?

[Andrew Simper]

On 29 May 2012 09:34, Tom Wiltshire <tom at electricdruid.net> wrote:
>
>
> On 28 May 2012, at 18:19, Andrew Simper wrote:
>
> > It can be made from any ota core and provides "stronger" high pass and
> > band pass responses than the Xpander style summing of output taps.
>
> What do you mean by "stronger" here, Andrew?
>
> Is the Synthex filter stronger because the Xpander method requires an accurate addition to sum the LP and straight signal to get HP?
>
> Aren't the transfer functions the same otherwise? (for at least the modes in question; 12dB HP and 12dB BP)
>
> Thanks,
> Tom
>

The transfer functions with resonance are different when summing the
taps of a 4 x one pole low pass cascade and summing those with
different weights than, especially for the four pole high pass, than
for having each section an individual low pass / high pass in the
cascade to form the filter. The resonance isn't as pronounced and also
when the filter is pushed into the non-linear region the high pass
achieved from summing taps sounds a bit thinner.

Here are some plots to show the difference in the linear case, the
resonance isn't above the passband most of the time so it doesn't
sounds as strong:

www.cytomic.com/files/dsp/cascade-hp4-vs-lp4hpxtaps.pdf

You can get some cool dual notch shapes by using the summing method, I
did this for a virtual synth product called Synth Squad, and then MOTU
ripped off the modes I came up with and even the names I used for
their product!

Andy