# [PD] Fwd: what fdn~ does?

Alexandre Torres Porres porres at gmail.com
Tue Jan 29 02:41:24 CET 2019

```I guess "bingo"?
https://ccrma.stanford.edu/~jos/pasp/First_Order_Delay_Filter_Design.html

Em seg, 28 de jan de 2019 às 22:54, Alexandre Torres Porres <
porres at gmail.com> escreveu:

>
> Em seg, 28 de jan de 2019 às 18:45, Peter P. <peterparker at fastmail.com>
> escreveu:
>
>> * Alexandre Torres Porres <porres at gmail.com> [2019-01-28 20:10]:
>> > I guess I'm figuring something out, but the secondary inlets are still
>> not
>> > clear to me, they set the decay time, but why is there a "low" and
>> "high"?
>> Could it be that decay times are different for high and low frequencies
>> and that these two are divided by a cutoff frequency? Just guessing....
>>
>
> Sort of... I finally made some sense of the code. it seems there's a 1pole
> filter whose parameters depend on these low/high values and the delay
> length! In the comment of the code we find a filter equation, something
> like:* "yn = (2*gl*gh ) / (gl+gh) x + (gl-gh) / (gl+gh) y[n-1]" *- where
> *gl* & *gh* are derived from these low and high values. Here's a code
> simplification of it
>
>
>
>     *for*(i = 0; i < x->x_ctl.c_order; i++){
>
>         gl = pow(10, -0.003 * x->x_ctl.c_length[i] / low);
>
>         gh = pow(10, -0.003 * x->x_ctl.c_length[i] / high);
>
>         x->x_ctl.c_gain_in[i] = 2*gl*gh / (gl+gh);
>
>         x->x_ctl.c_gain_state[i] = (gl-gh) / (gl+gh);
>
>
>
> I don't really get how this filter fully works yet, but I can sort of get
> the gist of it. I'm now in the quest to find what is the source of this
> filter, and maybe try it out independently to see how it behaves. But
> perhaps a more sophisticated method, with a settable crossover frequency
>
>
> And yeah, it seems vanilla's [rev2~] and [rev3~] are implementations of
> feedback delay networks like it's been said here on this thread.
>
>
> cheers
>
>
>
>
>
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