Bus width between registers in IIR

Sam (r�p. sans -no-sp-am) wrote:

> Thank you for all these answers !
> 
> The only thing I would know is how many bits wide should I take for the
> intermediate results when I use an IIR filter. The input data is 24 bit.
> Should I use more than 24 bits to calculate the partial products, or not ?
> 
> Thank you !
> 
> Sam

Yes
-- 
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"Sam (r&#4294967295;p. sans -no-sp-am)" <totalsam-no-sp-am@hotmail.com> wrote in message
news:4061ec72$0$702$5402220f@news.sunrise.ch...
> Thank you for all these answers !
>
> The only thing I would know is how many bits wide should I take for the
> intermediate results when I use an IIR filter. The input data is 24 bit.
> Should I use more than 24 bits to calculate the partial products, or not ?
>
> Thank you !
>
> Sam

There is no one answer for all IIR filters.  You need to figure out what the
maximum value *your system* can produce relative to the input scale for
*your* desired IIR structure and how many bits you need for *your* desired
IIR structure to get the stability and noise to acceptable levels.

> The only thing I would know is how many bits wide should I take for the
> intermediate results when I use an IIR filter. The input data is 24 bit.
> Should I use more than 24 bits to calculate the partial products, or not ?

You need to add some MSBs.
The number of them depends on the filter sharpness.
If the filter is rather selectable you have to add even 8-12 MSBs.
Therefore the best solution is to simulate the filter with real signals,
investigate the overflows,
and select the minimum bit number which does not make the overflow.

A.Ser.

If the overflows are internal only, ie. if the added MSBs are not propagated all
the way to the output, then you don't need to add them provided your number
system is modular (2's complement is).   The arithmetic operations in the filter
are such that if you change one bit of an operand, only that bit and bits to the
left of it are affected.  Therefore, overflow bits internally do not affect
output bits with less significance.

Anatoli Sergienko wrote:

> > The only thing I would know is how many bits wide should I take for the
> > intermediate results when I use an IIR filter. The input data is 24 bit.
> > Should I use more than 24 bits to calculate the partial products, or not ?
>
> You need to add some MSBs.
> The number of them depends on the filter sharpness.
> If the filter is rather selectable you have to add even 8-12 MSBs.
> Therefore the best solution is to simulate the filter with real signals,
> investigate the overflows,
> and select the minimum bit number which does not make the overflow.
>
> A.Ser.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930     Fax 401/884-7950
email ray@andraka.com
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