IIR filter reference designs

Hi all.

I'm struggling to implement my own codes for IIR
filter design by analog prototypes. The Butterworth
filter works, the Chebyshev type 1 doesn't.

The books I have available are Proakis & Manolakis' 3rd
edition, Oppenheim & Schafer 1975+1999, Leland B Jackson
1989. Most show, relatively crudely, the main steps
in filter design. Only P&M show any examples of Cheb 1
filters in sufficient detail to check that my code
runs correctly, but I don't trust the numbers in P&M.
The reason is that I found two grave typos in the
table of spectral transforms (how to transform a
lowpass prototype filter to highpass, bandpass, or
bandstop filters).

So when I can't reproduce the numbers from the examples
and my filter functions don't make sense, I just don't
know if my code is wrong or if there are more typos.

So, do anyone know of reference filter designs? I.e. the
analog coefficients that pop out of a given spec and
their discrete-time counterparts?

Rune

Rune Allnor wrote:
> Hi all.
>
> I'm struggling to implement my own codes for IIR
> filter design by analog prototypes. The Butterworth
> filter works, the Chebyshev type 1 doesn't.
...
> So, do anyone know of reference filter designs? I.e. the
> analog coefficients that pop out of a given spec and
> their discrete-time counterparts?

There are several ways on how to approximate a given analogue (s
domain) transfer function with a digital (z domain) transfer function.

rbj's cookbook uses bilinear transform (I think?), available at
http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt.

Is that useful?

Regards,
Andor

"Andor" <an2or@mailcircuit.com> wrote in message
news:1118349254.467837.166700@z14g2000cwz.googlegroups.com...
> Rune Allnor wrote:
> > Hi all.
> >
> > I'm struggling to implement my own codes for IIR
> > filter design by analog prototypes. The Butterworth
> > filter works, the Chebyshev type 1 doesn't.
> ...
> > So, do anyone know of reference filter designs? I.e. the
> > analog coefficients that pop out of a given spec and
> > their discrete-time counterparts?
>
> There are several ways on how to approximate a given analogue (s
> domain) transfer function with a digital (z domain) transfer function.
>
> rbj's cookbook uses bilinear transform (I think?), available at
> http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt.

Yes, he does use the bilinear transform, as described in the explanatory text.
I went through the trouble to try to duplicate one of his equations exactly
starting from first principles, and eventually got it to match after some trig
substitutions and a whole bunch of algebra.  At the end of the cookbook, he
gives some hints on the trig identities and procedure necessary to come up with
his final forms, which I found invaluable.

FWIW, here's an example of using the bilinear transform on an analog 
filter prototype; rbj uses sin/cos instead of tan, but they give the 
same results--obviously, you can rearrange the equations in any number 
of ways (I followed Zolzer's format).

http://www.earlevel.com/Digital%20Audio/Bilinear.html


In <11ahc7hs1pvibb4@corp.supernews.com> Jon Harris wrote:
> "Andor" <an2or@mailcircuit.com> wrote in message
> news:1118349254.467837.166700@z14g2000cwz.googlegroups.com...
>> Rune Allnor wrote:
>> ...rbj's cookbook uses bilinear transform (I think?), available at
>> http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt.
> 
> Yes, he does use the bilinear transform, as described in the 
> explanatory text. I went through the trouble to try to duplicate one 
> of his equations exactly starting from first principles, and 
> eventually got it to match after some trig substitutions and a whole 
> bunch of algebra...

in article 20050609173703992-0700@news.linkline.com, Nigel Redmon at
earlevel@earlevel.com wrote on 06/09/2005 20:37:

> FWIW, here's an example of using the bilinear transform on an analog
> filter prototype; rbj uses sin/cos instead of tan, but they give the
> same results--obviously, you can rearrange the equations in any number
> of ways (I followed Zolzer's format).

just FYI, the reason i use the sin() and cos() (unlike Udo) is because,
especially in fixed-point math, tan() is not your friend.  tan() likes to
purport he's your friend, but he'll betray you and stab you in the back.
tan() is your adversary.

beware of tan().

-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

Hehe--yeah, I always assumed that's why you used sin/cos. I've always 
had the good fortune of a UI running on a front end that did the 
calculations in floating point (either pre-calculated at compile time, 
or done at run-time), whether it be in an imbedded device with a host 
running a 56k or something like a Pro Tools plug-in with a prime-time 
cpu either doing the work in fp or passing the coefs to a 56k. (For that 
matter, I usually try to use something other than a biquad if the filter 
needs much in the way of realtime control.)

In <BECEA0FB.81CE%rbj@audioimagination.com> robert bristow-johnson  
wrote:
> in article 20050609173703992-0700@news.linkline.com, Nigel Redmon at
> earlevel@earlevel.com wrote on 06/09/2005 20:37:
> 
>> FWIW, here's an example of using the bilinear transform on an analog
>> filter prototype; rbj uses sin/cos instead of tan, but they give the
>> same results--obviously, you can rearrange the equations in any 
>> number of ways (I followed Zolzer's format).
> 
> just FYI, the reason i use the sin() and cos() (unlike Udo) is because,
> especially in fixed-point math, tan() is not your friend.  tan() likes 
> to purport he's your friend, but he'll betray you and stab you in the 
> back. tan() is your adversary.
>
> beware of tan().

