[sdiy] moog high pass flter

[Ian Fritz]

Actually, I've been thinking lately that HPFs are underappreciated and 
underutilized.

Many acoustic instrument sounds have frequency spectra whose fundamentals 
are weaker than several of the higher harmonics.

The reason for this is that the low frequencies need to be well trapped 
inside the acoustic waveguide structure in order for the non-linear engine 
that drives the sound production to operate efficiently.  What we hear is 
more like the exhaust!

A pretty example of this is shown in the recent book on mathematical 
modeling of acoustic instruments edited by Hirschberg, Kergomard and 
Weinreich (Mechanics of Musical Instruments, Springer-Verlag, 
1995).  Figure 1 of the Introduction  shows internal and external waveforms 
of a clarinet.  The internal wave is a rounded square wave, whereas the 
external waveform is extremely spikey, obviously dominated by high  harmonics.

As an other example, look at the simple digital waveguide models developed, 
for example, by  J. O. Smith at Stanford.  The wave reflected from the end 
of the waveguide (back to the excitation source) is LP filtered, whereas 
the transmitted (output) wave is HP filtered.

Of course, acoustic instruments have HF cutoffs, also, so LP filtering is 
indeed a natural part of music.  But HP filtering is every bit as important.

It is impossible to obtain spectra with higher harmonics dominating the 
fundamental by using standard synth waveforms plus LP filtering. So unless 
you have a cool waveshaper like the Wavolver, you will definitely need to 
include HP filtering in your acoustic arsenal.

As an interesting sidenote, the research done by H. Benade showed that the 
HF cutoff frequency of woodwinds is independent of pitch (in the lowest 
register).  An amazing result!  He and a student actually designed and 
built an isospectral clarinet (cutoff following pitch) and found that it 
was too dark and muffeled at low frequencies and too bright at high 
frequencies.  So non-tracking LP filtering is also an interesting synth 
programming option.

At 12:20 PM 12/8/2002, harrybissell wrote:

>and Hipass filters suck. oops did I say that outloud. Thats bad.. thats
>flamebait ;^P
>maybe blasphemy...

Ignorance is what sucks, IMO.

Best regards,

   Ian