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why do we need Polyphase decimatation filter?

Started by mary1234567 February 5, 2006
I did a decimator FIR filter simply by doing the convolution only once
every M' input sample. 
for example M=3:
 feed x0,feed x1, feed x2, convolution, feed x3,feed x4, feed x5,
convolution, feed x6,....

Then I find many people talk about polyphase decimation filter, I wonder
if it is more efficient than what I'm doing? What is the differrence?

thanks
mary  


mary1234567 wrote:

> I did a decimator FIR filter simply by doing the convolution only once > every M' input sample. > for example M=3: > feed x0,feed x1, feed x2, convolution, feed x3,feed x4, feed x5, > convolution, feed x6,.... > > Then I find many people talk about polyphase decimation filter, I wonder > if it is more efficient than what I'm doing? What is the differrence?
IMO, there is only a conceptual difference or notational difference. From the implementation point of view, both lead to identical complexity and identical results (in the case of decimation). Polyphase comes handy as a notational tool when systems get more complex than simple decimator. -- Jani Huhtanen Tampere University of Technology, Pori

mary1234567 wrote:

> I did a decimator FIR filter simply by doing the convolution only once > every M' input sample. > for example M=3: > feed x0,feed x1, feed x2, convolution, feed x3,feed x4, feed x5, > convolution, feed x6,.... > > Then I find many people talk about polyphase decimation filter, I wonder > if it is more efficient than what I'm doing? What is the differrence? >
There is no difference in your simple case. The polyphase filter is handy when you need fractional decimation ratios, and if the decimation is performed in the several stages. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Thanks for all the answers! Yes, I do have several decimate stages.(not
fractional)  I do every stage the same simple way. Will polyphase
decimator be more efficient then ? 

Thanks!


"mary1234567" <mjm0520@yahoo.com> wrote in message 
news:NNCdnc5Mvoq9jnvenZ2dnUVZ_sednZ2d@giganews.com...
>I did a decimator FIR filter simply by doing the convolution only once > every M' input sample. > for example M=3: > feed x0,feed x1, feed x2, convolution, feed x3,feed x4, feed x5, > convolution, feed x6,.... > > Then I find many people talk about polyphase decimation filter, I wonder > if it is more efficient than what I'm doing? What is the differrence?
A polyphase filter simply recognizes that one need not multiply each retained input sample by each filter coefficient for each output sample. Most often the filter ends up looking like a number of filters in parallel with inputs or outputs commutated at the sample rate. This means that only a subset of filter coefficients operate at each sample interval - and that's efficient. Then, if the idea is to decimate, some number of the outputs and possibly those paths can be eliminated. In my mind, polyphase is all about computational efficiency. Otherwise why bother mentioning it? It doesn't change the underlying filters. Fred
"mary1234567" <mjm0520@yahoo.com> wrote in message
news:P8-dndQs_ePX6XreRVn-iQ@giganews.com...
> Thanks for all the answers! Yes, I do have several decimate stages.(not > fractional) I do every stage the same simple way. Will polyphase > decimator be more efficient then ?
No. Since you only have simple decimation stages, you can do just as good with what you have. It doesn't sound as cool as saying "I used a polyphase filter" but it gets the job done and it essentially does the same thing a polyphase works out to doing. If you have sample rate changes where you have both interpolation and decimation (say a rate change of 9/7), then a polyphase filter would be an excellent choice. Cheers Bhaskar
mary1234567 wrote:
...
> Then I find many people talk about polyphase decimation filter, I wonder > if it is more efficient than what I'm doing? What is the differrence?
... You might want to look at dspGuru's "Multirate FAQ" at http://dspguru.com/info/faqs/mrfaq.htm. It covers a broad range of issues related to decimation, interpolation, and resampling in a format that's (hopefully) pretty practical and easy to understand. =g2 -- _____________________________________________________________________ Grant R. Griffin Publisher of dspGuru http://www.dspguru.com Iowegian International Corporation http://www.iowegian.com See http://www.iowegian.com/img/contact.gif for e-mail address