Most likely due to limited word length - your math is not the same as
theory.

The choices you make in grouping poles and zeroes into sections, and
in ordering the sections, influence the quantization errors in
practice.

Rule of thumb: avoid big swings in gain between sections by pairing
the pole nearest the unit cirle with the zero nearest to it - then
repeat for remaining pairs. The zero tends to pull down the gain of
the pole.

I explain this in web pages from a while back, starting at:

   http://www.bores.com/courses/intro/iir/5_quant.htm

The textbooks can seem misleading because they suggest that such
grouping and ordering makes no difference (prove by the transposition
theorem) but this does not take account of quantization errors which
can be large in systems with feedback.

Also, of course avoid quantization by choosing your original design
well - also explained in links from the above pages.

Chris
===========================
Chris Bore
BORES Signal Processing
www.bores.com
chris@bores.com

christian.scholz@alpha-com.biz (Christian) wrote in message news:<980b0231.0410210017.79b0d968@posting.google.com>...
> Hi there!
> 
> I've got a problem with my 8th order IIR filter.
> In fact, it's a elliptic high pass designed with matlab. Cutoff
> frequency is 350Hz, attenuation 40dB and sample rate 8000Hz. Pass band
> ripple was designed to 0.1dB, but after transforming it into biquad
> structure and programming it, the ripple is about 1.5dB. That is too
> much for me. Only 0.5dB are acceptable. Has anyone some sugestions for
> me?
> 
> Thanks, Christian.

Seems to be no rounding error at all.
Just got it. It was the hardware guy's part. The hardware itself
produced the ripple, the filter was correct... :-)

Greetings to all the 'softies' out there! He, he.

Christian


Jerry Avins <jya@ieee.org> wrote in message news:<2tq42cF21qhftU2@uni-berlin.de>...
> Christian wrote:
> 
> > Hi there!
> > 
> > I've got a problem with my 8th order IIR filter.
> > In fact, it's a elliptic high pass designed with matlab. Cutoff
> > frequency is 350Hz, attenuation 40dB and sample rate 8000Hz. Pass band
> > ripple was designed to 0.1dB, but after transforming it into biquad
> > structure and programming it, the ripple is about 1.5dB. That is too
> > much for me. Only 0.5dB are acceptable. Has anyone some sugestions for
> > me?
> > 
> > Thanks, Christian.
> 
> Roundoff error? Insufficient word length?
> 
> Jerry

In article <980b0231.0410210017.79b0d968@posting.google.com>, Christian
<christian.scholz@alpha-com.biz> wrote:

> Hi there!
> 
> I've got a problem with my 8th order IIR filter.
> In fact, it's a elliptic high pass designed with matlab. Cutoff
> frequency is 350Hz, attenuation 40dB and sample rate 8000Hz. Pass band
> ripple was designed to 0.1dB, but after transforming it into biquad
> structure and programming it, the ripple is about 1.5dB. That is too
> much for me. Only 0.5dB are acceptable. Has anyone some sugestions for
> me?
> 
> Thanks, Christian.

Hi
You need to check your transformation. There should not be any
noticeble difference.

Regards,

Lasse

Christian wrote:

> Hi there!
> 
> I've got a problem with my 8th order IIR filter.
> In fact, it's a elliptic high pass designed with matlab. Cutoff
> frequency is 350Hz, attenuation 40dB and sample rate 8000Hz. Pass band
> ripple was designed to 0.1dB, but after transforming it into biquad
> structure and programming it, the ripple is about 1.5dB. That is too
> much for me. Only 0.5dB are acceptable. Has anyone some sugestions for
> me?
> 
> Thanks, Christian.

Roundoff error? Insufficient word length?

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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Hi there!

I've got a problem with my 8th order IIR filter.
In fact, it's a elliptic high pass designed with matlab. Cutoff
frequency is 350Hz, attenuation 40dB and sample rate 8000Hz. Pass band
ripple was designed to 0.1dB, but after transforming it into biquad
structure and programming it, the ripple is about 1.5dB. That is too
much for me. Only 0.5dB are acceptable. Has anyone some sugestions for
me?

Thanks, Christian.