>On 10 Apr, 02:17, Rick Lyons <R.Lyons@_BOGUS_ieee.org> wrote:
>> On Thu, 09 Apr 2009 08:22:55 -0500, "bmh161" <bmh...@yahoo.com> wrote:
>> >Instead of going through all the trouble of designing complicated digital
>> >filters, why don't we just pick the frequencies that we want to supress and
>> >replace those bins in the FFT with zeros?
>> �I just wanted to say that
>> your question is sensible and that we, the DSP guys here,
>> should investigate the process of freq-domain filtering
>> in detail
>Rick, did we read the same post? The question is not why
>frequency domain filtering is a bad idea, but why one doesn't
>use the naive, 'obvious' specs for the filters. I commented on
>a similar question not too long ago:
>Such questions are covered in most DSP texts - I wouldn't be
>surprised if you touched on the subject already. If not, the issue
>might be worth half a page's comments in your 3rd edition, as
>your book is the first entry point into DSP for lots of users.
>Teachning students how to think 'right' is an admirable (and
>difficult!) task; warning against how to think 'wrong' can only
>help the effort.
Oh shoot. Maybe I misunderstood the poster's
question. For whatever reason, I thought he was
asking: "Can we perform an N-point FFT on an N-point
time sequence, zero-out the spectral components
that we wish to attenuate, and then perform an
N-point inverse FFT to obtain a "filtered"
If the original poster was asking about ways to
design a time-domain digital filter, then I
completely missed the point of his question.
(That wouldn't be the first time for me.)
PS. Rune, I wish you "calm seas."
>"bmh161" <email@example.com> a �crit dans le message de
>> This is probably a stupid question but, well, I'm not all that smart...
>> Instead of going through all the trouble of designing complicated digital
>> filters, why don't we just pick the frequencies that we want to supress
>> replace those bins in the FFT with zeros?
>Doing what you propose (FFT of the signal, multiplying with a frequency
>mask, IFFT) gives EXACTLY the same result mathematically speaking than a FIR
>filter, its just another way to calculate it. In a nutshell :
[snipped by Lyons]
are you sure about that? Let's say I have a
1024-sample time sequence on which I perform a 1024-pt
FFT. Next, I zero-out the FFT bins (samples) for the
freqs I want to attenuate. Finally I perform a 1024-pt
inverse FFT. My resultant "filtered" time sequence
is 1024 samples in length (just as was the original
Now ...what kind of time-domain digital filtering can
I perform on the original 1024-sample time sequence
where the filter output sequence is EXACTLY the same
1024 samples as the above freq-domain filtering output?
I believe the answer is: "There *IS* no such time-domain
As far as I know, the freq-domain filtering I discussed in
my first paragraph above is equivalent to time-domain circular
convolution, and time-domain filtering does
not perform circular convolution.
Robert, if I'm mistaken, then I'd sure like to
know about it.
> This is probably a stupid question but, well, I'm not all that smart...
> Instead of going through all the trouble of designing complicated digital
> filters, why don't we just pick the frequencies that we want to supress a=