Re: Variable low pass filtering?

On Aug 9, 4:54 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@My-Web-
Site.com> wrote:
On Sun, 9 Aug 2009 16:28:33 -0700 (PDT), "m...@xxxxxxxxx"



<m...@xxxxxxxxx> wrote:
On Aug 9, 3:48 am, Bobby Joe <bobbyjoe23...@xxxxxxxxx> wrote:
Standard audio low pass filters tend to work by adjusting the corner
instead of the roll off or in some sense a one sided Q. So your
standard treble control is a simple RC low pass filter with R being a
pot. We have been conditioned by years of use to understand how these
filters work but they are not the only ones.

What about changing the roll off rate instead or as a combination with
the corner?  Something that shifts the corner and increases the roll
off? I suppose this is hard to realize in hardware because it requires
a way to vary between a large number of poles smoothly? At least in
the analog domain.

I wonder what such filters would look like and sound like?

You can make such a beast uses voltage controllers amplifiers (really
variable OTA). Probably googling voltage controlled filter will get
you started on the search. Curtis Electromusic made such chips, but of
course DSP took the market.
http://en.wikipedia.org/wiki/Curtis_CEM
I'm sad to hear Doug died. I used to do contract work for him.

Crikey!  So did I.  I did a VCA on one of the _smallest_ Interdesign
Monochips for him.

How old was Doug?  When I worked with them we were all young ;-)

Damn
nice guy that would do back flips for you if you treated him right.
When he fired someone, trust me, they deserved it.
http://curtiselectromusic.com/Doug_Curtis.html

I think the variable Q is what you are after:
http://curtiselectromusic.com/uploads/CEM_3385_Prelim.pdf

                                        ...Jim Thompson
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 I love to cook with wine     Sometimes I even put it in the food

Doug had a nice business doing the heavy lifting for customers wanting
Interdesign or Exar analog gate arrays, in addition to his full custom
work. [These people needed more hand holding that an Exar or
Interdesign would provide, plus it made more sense to spend a bit more
up front money to employ an outfit familiar with the gate arrays.] ]
In the old days, the company was his guest house, and you were greeted
by coyotes. Eventually he got an office by the airport.

I'm guessing he was in his 60's. Hard to tell.
.

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