I don't know if this is the right place to ask, but there seemed to be a
fair amount of activity in this group, so at least someone might be able
to point me in the right direction.

I want to build an analog audio filter, and I have never done anything
like it before. It needs to be controlled by a digital device, so I also
need some advice on how to do that (that is, say the cutoff is
represented by a 16 bit binary number and needs to be converted to an
analog signal of some sort). I'd like to build high, low and bandpass
filters, of decent quality but not too complicated. Any pointers
appreciated. I am an almost complete beginner in audio electronics.


Bjarni

On Mon, 22 Nov 2004 22:08:52 +0100, Bjarni Juliusson
wrote:

I don't know if this is the right place to ask, but there seemed to be a
fair amount of activity in this group, so at least someone might be able
to point me in the right direction.

I'm sure we can help you here.


I want to build an analog audio filter, and I have never done anything
like it before. It needs to be controlled by a digital device, so I also
need some advice on how to do that (that is, say the cutoff is
represented by a 16 bit binary number and needs to be converted to an
analog signal of some sort). I'd like to build high, low and bandpass
filters, of decent quality but not too complicated. Any pointers
appreciated. I am an almost complete beginner in audio electronics.

Then this does not seem to be the right place to start, if you ask me.

However, if you tell us the application for your filter, we may be of
a lot more help! Will this filter be operating in the analog domain,
completely? At line level or something else? Why do you want to build
it yourself instead of buying a commercial product?

Per.

Per Stromgren wrote:
On Mon, 22 Nov 2004 22:08:52 +0100, Bjarni Juliusson
wrote:


I don't know if this is the right place to ask, but there seemed to be a
fair amount of activity in this group, so at least someone might be able
to point me in the right direction.


I'm sure we can help you here.

Utmärkt :-)

I want to build an analog audio filter, and I have never done anything
like it before. It needs to be controlled by a digital device, so I also
need some advice on how to do that (that is, say the cutoff is
represented by a 16 bit binary number and needs to be converted to an
analog signal of some sort). I'd like to build high, low and bandpass
filters, of decent quality but not too complicated. Any pointers
appreciated. I am an almost complete beginner in audio electronics.


Then this does not seem to be the right place to start, if you ask me.

However, if you tell us the application for your filter, we may be of
a lot more help!

General music filtering, for timbre effects. I'm a computer guy, both
hardware and software, and now I'd like to experiment with audio
synthesis. For that reason, I'd like to look into how one builds a
fairly simple filter. Any hints are welcome. I know very little of
analog electronics, and I want to learn.

Will this filter be operating in the analog domain,
completely?

What do you mean by that? The filter should be a simple analog device,
taking analog inputs and producing analog output. The digital parts are
only for a computer to be able to generate the analog controls.

At line level or something else?

If that is the particular voltage range used for "line out" as found in
typical household audio equipment, then yes, that would be nice. What is
the actual voltage? Did I mention I'm a beginner?

Why do you want to build
it yourself instead of buying a commercial product?

Building it yourself is more fun, and more educational.


Bjarni

On Tue, 23 Nov 2004 18:38:06 +0100, Bjarni Juliusson
wrote:


General music filtering, for timbre effects. I'm a computer guy, both
hardware and software, and now I'd like to experiment with audio
synthesis. For that reason, I'd like to look into how one builds a
fairly simple filter. Any hints are welcome. I know very little of
analog electronics, and I want to learn.

You *could* do this with analog filters, it was always done this way,
but the common technology today is of course to do all filtering in
the digital domain. Digital filters can do everything analog filters
can, and a lot they can't.

In both the analog and digital filter design you start with a
transmision function, based on what you want to do. Then you implement
this filter with analog devices or does it in software. Software is so
much easier, nowadays. There where a time (in the dark ages when I was
a young EE engineer in the early 70's) when this was expensive and not
so easy, but now the cheapest cell phone has more digital filtering
that you thought you needed...


Will this filter be operating in the analog domain,
completely?

What do you mean by that? The filter should be a simple analog device,

Well, it can't be *that* simple if you want to control it from your
software.

taking analog inputs and producing analog output. The digital parts are
only for a computer to be able to generate the analog controls.

