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From: Jerry Avins on 29 May 2010 18:19
On 5/29/2010 6:00 PM, Fred Marshall wrote:
> Jerry,
>
>>>
>>> I supposed that a half-band filter might fall short of the needed
>>> performance. Isn't the attenuation at fs/4 only 3 dB?
>>
>> Er, 6 dB. I do too much of that lately.
>>
>
> Well, yes it is. But then one hopes that the guard band in the signal is
> maybe 20% also. So, in that case it makes sense. If there is some
> bandwidth margin then the cutoff frequency (where the passband and/or
> stopband departs from its normal / average value beyond the ripple
> value) can be at the edge of the margin.
>
> If there is *no* margin then one may not be sure what one is working
> with in the first place as it has all the appearance of not meeting the
> sampling requirements.
>
> Either one is willing to get some aliasing or one must be willing to do
> some pre-processing filtering to get some band margin in order to rather
> guarantee acceptable aliasing or *no* aliasing if that's possible....
>
> Then, you use a half band filter when efficiency of implementation is
> important. And, this might also lead to a polyphase implementation of same.
>
> I note that "polyphase" filtering is an implementation technique and not
> a filter design technique - at least where coefficients are concerned.
> You first have to design a filter with coefficients such that the filter
> *can* be implemented as a polyphase structure and then you can use a
> polyphase structure.

Note to Joe: Here, polyphase boils down to this: since, in decimating
you discard half the samples, there's no need to compute them in the
first place. When interpolating, half the samples will be zero, so
there's no reason to carry out any multiplication on them.

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