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@xxxxxxxxxxx> 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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
