FPGA Central

Im have a few question about polyphase filtering.

Lets say you have a downsampling application. and in general wit
polyphase filtiering yoou wish to exploit the efficiency assocationw i
downsampling before invoking the lowpass filter.

If you didnt do ployphase filtering, lets say you implimented a lowpas
that was to reject frequencies above .2Fs and then did the downsampling t
avoid aliasing.

If you were poly phase filtering, you have a bank of M filters, wit
inputs delayed and downsampled accordingly.. now you wish to lowpas
filter. is the lowpass filter in the filter banks a .2Fs filter just at th
lower sampling rate?


Meanwhile, in trying to answer that question, there lies a bigger one..
in a filter bank, where the input to each filter is a subsequence of
larger sequence, wat is the relationship between each subsequence to th
original. in general each subsequence input to the filterbacnk will be
badly aliased version of the origional sequence... how then do we get wha
we want out of the filter?

On Thu, 10 Sep 2009 10:12:13 -0500, westocl wrote:

> Im have a few question about polyphase filtering.
>
> Lets say you have a downsampling application. and in general with
> polyphase filtiering yoou wish to exploit the efficiency assocationw it
> downsampling before invoking the lowpass filter.
>
> If you didnt do ployphase filtering, lets say you implimented a lowpass
> that was to reject frequencies above .2Fs and then did the downsampling
> to avoid aliasing.
>
> If you were poly phase filtering, you have a bank of M filters, with
> inputs delayed and downsampled accordingly.. now you wish to lowpass
> filter. is the lowpass filter in the filter banks a .2Fs filter just at
> the lower sampling rate?
>
>
> Meanwhile, in trying to answer that question, there lies a bigger one...
> in a filter bank, where the input to each filter is a subsequence of a
> larger sequence, wat is the relationship between each subsequence to the
> original. in general each subsequence input to the filterbacnk will be a
> badly aliased version of the origional sequence... how then do we get
> what we want out of the filter?

You are confused. You don't downsample before the filter. Instead, you
only run the filter for the output samples you want. Furthermore, if the
ratio of the two sampling rates is harmonic (i.e. 2:1, 3:1, etc.) then
you don't need to use polyphase filtering at all -- you can just run the
one 'correct' filter.

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