# [PD] from poles/zeros to biquad coefficients - how to? (something like max's z-plane)

Funs Seelen funsseelen at gmail.com
Tue Sep 24 10:46:16 CEST 2013

```Hi Alexandre,

This is the online tool: http://kmt.hku.nl/~pieter/cgi-bin/resp/nph-PZT.cgi.

It starts with an example and every time you refresh the page it gives you
a new one. If you scroll down there's a link that tells you how the
coefficients were calculated, e.g.:
2 zeros give 3 coefficients: *a0* = G
*a1* = -G(Z0 + Z1)
*a2* = G(Z0*Z1)

2 poles give 3 coefficients: *b0* = 1
*b1* = -(P0 + P1)
*b2* = (P0*P1)

The linear difference equation is derived from these as you can see.

Regards,
--Funs

On Tue, Sep 24, 2013 at 7:36 AM, Alexandre Torres Porres
<porres at gmail.com>wrote:

> for what i see, it's not some sort of straight formula, right? seems a bit
> more complicated than that.
>
> cheers
>
>
> 2013/9/23 Funs Seelen <funsseelen at gmail.com>
>
>> On Mon, Sep 23, 2013 at 5:35 PM, Alexandre Torres Porres <
>> porres at gmail.com> wrote:
>>
>>>
>>> thanks, here's a pic of what I have so far
>>>
>>>
>>> https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-prn1/11212_10151872996046683_1825736206_n.jpg
>>>
>>
>> Cool.
>>
>>
>>>
>>> > For extra inspiration you could have a look at PoZeTools
>>>
>>> It sure does look like what I need. Thanks. But extracting what I need
>>> to know about the math of converting from coordinates to coefficients was
>>> just over my head :P unfortunately, sorry.
>>>
>>> I was hoping for something simpler, like just the operations needed. If
>>> the info is in code, I need it to more explicit. I'd really appreciate if
>>> anyone knows how to read from this and just points it out for me so I can
>>> put it in a patch.
>>>
>>> I'm assuming it's rather simple math
>>>
>>
>> I remember I once learned how to do this but never repeated the practice.
>> If I find time to do that I would gladly try to figure it out again, but if
>> someone more experienced feels the urge to chime in before that time I
>> would be very happy too :).
>>
>
>
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