At line level or something else?

If that is the particular voltage range used for "line out" as found in
typical household audio equipment, then yes, that would be nice.

OK

What is
the actual voltage?

Around 1 volt.

Did I mention I'm a beginner?

Yes, I realize that.

Why do you want to build
it yourself instead of buying a commercial product?

Building it yourself is more fun, and more educational.

OK. Perhaps you should start building the filters without the software
control, in order to learn how they work? If I interpret your e-mail
address correctly, there should be plenty information at the EE
department of your university!

But I still think you should do your filter experiments in software,
it's right there at your fingertips!

Introduktion to digital filters:
http://www.dsptutor.freeuk.com/dfilt1.htm

Per,
EE from KTH

Per Stromgren wrote:
On Tue, 23 Nov 2004 18:38:06 +0100, Bjarni Juliusson
wrote:

General music filtering, for timbre effects. I'm a computer guy, both
hardware and software, and now I'd like to experiment with audio
synthesis. For that reason, I'd like to look into how one builds a
fairly simple filter. Any hints are welcome. I know very little of
analog electronics, and I want to learn.

You *could* do this with analog filters, it was always done this way,
but the common technology today is of course to do all filtering in
the digital domain. Digital filters can do everything analog filters
can, and a lot they can't.

....which is a typical news answer it seems. But it doesn't answer the
question. I want to make it clear that I'm not really interested in
getting optimal sound quality or learning the math or learning how to
program. See below.

Will this filter be operating in the analog domain,
completely?

What do you mean by that? The filter should be a simple analog device,

Well, it can't be *that* simple if you want to control it from your
software.

That interface should hardly complicate matters much. The filter itself
is the same, and the interface points ough to be pretty much vanilla
circuitry. Besides, when I say "simple", I don't mean two transistors
simple; I'm prepared to build a somewhat more complicated circuit than
that :-).

What is the actual voltage?

Around 1 volt.

OK. Thanks.

OK. Perhaps you should start building the filters without the software
control, in order to learn how they work?

Well sure, I could do that. Do you have any knowledge that will help me
to build it, or should I look elsewhere?

If I interpret your e-mail
address correctly, there should be plenty information at the EE
department of your university!

That is one place I could look, but I thought I'd ask here first because
it's less work and I figured I was more likely to get a good answer.

But I still think you should do your filter experiments in software,
it's right there at your fingertips!

Yes, but I had that much figured out by myself. Naturally I could do it
in software, but I'm not interested in that.

Introduktion to digital filters:
http://www.dsptutor.freeuk.com/dfilt1.htm

OK, thanks. The request remains however :-).


Bjarni

Both Analog Devices www.analog.com and Microchip Technology
www.microchip.com have application notes on the use of use of digital
potentiometers for filtration. The interface can be I2C, SPI, etc. Some of
the things you have to be mindful of are the noise and bandwidth of these
chips. TI also has a line of digital controls -- very high quality units.

Maxim Technology now owns Dallas Semiconductor - they have a line of digital
potentiometers which can be "manually controlled".

You don't have to bother working the transfer functions to design the
filters -- you can download Texas Instruments "Filter Pro" software from
their website -- it is a really wonderful tool. Analog also has an
interactive filter tool on their site, but I find it a little less
user-friendly.

Jack

"Bjarni Juliusson" wrote in message
...
I don't know if this is the right place to ask, but there seemed to be a
fair amount of activity in this group, so at least someone might be able
to point me in the right direction.

I want to build an analog audio filter, and I have never done anything
like it before. It needs to be controlled by a digital device, so I also
need some advice on how to do that (that is, say the cutoff is
represented by a 16 bit binary number and needs to be converted to an
analog signal of some sort). I'd like to build high, low and bandpass
filters, of decent quality but not too complicated. Any pointers
appreciated. I am an almost complete beginner in audio electronics.


