> Really good info; in fact some time ago I tried to figure out how to
> analize and reproduce in the digital realm a sound or fx.
>
> But, as Ken, my skills in DSP and programming aren't that good to try
> to help in such a deep way. I think I'd be more usefull in analysing
> by ear or to contribute with some ideas.
>
> However, what I could do is to provide some more data from a Wah pedal
> (borrowed), along with Julien's. In fact, I was considering to buy one
> some time ago, but I didn't decided nor what kind (Wah, Cry baby...)
> Maybe this is a sign ;)
>
> 2009/7/21, Ken Restivo :
> > On Mon, Jul 20, 2009 at 11:51:07AM +0200, Fons Adriaensen wrote:
> >> On Sun, Jul 19, 2009 at 08:10:47PM -0700, Ken Restivo wrote:
> >>
> >> > Just a quick update on the wah research.
> >> >
> >> > A friend owns a Dunlop "Jimi Hendrix Wah", which says it is the
> >> > "Original Thomas Design", by which I assume they mean to claim it's the
> >> > same design as the Thomas Organ Wah, formerly Vox.
> >> >
> >> > This website's describes the frequency response as a lowpass with a
> >> > resonant peak:
> >> >
http://www.geofex.com/Article_Folders/wahpedl/wahped.htm
> >> >
> >> > So here is what JAPA says it does (and I believe JAPA more than some
> >> > random website):
> >> >
> >> > When fully closed, it's a bandpass, with a VERY high Q!
> >> >
http://restivo.org/misc/lowend-jimi.png
> >> >
> >> > But, wait, when I open it up, suddenly it becomes more like a highpass,
> >> > but with a lot of resonance:
> >> >
http://restivo.org/misc/midrange-jimi.png
> >> >
> >> > When it's fully opened, it's definitely a highpass, but with a helluva
> >> > peak:
> >> >
http://restivo.org/misc/high-jimi.png
> >> >
> >> > So, not only is the opposite of what that article says, but it's also
> >> > kind of non-linear. I'll poke around the various LADSPA plugins and see
> >> > if I can find something nearly like this.
> >> >
> >> > Another guitar-player friend has a different wah (IIRC, either a "Cry
> >> > Baby", or a Morley), and I'll see if I can run his through this and see
> >> > what it comes up looking like.
> >>
> >>
> >> AFAICS this is a resonant (which is not the same as bandpass) filter.
> >> If the response near Fs/2 bcomes flat, that does not mean it is a
> >> highpass.
> >>
> >> Remember that any digital filter is 'mirrored' to the other side
> >> of Fs/2. Also the magnitude of the response must be continuous or
> >> zero at all points (for finite order).
> >>
> >> The result of all this is that at Fs/2 the response must be either
> >> zero or have a zero derivative, i.e. be horizontal.
> >>
> >> In a high order filter you can make the 'roundoff' region near
> >> Fs/2 very small, but it's always there, unless the response is
> >> zero at that frequency.
> >>
> >> You can probably get this type of response using the MOOG VCF
> >> by taking the output at a different point in the algorithm.
> >>
> >> The MOOG VCF is 4th order, this is overkill as the analog
> >> circuit is very likely to be just 2nd order.
> >>
> >
> > Thanks. Alas, that seems like a very concise explanation, but I don't have
> > the mathematical background to implement that.
> >
> > If someone feels like modifying the Moog VCF to make it a Vox/Thomas Wah,
> > I'd be eternally grateful. But it's pretty clear I don't have the skills to
> > take this over the finish line.
> >
