Analog filter

Archived from groups: rec.audio.tech (More info?)

On Mon, 22 Nov 2004 22:08:52 +0100, Bjarni Juliusson
<bjarni.ferret@update.ferret.uu.se> 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.

Archived from groups: rec.audio.tech (More info?)

On Tue, 23 Nov 2004 18:38:06 +0100, Bjarni Juliusson
<bjarni@update.uu.se> 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

Archived from groups: rec.audio.tech (More info?)

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" <bjarni.ferret@update.ferret.uu.se> wrote in message
news:cntkh9$m0p$1@Tempo.Update.UU.SE...
> 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

Archived from groups: rec.audio.tech (More info?)

On Wed, 24 Nov 2004 21:46:44 +0100, Bjarni Juliusson
<bjarni@update.uu.se> 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.

Archived from groups: rec.audio.tech (More info?)

Bjarni Juliusson <bjarni@update.uu.se> wrote in message news:<co50uu$e7p$1@Tempo.Update.UU.SE>...
> 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

Archived from groups: rec.audio.tech (More info?)

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

Archived from groups: rec.audio.tech (More info?)

Bjarni Juliusson <bjarni@update.uu.se> wrote in message news:<coas6g$tl0$1@Tempo.Update.UU.SE>...
> 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

Archived from groups: rec.audio.tech (More info?)

Sorry, that's DAC of course.

Archived from groups: rec.audio.tech (More info?)

Bjarni Juliusson <bjarni@update.uu.se> wrote in message news:<coas6g$tl0$1@Tempo.Update.UU.SE>...
> 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! :-)

Archived from groups: rec.audio.tech (More info?)

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

Archived from groups: rec.audio.tech (More info?)

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