[LAU] Wah update

Justin Smith noisesmith at gmail.com
Mon Jul 20 01:37:10 EDT 2009

On Sun, Jul 19, 2009 at 10:33 PM, Justin Smith<noisesmith at gmail.com> wrote:
> On Sun, Jul 19, 2009 at 10:24 PM, Ken Restivo<ken at restivo.org> wrote:
>> On Sun, Jul 19, 2009 at 09:32:21PM -0700, Justin Smith wrote:
>>> On Sun, Jul 19, 2009 at 8:10 PM, Ken Restivo<ken at restivo.org> 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.
>>> >
>>> Cool. Nice to see some good open source DSP design in process. Is your
>>> goal to make something like a wah with a combination of LADSPA plugins
>>> or would you also consider making a new plugin emulating this
>>> behavior? It may be worth considering trying to build something in
>>> puredata or csound, given that they have much more fine grained
>>> control for building customized processing chains than could be done
>>> in a single instance of jack-rack.
>> I'm either going to find a LADSPA plugin which does this, or hack something together in JACK-RACK or via some custom C using existing plugins.
>> I haven't the maths skills to write any serious DSP in C, and probably not in PD or Csound either.  Whatever I end up with will have to be in C, however, and efficient too, so it can run on a netbook along with many other synths and plugins.
>>> I think the best way to mimic the wah behavior will probably be to
>>> manipulate the Q and center frequency based on a pair of table
>>> lookups, or additionally/alternatively you could have a pair of
>>> filters and crossfade betwengn them based on your virtual pedal
>>> position.
>> Tonight I tried every LADSPA bandpass filter I could find, and none came even close AFAICT. Comparing its shape to those of other LADSPA plugins indicates to me that the "Thomas Organ" wah circuit isn't a band-pass after all, just a high-pass with a VERY high Q. So, yeah, I might be able to get pretty close with one ofthe existing LADSPA HPF's and varying the Q, as you suggested.
>> -ken
> Just to clarify, the pd or csound solution would only have to have
> premade filters in it, plus some parameter routing, the issue with
> jack-rack is that you cannot tie parameters to one another in
> jack-rack itself (and as far as I know you cannot even have parallel
> signal paths), and any external solution you hack together is going to
> be less efficient and more fragile than pd, csound, or the like. When
> you say whatever you make would have to be written in c I hope you
> understand that pd and csound are written and c and will have less
> overhead than jack-rack plus whatever midi preprocessing and shell
> scripting you need for the alternative approach (though I will not
> argue the fact that coding the whole thing in c would be even more
> efficient, but also more time consuming and error prone than a
> dedicated DSP language).
> Suddenly it occurs to me that maybe your goal is to get me so
> opinionated about the right way to do this that I just do it for you
> myself :)

Never mind, I replied too soon, reading what you wrote more carefully,
I think what I said was mostly moot.

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