[sdiy] Vocoder dumb idea of the night

[jhaible at debitel.net]

> On Wed, May 14, 2003 at 10:04:12AM +0200, Czech Martin wrote:
> > A 2N order bandpass has N*6dB fallof for higher, lower frequencies.
> > 2nd order means 6dB, 4th order means 12dB. Arround the peak it can be more
> > due to high Q.
> 
> I dont know why I always get Order and Pole confused. *sigh*

I also think Martin's definition is right. (a SVF for instance doesn't
change its order if I go from the 12dB LPF output to the 6dB BPF
output !) But nevermind: One of my filter design programs also
uses a different definition (where a 12dB BPF is called "2nd order")

Maybe there are two competing definitions.
The SVF example supports the "2nd order BPF = 6dB/Oct slope" point
of view. But there is also a design method for BPFs and HPFs that
starts with a LPF and then does a LPF->BPF or LPF->HPF transformation.
While in the LPF->HPF case the order and slope is preserved, I
_think_ (please double check!) in the LPF->BPF transformation
the slope is preserved and the order would then be doubled.
And here I can see a reasoning that you would define "order"
in an alternative way, where LPF->BPF transformation also preserves
the filter order.
I don't know if there is a definition / argument for that second
case. But while I could understand the reasoning behind it, I feel
more comfortable with the definition that just counts filter poles.

JH.

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