[PD] [lop~] coefficient calculation
Joe White
white.joe4 at gmail.com
Fri May 16 13:58:31 CEST 2014
Hi,
I've been looking at the [lop~] implementation (Pd-0.45-4) and noticed
something that seem weird to me.
In d_filter, line 176:
static void siglop_ft1(t_siglop *x, t_floatarg f)
{
if (f < 0) f = 0;
x->x_hz = f;
x->x_ctl->c_coef = f * (2 * 3.14159) / x->x_sr;
if (x->x_ctl->c_coef > 1)
x->x_ctl->c_coef = 1;
else if (x->x_ctl->c_coef < 0)
x->x_ctl->c_coef = 0;
}
Is it correct that for:
y[n] = x[n] * a + y[n-1] * b
*a = 2π * Fc / Fs*
b = 1.0 - a
where Fc is the cut-off frequency and Fs the sampling frequency.
I appreciate the a coefficient is bounded afterwards but wouldn't that mean
that Fc values greater than Fs / 2π will have no impact on the sound being
processed.
For example if Fs is 44100, then Fc values above ~7020Hz will not affect
the filter.
Have I missed something crucial or could this a bug in the code?
The simple IIR filter described in
http://en.wikipedia.org/wiki/Low-pass_filter suggests that the actual
coefficient calculation should be more like:
a = 2π*Fc / (2π*Fc + Fs)
Looking forward to understand this more!
Cheers,
Joe
--
Follow me on Twitter @diplojocus
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