Bjarni

John Walton wrote:
Both Analog Devices www.analog.com and Microchip Technology
www.microchip.com have application notes on the use of use of digital
potentiometers for filtration. The interface can be I2C, SPI, etc. Some of
the things you have to be mindful of are the noise and bandwidth of these
chips. TI also has a line of digital controls -- very high quality units.

Now that sounds like something I could use. I will have a look.

You don't have to bother working the transfer functions to design the
filters -- you can download Texas Instruments "Filter Pro" software from
their website -- it is a really wonderful tool. Analog also has an
interactive filter tool on their site, but I find it a little less
user-friendly.

Cool, I will check them out.

Thanks for your help!


Bjarni

On Wed, 24 Nov 2004 21:46:44 +0100, Bjarni Juliusson
wrote:


...which is a typical news answer it seems. But it doesn't answer the
question. I want to make it clear that I'm not really interested in
getting optimal sound quality or learning the math or learning how to
program. See below.

So you are interested in exactly what? Designing analog filters?
Building boxes that sound hip? Impressing your girl friends? :-)

No, seriously, if your interest is in the filter design itself, you
can play with this without pulling out you soldering iron. In fact,
you do all your design on paper, it's pure math, nothing else. You
*have* to learn the math, if you want to learn anything in this part
of science! The theory behind all this was pretty much settled 60-70
years ago, with digital entering in the 60's. The telecoms industry
knows everything about, be it analog or digital.

If you want to listen to your filters, it is nice to know that they
will sound the same both in analog and digital design.


That interface should hardly complicate matters much. The filter itself
is the same, and the interface points ough to be pretty much vanilla
circuitry. Besides, when I say "simple", I don't mean two transistors
simple; I'm prepared to build a somewhat more complicated circuit than
that :-).

Good. I saw another poster giving you pointers.


Well sure, I could do that. Do you have any knowledge that will help me
to build it, or should I look elsewhere?

I have my old litterature from the university, of course, but your
university library is probably full of books on the matter.


That is one place I could look, but I thought I'd ask here first because
it's less work and I figured I was more likely to get a good answer.

What? After reading my posts? :-) My prof would be proud...

Take my advice, go talk to them, they will gladly help an interested
student!


But I still think you should do your filter experiments in software,
it's right there at your fingertips!

Yes, but I had that much figured out by myself. Naturally I could do it
in software, but I'm not interested in that.

Can't you tell us why?

Per.

Per Stromgren wrote:
On Wed, 24 Nov 2004 21:46:44 +0100, Bjarni Juliusson
wrote:

...which is a typical news answer it seems. But it doesn't answer the
question. I want to make it clear that I'm not really interested in
getting optimal sound quality or learning the math or learning how to
program. See below.

So you are interested in exactly what? Designing analog filters?
Building boxes that sound hip? Impressing your girl friends? :-)

Playing. Not interested in being hip or impressing a hypothetical
girlfriend :-).

No, seriously, if your interest is in the filter design itself, you
can play with this without pulling out you soldering iron. In fact,
you do all your design on paper, it's pure math, nothing else. You
*have* to learn the math, if you want to learn anything in this part
of science! The theory behind all this was pretty much settled 60-70
years ago, with digital entering in the 60's. The telecoms industry
knows everything about, be it analog or digital.

Well, yes, but I'm not interested in *implementing* the filters in
software even if software can help in the design. It's playing with
analog electronics I'm interested in. I'll have to do some math to
design filters, but I don't want to learn about filters as a way to
learn lots of math.

If you want to listen to your filters, it is nice to know that they
will sound the same both in analog and digital design.

I know perfectly well that a computer at all approaching turing
completeness can be programmed to approximate an analog filter, but
that's not the kind of fun I'm looking for.

Well sure, I could do that. Do you have any knowledge that will help me
to build it, or should I look elsewhere?

I have my old litterature from the university, of course, but your
university library is probably full of books on the matter.

Quite possible. I can probably find some professor to point me in the
right direction not far from where I am sitting now. I'll go ask.

But I still think you should do your filter experiments in software,
it's right there at your fingertips!

Yes, but I had that much figured out by myself. Naturally I could do it
in software, but I'm not interested in that.

Can't you tell us why?

