[LAU] Wah update

Ken Restivo ken at restivo.org
Mon Jul 20 01:24:50 EDT 2009

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. 


More information about the Linux-audio-user mailing list