Hallo, Geoff hat gesagt: // Geoff wrote:
The DSP book I have read gives a simple lowpass filter function as
g(n) = (f(n-1) + f(n) + f(n+1))/3
i.e the average value of three consecutive samples. I understand how
this is in effect a lowpass filter.How do I implement that in PD?
A lot of that is explained in Miller's book: http://crca.ucsd.edu/~msp/techniques/latest/book-html/node127.html You better read the previous chapter on Delays first, it contains important info as well.
Generally the basic objects for filter design are [rzero~], [rpole~] and [czero~], [cpole~] and another one is [biquad~].
With the zero/pole objects you directly give the location of poles and zeroes of your filter while the biquad~ is an alternative way to express two zero and two pole objects in one.
I don't know the meaning of the variables in your formula, especially the f(n+1) looks strange: Is this a value of the future or an output value, like the "y" variables in many other filter equations? Anyway you can sum up the current and the previous samples easily with rzero~:
[sig~ 1] | [rzero~ -1] | [snapshot~] | [2 \
For more complex filters check out Miller's book and the examples included in Pd.
I recently also wrote a lot of filter abstractions for the RjDj library: see http://trac.rjdj.me/browser/trunk/rjlib/rj The interesting objects there are e_beequad.pd as a biquad~ clone and these for calculating various coefficients: u_lowpass, u_lowpassq, u_highpass, u_highpassq, ...
e_lop4.pd also is interesting: It's a 4 pole resonant lowpass filter, good for moogish synths, when combined with the bandlimited oscillators of s_osc.pd there. All these filtes use zero/pole objects inside.
RjDj abstractions are for Pd vanilla, so they run everywhere.
Ciao
Frank Barknecht