No, it's not an intellectual choice. It is an actual live example of
interest. Play with programming and math all days, and even digital
electronics; felt like playing with some analog electronics next.


Bjarni

Bjarni Juliusson wrote in message ...
John Walton wrote:
Both Analog Devices www.analog.com and Microchip Technology
www.microchip.com have application notes on the use of use of digital
potentiometers for filtration. The interface can be I2C, SPI, etc. Some of
the things you have to be mindful of are the noise and bandwidth of these
chips. TI also has a line of digital controls -- very high quality units.

Now that sounds like something I could use. I will have a look.

You don't have to bother working the transfer functions to design the
filters -- you can download Texas Instruments "Filter Pro" software from
their website -- it is a really wonderful tool. Analog also has an
interactive filter tool on their site, but I find it a little less
user-friendly.

Cool, I will check them out.

Thanks for your help!


Bjarni

If you don't already have it, you should take a look at "The Art of
Electronics" by Horowitz & Hill. It's a pretty good electronics book,
designed for beginniners.

I think what you will find, though, is that unless you are some kind
of math genius, you will end up copying someone else's analog filter
design. Basically, the only work you will be doing is soldering.

Or you could just buy a single chip that does everything you need!

Overdog

Try Googling for "DIY synth circuits". There are loads of people
experimenting like this and loads of diagrams and explanations too. There's
not much on the digital control side of things, but most of the circuits
you'll find are voltage controlled anyway.

For the digital side I suggest you read up on Programming Microcontrollers.
The 8051 family are powerful, cheap and well supported.


Gareth.



"Bjarni Juliusson" wrote in message
...
I don't know if this is the right place to ask, but there seemed to be a
fair amount of activity in this group, so at least someone might be able to
point me in the right direction.

I want to build an analog audio filter, and I have never done anything
like it before. It needs to be controlled by a digital device, so I also
need some advice on how to do that (that is, say the cutoff is represented
by a 16 bit binary number and needs to be converted to an analog signal of
some sort). I'd like to build high, low and bandpass filters, of decent
quality but not too complicated. Any pointers appreciated. I am an almost
complete beginner in audio electronics.


Bjarni

Overdog wrote:
If you don't already have it, you should take a look at "The Art of
Electronics" by Horowitz & Hill. It's a pretty good electronics book,
designed for beginniners.

I will. Thanks.

I think what you will find, though, is that unless you are some kind
of math genius, you will end up copying someone else's analog filter
design. Basically, the only work you will be doing is soldering.

If I can find a design that fits my needs, that's fine for a start. I
don't mean that I won't learn the math, but I don't want to just write
some program to do the filtering - especially since it's for a home
built computer and it would be kinda silly to have more computing power
in the filter than in the CPU :-).

Or you could just buy a single chip that does everything you need!

If what you say is true, this "chip" could be dangerous!


Bjarni

Gareth Magennis wrote:
Try Googling for "DIY synth circuits". There are loads of people
experimenting like this and loads of diagrams and explanations too. There's
not much on the digital control side of things, but most of the circuits
you'll find are voltage controlled anyway.

Yes, I've been googling already. I think I'll just pick one of the
designs and try it out! Just as you say, I haven't found anything about
digital control, but that's important to me. How do I generate a voltage
digitally? Perhaps by a counter counting really fast and flipping a
flip-flop connected to a transistor that feeds a capacitor :-). Isn't
that how switching power supplies work? Anyway, there is naturally a
simpler way...

For the digital side I suggest you read up on Programming Microcontrollers.
The 8051 family are powerful, cheap and well supported.

Er, the 8051 has no analog outputs does it? Anyway, the digital side
isn't really a problem. I'm a computing science student and all that,
it's just the conversion of 16 bits of TTL into a voltage that I don't
know how to do.

Anyway, thanks everybody for your help! I got some pointers, and I think
I know what to do, except the digital-analog part; I'll go google.


Bjarni

Bjarni Juliusson wrote in message ...
Gareth Magennis wrote:
Try Googling for "DIY synth circuits". There are loads of people
experimenting like this and loads of diagrams and explanations too. There's
not much on the digital control side of things, but most of the circuits
you'll find are voltage controlled anyway.

