[PD] Fwd: what fdn~ does?

Julian Brooks jbeezez at gmail.com
Thu Jan 31 17:18:08 CET 2019

```Nice sleuthing Holmes

On Tue, 29 Jan 2019 at 01:43, Alexandre Torres Porres <porres at gmail.com>
wrote:

> 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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