Reply by robert bristow-johnson●October 16, 20072007-10-16
On Oct 16, 2:17 pm, right05 <vidu...@gmail.com> wrote:
> Thanks Robert. Thats a complex math for a fourth order LPF. Wish there
> is a generic formula for the coefficients.This website does
> http://www.apicsllc.com/apics/Sr_3/Sr_3.htm but it's not right.
take a look at:
http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
see how it spells out the coefficient values for a 2nd-order LPF
section of known resonant frequency and known Q. for an Nth-order
Butterworth (where N is even) the general formula for the Q of the nth
section is:
1/Q = 2*cos(pi/N*(n+1/2)) where 0 <= n < N/2
and for all sections, the resonant frequency, f0 (or w0), is the same
for a Butterworth (this is not the case for Tchebyshev or Elliptical
or anything other than Butterworth, as far as i know).
it's all spelled out, at least for even-order Butterworth implemented
as cascaded 2nd-order sections (which is the manner you should
implement it), and will not get less complex.
r b-j
Reply by Jerry Avins●October 16, 20072007-10-16
right05 wrote:
> On Oct 16, 2:21 pm, Jerry Avins <j...@ieee.org> wrote:
>> right05 wrote:
>>> Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working
>>> out a formula for the co-efficients for the fourth order low pass
>>> filter.
>>> But if I have 2 second order low pass filters in cascade, can I use
>>> the second order co-efficients for both the filters..I mean using the
>>> second order formula.
>> No. To achieve a Butterworth response, each section has its own Q.
>>
>> Jerry
>> --
>> Engineering is the art of making what you want from things you can get.
>> �����������������������������������������������������������������������
>
> But for a low pass filter ,all it needs is a cut off frequency. ???
Not so. "Butterworth" specifies a maximally flat shape in the passband,
There are many 4th order low-pass filters with a particular cutoff
frequency. A Butterworth filter is a specific one of those.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Tim Wescott●October 16, 20072007-10-16
On Tue, 16 Oct 2007 19:24:53 +0000, right05 wrote:
> On Oct 16, 2:21 pm, Jerry Avins <j...@ieee.org> wrote:
>> right05 wrote:
>> > Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working
>> > out a formula for the co-efficients for the fourth order low pass
>> > filter.
>> > But if I have 2 second order low pass filters in cascade, can I use
>> > the second order co-efficients for both the filters..I mean using the
>> > second order formula.
>>
>> No. To achieve a Butterworth response, each section has its own Q.
>>
>> Jerry
>> --
>> Engineering is the art of making what you want from things you can get.
>>
>
> But for a low pass filter ,all it needs is a cut off frequency. ???
If that statement were correct, you wouldn't specify a Butterworth or a
Chebychev or an Elliptical filter -- you'd just specify a "low pass".
So, no. Actually if you look at the page you quoted you'll see that the
denominator polynomial for the 4th order system is already split into two
dissimilar polynomials.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Reply by Vladimir Vassilevsky●October 16, 20072007-10-16
right05 wrote:
>>>Thats a complex math for a fourth order LPF.Wish there
>>>is a generic formula for the coefficients.
>>
>>I can give you the formula. The cost is $500.
>>
> Thanks.With the help of the friends online ,I should be able to work
> out the co-efficients myself and then I will posit it for free!
No you won't. It is too tough for a seeker of a free canned solutions.
VLV
Reply by right05●October 16, 20072007-10-16
On Oct 16, 2:24 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:
> right05 wrote:
> > Thats a complex math for a fourth order LPF.Wish there
> > is a generic formula for the coefficients.
>
> I can give you the formula. The cost is $500.
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
Thanks.With the help of the friends online ,I should be able to work
out the co-efficients myself and then I will posit it for free!
Reply by Vladimir Vassilevsky●October 16, 20072007-10-16
right05 wrote:
> Thats a complex math for a fourth order LPF.Wish there
> is a generic formula for the coefficients.
I can give you the formula. The cost is $500.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by right05●October 16, 20072007-10-16
On Oct 16, 2:21 pm, Jerry Avins <j...@ieee.org> wrote:
> right05 wrote:
> > Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working
> > out a formula for the co-efficients for the fourth order low pass
> > filter.
> > But if I have 2 second order low pass filters in cascade, can I use
> > the second order co-efficients for both the filters..I mean using the
> > second order formula.
>
> No. To achieve a Butterworth response, each section has its own Q.
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF
But for a low pass filter ,all it needs is a cut off frequency. ???
Reply by Jerry Avins●October 16, 20072007-10-16
right05 wrote:
> Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working
> out a formula for the co-efficients for the fourth order low pass
> filter.
> But if I have 2 second order low pass filters in cascade, can I use
> the second order co-efficients for both the filters..I mean using the
> second order formula.
No. To achieve a Butterworth response, each section has its own Q.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Clay●October 16, 20072007-10-16
On Oct 16, 2:17 pm, right05 <vidu...@gmail.com> wrote:
> Thanks Robert. Thats a complex math for a fourth order LPF.Wish there
> is a generic formula for the coefficients.This website doeshttp://www.apicsllc.com/apics/Sr_3/Sr_3.htmbut its not right.
Thanks Robert. Thats a complex math for a fourth order LPF.Wish there
is a generic formula for the coefficients.This website does
http://www.apicsllc.com/apics/Sr_3/Sr_3.htm but its not right.