I could be wrong, but I don't think it's quite the same thing.  I believe the signal would be out of phase negating many of the effects of the filter.  I would recommend using [biquad~] and in pd-extended there is a [notch] object which takes care of the coefficients.  This sounds much cleaner and more notch-like to my ear than subtracting the filtered output.  
 There is an explanation in Miller's book if you like unit circle math: http://msp.ucsd.edu/techniques/latest/book-html/node144.html
-----------
Message: 4
Date: Tue, 22 Apr 2014 01:59:07 -0300
From: Alexandre Torres Porres <porres@gmail.com>
Subject: Re: [PD] WG: Inverse bandpass filter
To: Ingo <ingo@miamiwave.com>
Cc: pd-list <pd-list@iem.at>
Message-ID:
<CAEAsFmhD0HanLmv9vutcSQZjkZY69i7wMeBqQ+20S2riWYaakw@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

isn't it just subtract the audio from the filtered output?

I guess you can get inverse freq response just by that

cheers


2014-04-18 17:21 GMT-03:00 Ingo <ingo@miamiwave.com>:

You could send the original signal in parallel and invert the phase by
multiplying with -1. You might have to delay the original signal in case
that the processed signal gets also delayed by one or more blocks.

Ingo

_______________________________________
> Von: pd-list-bounces@iem.at [mailto:pd-list-bounces@iem.at] Im Auftrag
von
> AP Vague
> Gesendet: Freitag, 18. April 2014 18:49
> An: pd-list@iem.at
> Betreff: [PD] Inverse bandpass filter
>
> Is there a simple way to make [bp~] or [vcf~] have an inverse function?
To
> filter out, rather than pass a changing frequency value. Is the easiest
> way to do this with a combination of [lop~] and [hip~]?


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