Low pass filter at half Nyquist

"Randy Yates" <yates@ieee.org> wrote in message 
news:m3zltg9dem.fsf@ieee.org...
> "VelociChicken" <bob@yahoob.com> writes:
> I've never heard of "two-path" polyphase filters, but the operation of
> polyphase decimation filters is very straightforward: since you're going
> to throw away M-1 of M samples when decimating by M, simply don't
> compute them. The rest is bookkeeping.
> -- 

Exactly.  Polyphase is only about the internal implementation of some 
filters and not about filter design - as in coefficient selection.

Fred 

"VelociChicken" <bob@yahoob.com> wrote in message 
news:uUYyj.83377$jH4.73721@fe1.news.blueyonder.co.uk...
>>> If you google 'polyphase filter' you'll find better
>>> explanations than I can give. I've optimised this 
>>> codehttp://www.musicdsp.org/archive.php?classid=3#39which states that it 
>>> can
>>> achieve up to '150db rejection' with a transition band of .05
>
>>150 dB attenuation, that's on the order of 10e-7, right?
>>I'm not sure I believe that - one is pushing the limits
>>of what can be done with single-precision floating point
>>numbers...
>>Rune
>
> Single-precision floating point?    *pfft*  - nobody uses those anymore! 
> : )
>

After reading through your references I wonder if it might not be a good 
idea for you to tell us what you think a "halfband filter" is.  Because, it 
isn't simply a filter with cutoff at fs/4.

In order to get a halfband filter of any reasonable length and 150dB 
rejection, the transition would have to be awfully wide I would think.

Fred 

Fred Marshall wrote:
> "Randy Yates" <yates@ieee.org> wrote in message 
> news:m3zltg9dem.fsf@ieee.org...
>> "VelociChicken" <bob@yahoob.com> writes:
>> I've never heard of "two-path" polyphase filters, but the operation of
>> polyphase decimation filters is very straightforward: since you're going
>> to throw away M-1 of M samples when decimating by M, simply don't
>> compute them. The rest is bookkeeping.
>> -- 
> 
> Exactly.  Polyphase is only about the internal implementation of some 
> filters and not about filter design - as in coefficient selection.

It's a different and idiosyncratic use of "polyphase". Mr. Chicken was 
misled by the author 0f the URL he quoted. Don't cluck at him.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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