I was working on some patches with bandpass filters that were initially [vcf~], but I ended up deciding not to vary the center frequency, so I replaced them with [bp~]. Now I find that [bp~] sounds different; e.g., a [phasor~ 100] going through a [bp~ 440 3] sounds different from one going out the left outlet of a [vcf~ 3] with [sig~ 440] going into its center inlet. What's the deal? How do I replace a non-varying [vcf~] with a [bp~] that sounds the same?

interesting question.

if someone here doesn't know the answer, you may get more help on this from the pd mailing list.

• it seems to me that [bp~] is the normal band pass eq (parametric or semi-parametric if you set center frequency as creation argument), while the [vcf~] seems to be born for a flanger comb-filter (it is specified you could change it's value in time).
• another possible difference is that [vcf~] have to be used with two audio signals, I think you can also use an audio stream in order to link the eq of the main signal with the "formant" one (hope it's clear enough) while [bp~] is limited to floats in center frequency setting (ie it's not possible use audio to control the eq behaviour).

in conclusion [vcf~] seems to be more expensive than [bp~] by the programmers' admission but more reliable to create flanging effects with moving combs.
this is what i get from help files, if someone has some better explanations pliz share