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