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'[EE]: Anyone knows how to build a VCF?'
2000\11\22@100154 by Edson Brusque

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Hello,

   I need to build a Low-Pass VCF (Voltage Controlled Filter) for a
project, but can't find any squematics about how it's done.

   I want to use a PWM output on a PIC to generate 0-5V, and with this
voltage, I want to be able to make the filter cut selected high frequencies
in an audio signal, say with 0V, it will cut at 100 Hz, with 5V, at 5kHz.

   Can anyone help me with this? Or point me some directions?

   Thank you very much,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\22@101433 by D Lloyd

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Hi,

Can you not use one of the switched capacitor Maxim parts (Max 291?) where
you throw in a clock frequency 100x the cut off required and it does the
rest? This, arguably, would be preferable to generating a PWM; you just
provide, say, a 100kHz clock for a 1kHz cut off.

I think the Max 291 is an 8th order device - you will need a smoothing
filter on the back end of it to get rid of some of the noise, probably.

Dan





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Hello,

   I need to build a Low-Pass VCF (Voltage Controlled Filter) for a
project, but can't find any squematics about how it's done.

   I want to use a PWM output on a PIC to generate 0-5V, and with this
voltage, I want to be able to make the filter cut selected high frequencies
in an audio signal, say with 0V, it will cut at 100 Hz, with 5V, at 5kHz.

   Can anyone help me with this? Or point me some directions?

   Thank you very much,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\22@114749 by Andy Jancura

picon face
>
>Hello,
>
>     I need to build a Low-Pass VCF (Voltage Controlled Filter) for a
>project, but can't find any squematics about how it's done.
>
>     I want to use a PWM output on a PIC to generate 0-5V, and with this
>voltage, I want to be able to make the filter cut selected high frequencies
>in an audio signal, say with 0V, it will cut at 100 Hz, with 5V, at 5kHz.
>
>     Can anyone help me with this? Or point me some directions?
>
>     Thank you very much,
>
>     Brusque
>

Hi Brusque,

don't know, but may be usefull too. Look at MChip digital pots and app.
notes describing programable low pass filter.

Andrej

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2000\11\22@125633 by Edson Brusque

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Hello Dan,

> Can you not use one of the switched capacitor Maxim parts (Max 291?) where
> you throw in a clock frequency 100x the cut off required and it does the
> rest? This, arguably, would be preferable to generating a PWM; you just
> provide, say, a 100kHz clock for a 1kHz cut off.

   This idea is really good. I've seen Maxim site about these filters and I
think with a F873@20MHz PWM output I think I can make tune it from about
12Hz to 2kHz. That's not the perfect frequency spectra, but very usefull for
my application.

   The sad side is that it seens to be costly and hard-to-find part...

   I think I would still preffer an squematics of a filter like the one
used on older analog musical keyboards.

   Thank you very much,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
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2000\11\22@130519 by D Lloyd

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Hi,

MAX 291s are $1.80, according to http://www.j3t.com/STOCK.txt.
I have only played with one so I'm not sure about how good they really are,
apart from providing the tasty roll off. Phase shift appeared to be linear
enough.

I know you will need +/- 5V and will want to probably use the uncommitted
(although it would be committed when you use it ;-) amp on board for lp
smoothing.

Anyway, good luck

Dan








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Hello Dan,

> Can you not use one of the switched capacitor Maxim parts (Max 291?)
where
> you throw in a clock frequency 100x the cut off required and it does the
> rest? This, arguably, would be preferable to generating a PWM; you just
> provide, say, a 100kHz clock for a 1kHz cut off.

   This idea is really good. I've seen Maxim site about these filters and
I
think with a F873@20MHz PWM output I think I can make tune it from about
12Hz to 2kHz. That's not the perfect frequency spectra, but very usefull
for
my application.

   The sad side is that it seens to be costly and hard-to-find part...

   I think I would still preffer an squematics of a filter like the one
used on older analog musical keyboards.

   Thank you very much,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\22@130525 by Olin Lathrop

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>     I need to build a Low-Pass VCF (Voltage Controlled Filter) for a
> project, but can't find any squematics about how it's done.
>
>     I want to use a PWM output on a PIC to generate 0-5V, and with this
> voltage, I want to be able to make the filter cut selected high
frequencies
> in an audio signal, say with 0V, it will cut at 100 Hz, with 5V, at 5kHz.

This will be easier to do digitally than in analog.  Even a low end DSP
should be able to handle this just fine.  This also means you wouldn't
need the PWM because the cutoff value could be passed digitally to the
DSP.  A DSP also allows for a much more "fancy" filter with sharper
cutoff, better out of band attenuation, or whatever.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts

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2000\11\22@133004 by Edson Brusque

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Hello Andy,

> don't know, but may be usefull too. Look at MChip digital pots and app.
> notes describing programable low pass filter.