Nigel Redmon wrote:
> Hehe--yeah, I always assumed that's why you used sin/cos. I've always
> had the good fortune of a UI running on a front end that did the
> calculations in floating point (either pre-calculated at compile time,
> or done at run-time), whether it be in an imbedded device with a host
> running a 56k or something like a Pro Tools plug-in with a prime-time
> cpu either doing the work in fp or passing the coefs to a 56k. (For that
> matter, I usually try to use something other than a biquad if the filter
> needs much in the way of realtime control.)

Hi Nigel,

what do you try to use instead of biquad? Is this method suited for
fixed or for floating (or for both)?

Regards,
Andor

Rune Allnor wrote:
> Hi all.
>
> I'm struggling to implement my own codes for IIR
> filter design by analog prototypes. The Butterworth
> filter works, the Chebyshev type 1 doesn't.
>
> The books I have available are Proakis & Manolakis' 3rd
> edition, Oppenheim & Schafer 1975+1999, Leland B Jackson
> 1989. Most show, relatively crudely, the main steps
> in filter design. Only P&M show any examples of Cheb 1
> filters in sufficient detail to check that my code
> runs correctly, but I don't trust the numbers in P&M.
> The reason is that I found two grave typos in the
> table of spectral transforms (how to transform a
> lowpass prototype filter to highpass, bandpass, or
> bandstop filters).
>
> So when I can't reproduce the numbers from the examples
> and my filter functions don't make sense, I just don't
> know if my code is wrong or if there are more typos.
>
> So, do anyone know of reference filter designs? I.e. the
> analog coefficients that pop out of a given spec and
> their discrete-time counterparts?
>
> Rune

Thank, folks, for your suggestions. I'll have to check
out the details in RBJ's cookbook. As of yet, I'm happy
just to be able to use matlab to find the filter coefficients.
And no, the signal processing toolbox is not an option.
It will cost me the equivalent of $1200 + VAT, which I
have better use for. I prefer to do the implementations
myself.

Anyway, I did dig up an old book from the archive boxes
in the attic above the garage. van Valkenburg's "Analog
Filter Design" from 1982 had just the tables I was looking
for: Tables 6.1 and 6.2 (page 165) contain the pole
locations and filter coefficients for reference Butterworth
filters for orders 10 and below, table 8.1 on page 248
contains pole locations for an assorted number of low
order Cheb 1 filters, while table 8.2, page 249, contains
a comparision between the design procedures for Butterworth
and Cheb 1 filters.

Just what I needed! Now, if somebody could compute and
publish similar reference tables for the bilinear
transform and frequency transforms... 

Rune

Hi Andor,

One handy filer for audio is the state variable (some comments from my 
badly neglected site: http://www.earlevel.com/Digital%20Audio/StateVar.
html). Independent Fc and Q--single parameter for each, simple 
computation--simulaneous low, high, notch, and bandpass output. 
Extremely good at low frequencies with limited coefficient word length (
unlike biquads), and great as a low frequency sine generator. Integer or 
floating point. The bad news is that the top end for Fc is only about a 
third of Nyquist--you must oversample if you want more.

Also, filters from single-order sections including, all-pole 
arrangements, simplify the coefficients versus biquads and make 
calculations and moving coefficients a lot easier. Besides the state-
variable, another popular synth filter is the Moog-style filter 
suggested by Stilson (http://ccrma-www.stanford.edu/~stilti/papers/
moogvcf.pdf); implemetations available at musicdsp.org.

In <1118393122.208868.285770@g44g2000cwa.googlegroups.com> Andor wrote:
> Nigel Redmon wrote:
>> For that matter, I usually try to use something other than a biquad
>> if the filter needs much in the way of realtime control.
> 
> Hi Nigel,
> 
> what do you try to use instead of biquad? Is this method suited for
> fixed or for floating (or for both)?
> 
> Regards,
> Andor

Hello Rune,

Using a normalized Butterworth filter (cutoff is at 1), the zeroes of
the transfer equation (LaPlace form) are on the unit circle. You can
grab pairs (taken symmetrically above and below the real axis) of the
zeros and form your transfer equation. The full set of zeroes are the
2n roots of 1. For just your filter, pick the ones in the left hand
plane (causal).

I.e., for 1st order, the 2n roots are 1, and -1. Using the n roots in
the left hand plane, we find just -1. So our transfer equation is
simply  1/(s--1) or simply 1/(s+1)

For 2nd order, the 4 roots are:

0.707+0.707j
0.707-0.707j
-0.707+0.707j
-0.707-0.707j

( Really the 0.707 is an approx. to sqrt(2)/2  ))

The two zeros in the left hand plane are

-0.707+707j and -0.707-0.707j

Form a quadratic (product of the two conjugate factors)

(s - (-0.707+0.707j) )*(s - (0.707-0.707j))

= s^2 + 1.414s + 1

So your 2nd order filter is

1 /  ( s^2 + 1.414s + 1)

This can be carried out easily for any order. I wrote a little c
program years ago that will give you these functions.

For Chebyshev, the method is similar, except instead of a unit circle,
you now have ponts on an ellipse. The major and minor axes are given by
sinh() and cosh() functions of the passband wobble.

When I get back to my house, I can find my notes where I have this all
worked out.

Are you interested in these details?

Clay