On 11 Okt, 02:01, "Zeph80" <surabhi_tal...@[EMAIL PROTECTED]
> wrote:
> In windowing method for FIR design, you specify the frequency response
of
> the desired filter at sample points fs/N, and perform an IDFT to obtain
> h(n).
> So doesnt this mean that you want fs to be as low as possible to specify
> better frequency resolution. But Fs is generally many times the cut-off
> frequency - more than the minimum of twice the cut off needed.
No. One doesn't select the sampling frequency quite as
freely as that. The sampling frequency is the dominating
system parameter, it's the hardest parameter to change.
So one chooses it with a lot of safety margin just to
be sure one doesn't loose soemthing that might be
im****tant.
So once you get to filtering one particular signal you
have to accept whatever sampling frequency was used, and
go on from there.
> Why is this?Isn't the frequency resolution that I specify for the filter
> response going to be poorer as I choose higher Fs?
No. The 'frequency resolution' is compensated by using
more coeffcients in the filter. If you do the math
you will find that it is the tem****al duration of the
filter that is im****tant, not the number of coefficients.
In other words, if a filter with sampling frequency f'
requires 9 samples to meet a (physical frequency) spec,
a filter with sampling frequency f" = 10*f' will require
10*9 = 90 samples to meet the same (physical frequency) spec.
The two filters have different numbers of coeffcinets but
their impulse responses last for the same (physical) time.
Rune
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