   I've seen these notes. Althought very interesting, they're some form of
fixed filter design, but using digital pots to make if variable. It's not a
true VCF. And I think a coulpe of digital pots will be more expensive than
the MAX291.

   Thanks anyway,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
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2000\11\22@134215 by Edson Brusque

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Hello Dan,

> MAX 291s are $1.80, according to http://www.j3t.com/STOCK.txt.
> I have only played with one so I'm not sure about how good they really
are,
> apart from providing the tasty roll off. Phase shift appeared to be linear
> enough.

   The MAX291 still seens to be a very good solution for my problem.

   I've asked a couple of pieces at http://www.j3t.com and some samples from
MAXIM.
Althought the price at DigiKey is above US$7.00 a piece.

> I know you will need +/- 5V and will want to probably use the uncommitted
> (although it would be committed when you use it ;-) amp on board for lp
> smoothing.

   Yes, probably...

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
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2000\11\22@134224 by Edson Brusque

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Hello Olin,

> This will be easier to do digitally than in analog.  Even a low end DSP
> should be able to handle this just fine.  This also means you wouldn't
> need the PWM because the cutoff value could be passed digitally to the
> DSP.  A DSP also allows for a much more "fancy" filter with sharper
> cutoff, better out of band attenuation, or whatever.

   My application is a digital tunner for musical instruments. My idea is
to get the sound (or body vibrations) of the instrument and filter the
harmonics. Let's say I'll want to tune an A (440Hz), so I'll make a very
sharp filter at about 660Hz to trash the harmonics and count how many cicles
(clipped) the PIC seens in a given time to calculate the frequency and show
the user what note the instrumment is playind and how much sharp or flat
this note is.

   I think a DSP would be more inteligent than a PIC with external filters,
but I know nothing about designing with DSP and programming it, and have no
time to study it, as I have urgency with this project. :*(

   Can you tell me more about how a DSP could make it?

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
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2000\11\22@134836 by Bob Blick

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Hi Edson,

If you get the data sheet for the National Semiconductor LM13600 there is
an application with a voltage controlled lowpass filter. It uses one half
of the chip, six resistors and one capacitor.

The chips are inexpensive and work quite well.

Cheers,

Bob

At 12:05 PM 11/22/2000 -0200, you wrote:
{Quote hidden}

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2000\11\23@150400 by Olin Lathrop

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>     My application is a digital tunner for musical instruments. My idea is
> to get the sound (or body vibrations) of the instrument and filter the
> harmonics. Let's say I'll want to tune an A (440Hz), so I'll make a very
> sharp filter at about 660Hz to trash the harmonics and count how many
cicles
> (clipped) the PIC seens in a given time to calculate the frequency and
show
> the user what note the instrumment is playind and how much sharp or flat
> this note is.
>
>     I think a DSP would be more inteligent than a PIC with external
filters,
> but I know nothing about designing with DSP and programming it, and have
no
> time to study it, as I have urgency with this project. :*(
>
>     Can you tell me more about how a DSP could make it?

That's a big question.  Programming a DSP is just like programming any other
microcontroller.  What distiguishes a DSP is usually instructions optimized
for doing convolutions.  This includes multiply-accumulates with the
associated address and counter logic so that one MACC can be done every
cycle.

If you're asking how to implement a digital filter, then you need to read up
on the theory.  It is not something easily answered in a few paragraphs.
Often with DSPs you transform the problem from frequency space to time
space.  So instead of doing a fourier transform, messing with the
frequencies, then doing the inverse fourier transform, you do the whole
thing in one convolution.  I'm sure there are lots of references about the
theory and implementation of digital filters you can look this up in.

If you need to get this done NOW, then you probably want to chose a
different approach or get someone familiar with this stuff to do the filter
part.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, KILLspamolinspamBeGonespamembedinc.com, http://www.embedinc.com

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2000\11\24@032653 by Edson Brusque

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Hello Olin,

> That's a big question.  Programming a DSP is just like programming any
other
> microcontroller.  What distiguishes a DSP is usually instructions
optimized
> for doing convolutions.  This includes multiply-accumulates with the
> associated address and counter logic so that one MACC can be done every
> cycle.

   What is MACC?

> If you're asking how to implement a digital filter, then you need to read
up
> on the theory.  It is not something easily answered in a few paragraphs.
> Often with DSPs you transform the problem from frequency space to time
> space.  So instead of doing a fourier transform, messing with the
> frequencies, then doing the inverse fourier transform, you do the whole
> thing in one convolution.  I'm sure there are lots of references about the
> theory and implementation of digital filters you can look this up in.

