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Hye,

 I have a question to try to find a way to improve my processing.

I use a multirate filter with decimation.
There is a decimation by 40. One FIr is used then decimation, and the 1
IIR with decimation, then a second IIR with decimation, then again oner
IIR and decimation and at last one FIr .

I use a fix point DSP. The filter are designed in 16 bits inputs / outputs
(32 bits for computation).
It appears that the results is that i have in output a 12bits resoution
and 4 lsb bits are lost during the stages.

How can i improve this? 
Should i change the IIR design? or thedecimation?


thank you


		
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domistep wrote:

> Hye,
> 
>  I have a question to try to find a way to improve my processing.
> 
> I use a multirate filter with decimation.
> There is a decimation by 40. One FIr is used then decimation, and the 1
> IIR with decimation, then a second IIR with decimation, then again oner
> IIR and decimation and at last one FIr .
> 
> I use a fix point DSP. The filter are designed in 16 bits inputs / outputs
> (32 bits for computation).
> It appears that the results is that i have in output a 12bits resoution
> and 4 lsb bits are lost during the stages.
> 
> How can i improve this? 
> Should i change the IIR design? or thedecimation?
> 
> 
This sort of problem is generally caused by an implementation detail 
that is not easy to track down without looking at code -- and many folks 
(certainly I) generally don't look at code in newsgroup postings unless 
there's less than 5 or 10 lines.

Check to make sure that you're using the correct gains going into your 
internal states.  You should have gains that will neither cause your 
states to overflow or underflow.  The best way to do this is to write 
out the difference equations from your input to your states, find the 
transfer function, then do a frequency sweep.  This process is outlined 
for somewhat different systems in 
http://www.wescottdesign.com/articles/zTransform/z-transforms.html, but 
it works fine for filters.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Why did you pick IIR filters?

Dirk

domistep wrote:
> Hye,
>
>  I have a question to try to find a way to improve my processing.
>
> I use a multirate filter with decimation.
> There is a decimation by 40. One FIr is used then decimation, and the 1
> IIR with decimation, then a second IIR with decimation, then again oner
> IIR and decimation and at last one FIr .
>
> I use a fix point DSP. The filter are designed in 16 bits inputs / outputs
> (32 bits for computation).
> It appears that the results is that i have in output a 12bits resoution
> and 4 lsb bits are lost during the stages.
>
> How can i improve this?
> Should i change the IIR design? or thedecimation?
>
>
> thank you
>
>
>
> This message was sent using the Comp.DSP web interface on
> www.DSPRelated.com

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