Yes, I've been googling already. I think I'll just pick one of the
designs and try it out! Just as you say, I haven't found anything about
digital control, but that's important to me. How do I generate a voltage
digitally? Perhaps by a counter counting really fast and flipping a
flip-flop connected to a transistor that feeds a capacitor :-). Isn't
that how switching power supplies work? Anyway, there is naturally a
simpler way...

For the digital side I suggest you read up on Programming Microcontrollers.
The 8051 family are powerful, cheap and well supported.

Er, the 8051 has no analog outputs does it? Anyway, the digital side
isn't really a problem. I'm a computing science student and all that,
it's just the conversion of 16 bits of TTL into a voltage that I don't
know how to do.

Anyway, thanks everybody for your help! I got some pointers, and I think
I know what to do, except the digital-analog part; I'll go google.


Bjarni

Just buy a PIC. They're made by Microchip.

PICs have built in ADCs and DACs, and onboard flash memory so you
don't have to solder an external memory chip.

Your main problems will be finding a compiler and finding a
"programmer" that will download the code to the chip.

Overdog

Er, the 8051 has no analog outputs does it? Anyway, the digital side isn't
really a problem. I'm a computing science student and all that, it's just
the conversion of 16 bits of TTL into a voltage that I don't know how to
do.


Yes you're right about that, but there are plenty of other microcontrollers
that do. Thre isn't much to converting 16 bits to analogue using an ADC
though. It can be as simple as writing "LDADC xx". The ADC does everything
for you.

Good luck.

Gareth.

Sorry, that's DAC of course.

"Overdog" wrote ...
Just buy a PIC. They're made by Microchip.

PICs have built in ADCs and DACs, and onboard flash
memory so you don't have to solder an external memory
chip.

The analog inputs and outputs in microcontroller chips
(like PICs, etc.) aren't suitable for audio, IMHO. They
are intended for industrial control kinds of functions.
There are programmable DSP chips, OTOH, that are
actually made for audio (and even video) processing.

OTOH, if the OP was actually looking for digitally-
controlled analog processing, there were chips made
for applications like 1-octave "graphic equalizer"
functions that were controlled with some popular
2-wire chip-to-chip interface protocols. these days,
though, DSP is becoming to inexpensive and easy to
use, they may not even make such hybrid ******* chips
anymore(?).

Bjarni Juliusson wrote in message ...
Gareth Magennis wrote:
Try Googling for "DIY synth circuits". There are loads of people
experimenting like this and loads of diagrams and explanations too. There's
not much on the digital control side of things, but most of the circuits
you'll find are voltage controlled anyway.

Yes, I've been googling already. I think I'll just pick one of the
designs and try it out! Just as you say, I haven't found anything about
digital control, but that's important to me. How do I generate a voltage
digitally? Perhaps by a counter counting really fast and flipping a
flip-flop connected to a transistor that feeds a capacitor :-). Isn't
that how switching power supplies work? Anyway, there is naturally a
simpler way...

Well, how about a simple D/A converter: digital in, analog output
voltage proportional to the digital input value. 8 bit monolithic
D/A converters of sufficient quality to fit your needs can be had
for very cheap: you don't need much performance: since you're using
it as a source of control voltage and not passing audio through it,
you don't care much about speed or anything like that.

For the digital side I suggest you read up on Programming Microcontrollers.
The 8051 family are powerful, cheap and well supported.

Er, the 8051 has no analog outputs does it? Anyway, the digital side
isn't really a problem. I'm a computing science student and all that,
it's just the conversion of 16 bits of TTL into a voltage that I don't
know how to do.
Anyway, thanks everybody for your help! I got some pointers, and I think
I know what to do, except the digital-analog part; I'll go google.

Hmmm, let's figure this mystifying puzzle out. You have a DIGITAL
value that your want to CONVERT to an ANALOG signal.

Hmm... you need some sort of CONVERTER from DIGITAL TO ANALOG.

Hmm.. you need a ...

DIGITAL TO ANALOG CONVERTER!