   I think there's libraries for each family of DSPs. Like one library with
audio processings (echo, reverb, chorus) for Texas DSPs(?).

> If you need to get this done NOW, then you probably want to chose a
> different approach or get someone familiar with this stuff to do the
filter
> part.

   If there's someone here who is very familiar with audio processing with
DSPs and want to colaborate with me, please send me email. Maybe we can work
together.

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\25@081302 by Edson Brusque

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Hello Bob Blick,

> If you get the data sheet for the National Semiconductor LM13600 there is
> an application with a voltage controlled lowpass filter. It uses one half
> of the chip, six resistors and one capacitor.

   This chip is very interesting, althought National says it's obsolete.
Can't find a substitute. :(

   Regarads,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\25@082341 by staff

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Edson Brusque wrote:
>
> Hello Bob Blick,
>
> > If you get the data sheet for the National Semiconductor LM13600 there is
> > an application with a voltage controlled lowpass filter. It uses one half
> > of the chip, six resistors and one capacitor.
>
>     This chip is very interesting, althought National says it's obsolete.
> Can't find a substitute. :(


Hi, tone controls in audio equipment use a varable resistor
and fixed capacitor. There work pretty well as low pass
adjustable filters if the cap is to ground.

Someone here mentioned a bi-directional fet thingy that works
good as an adjustible resistor with audio, this would be ideal
and would give you voltage controlled low pass filter I think.
:o)
-Roman

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2000\11\25@110442 by Olin Lathrop

face picon face
> > That's a big question.  Programming a DSP is just like programming any
> other
> > microcontroller.  What distiguishes a DSP is usually instructions
> optimized
> > for doing convolutions.  This includes multiply-accumulates with the
> > associated address and counter logic so that one MACC can be done every
> > cycle.
>
>     What is MACC?

Multiply-accumulate.

> > If you're asking how to implement a digital filter, then you need to
read
> up
> > on the theory.  It is not something easily answered in a few paragraphs.
> > Often with DSPs you transform the problem from frequency space to time
> > space.  So instead of doing a fourier transform, messing with the
> > frequencies, then doing the inverse fourier transform, you do the whole
> > thing in one convolution.  I'm sure there are lots of references about
the
> > theory and implementation of digital filters you can look this up in.
>
>     I think there's libraries for each family of DSPs. Like one library
with
> audio processings (echo, reverb, chorus) for Texas DSPs(?).

I don't know, but checking the TI web site seems like an obvious first thing
to try.

> > If you need to get this done NOW, then you probably want to chose a
> > different approach or get someone familiar with this stuff to do the
> filter
> > part.
>
>     If there's someone here who is very familiar with audio processing
with
> DSPs and want to colaborate with me, please send me email. Maybe we can
work
> together.

Is this a real budgeted project, or are you expecting people to donate their
time?


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, EraseMEolinspamEraseMEembedinc.com, http://www.embedinc.com

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2000\11\25@120915 by Edson Brusque

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Hello Olin,

> Is this a real budgeted project, or are you expecting people to donate
their
> time?

   It's a budgeted project, but it's more on the *low budget* side. I could
allocate some money for a development partner, but my resources are limited.

   Regards,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\25@121744 by Bob Blick

face
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OK, How about some other transconductance amplifier. I'm sure National has
a new low voltage version. Or maybe the old guitar gadget favorite the
CA3080(LM3080) is still available. Hmm. I don't see any other parts in the
1993 National Opamp book(the newest I have at home). I'll see what else I
can find.

Actually, you can use just a plain JFET transistor but the control voltage
is not as linear. Not a problem if you use a lookup table.

Cheers,

Bob



At 11:12 AM 11/25/2000 -0200, you wrote:
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2000\11\28@070914 by Edson Brusque

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Hello,

   I'm having a tough time trying to find a MAX291CPA or MAX295CPA. The
company Dan told me (http://www.j3t.com) doesn't have it in stock. Digi-Key doesn't
have in stock and only sells minimun of 50 pieces.

   Can someone help me find this? PLEASE!!!

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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2000\11\28@072138 by D Lloyd

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part 1 1213 bytes content-type:text/plain; charset=us-ascii
Hi,

I know they have a ton of these up at the University. I could always "half
inch" one, if you are desperate.

Dan




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Subject:  Re: [EE]: Anyone knows how to build a VCF?

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Hello,

   I'm having a tough time trying to find a MAX291CPA or MAX295CPA. The
company Dan told me (http://www.j3t.com) doesn't have it in stock. Digi-Key
doesn't
have in stock and only sells minimun of 50 pieces.

   Can someone help me find this? PLEASE!!!

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------

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