On Tue, Jul 21, 2009 at 05:56:42PM +0200, Carlos Sanchiavedraz wrote: > 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. > > I received a suggestion off-list to try the Guitarix plugins, which I did, and got very good results from it: http://restivo.org/projects/wah/high-jimi.png http://restivo.org/projects/wah/guitarixcry-high.png http://restivo.org/projects/wah/lowend-jimi.png http://restivo.org/projects/wah/guitarixcry-low.png http://restivo.org/projects/wah/midrange-jimi.png http://restivo.org/projects/wah/guitarixcry-mid.png Part of the difference in the plots is that I wasn't using the correct JAPA settings for the plots of the actual "Jimi Hendrix Thomas" wah. The frequencies are different, but that might just be because the Guitarix plugin sweeps higher and lower than the Jimi pedal. It sounds great. Now I'm either going to hack together an auto-wah via AMS or something and an envelope follower, or build myself an Arduino pedal footcontroller. Funky and fun. -ken _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
