[LAU] Wah update

Carlos Sanchiavedraz csanchezgs at gmail.com
Tue Jul 21 11:56:42 EDT 2009

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 at restivo.org>:
> 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
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