Try looking THAT up! :-)

Dick Pierce wrote:
Bjarni Juliusson wrote in message ...

Well, how about a simple D/A converter: digital in, analog output
voltage proportional to the digital input value. 8 bit monolithic
D/A converters of sufficient quality to fit your needs can be had
for very cheap: you don't need much performance: since you're using
it as a source of control voltage and not passing audio through it,
you don't care much about speed or anything like that.

Sure, it's just me being such a noob about everything not digital. So
using a typical DAC will do the right thing then? I mean, they output a
voltage, not a current or a resistance or whatever, and suitable for the
control signals in question... sorry about being such a dope. Found
something called an R-2R network that seems to do what I want and be
easy to build. And before you say it: yes, I can also buy a DAC on a
chip, and I will if they do the right thing. I suppose there are
different kinds and it'll just say what type of output it provides.
Hmm... I may be a bit extra dopey right now due to sleep depravation :-|.

Hmmm, let's figure this mystifying puzzle out. You have a DIGITAL
value that your want to CONVERT to an ANALOG signal.

Hmm... you need some sort of CONVERTER from DIGITAL TO ANALOG.

Hmm.. you need a ...

DIGITAL TO ANALOG CONVERTER!

Try looking THAT up! :-)

Yes, Dick. Good boy :-P.


Bjarni

Gareth Magennis wrote:
Er, the 8051 has no analog outputs does it? Anyway, the digital side isn't
really a problem. I'm a computing science student and all that, it's just
the conversion of 16 bits of TTL into a voltage that I don't know how to
do.

Yes you're right about that, but there are plenty of other microcontrollers
that do. Thre isn't much to converting 16 bits to analogue using an ADC
though. It can be as simple as writing "LDADC xx". The ADC does everything
for you.

To clarify the digital bit: I already have a whole computer, and I am
well familiar with the purely digital aspects, including processors and
so forth. The complexity in software if I use, say, a 6502 is no
different from what it is if I use a microcontroller with the DAC built
in; instead of the LDADC I just STA to the address of the DAC. I am not
interested in any discussion of microcontrollers or of software, or just
about anything concerning the digital side of things. You can just
assume we have the 16 (in this case) bits of TTL data available at all
times, and all we need to do is output a suitable control voltage to
manipulate the filter circuit.

So, PICs are out of the question. Would it be at all feasible to build
something as simple as one op-amp, a couple of caps and resistors, and a
DAC or two for control? If I have a circuit that uses pots for control,
how can I interface that? Do I replace the pot with a DAC and a transistor?

Thanks everybody for your patient help on the matter despite my dumb
questions :-).


Bjarni

I would find some 16 bit DAC and ADC chips and then go to the manufacturers
websites and look at the spec sheets. Some will provide application notes
which sound like the kind of thing you are really asking for. Also some
basic electronics knowledge would come in handy for when you have to build
or otherwise source power supplies and reference voltages to drive the
chips. Not forgetting input and output buffers etc.

Google some more for DAC and ADC circuits.

16 bits sounds a bit over the top for contolling a filter by the way.






So, PICs are out of the question. Would it be at all feasible to build
something as simple as one op-amp, a couple of caps and resistors, and a
DAC or two for control? If I have a circuit that uses pots for control,
how can I interface that? Do I replace the pot with a DAC and a
transistor?

Thanks everybody for your patient help on the matter despite my dumb
questions :-).


Bjarni

Gareth Magennis wrote:
I would find some 16 bit DAC and ADC chips and then go to the manufacturers
websites and look at the spec sheets. Some will provide application notes
which sound like the kind of thing you are really asking for. Also some
basic electronics knowledge would come in handy for when you have to build
or otherwise source power supplies and reference voltages to drive the
chips. Not forgetting input and output buffers etc.

I think that I might be able to manage power supplies, but there seems
to be plenty of schematics on the web too.

Google some more for DAC and ADC circuits.

16 bits sounds a bit over the top for contolling a filter by the way.

I don't doubt it. 16 bits was just an example for clarity and it's a 16
bit machine.

Thanks.


Bjarni