# Re: Relation between delay in FIR filter and the cutoff frequency...

*From*: Jerry Avins <jya@xxxxxxxx>*Date*: Thu, 08 Oct 2009 11:46:25 -0400

brent wrote:

On Oct 7, 11:55 pm, Fully Half Baked <jfez...@xxxxxxxxxxxxxx> wrote:On Oct 7, 5:21 pm, brent <buleg...@xxxxxxxxxxxxxxx> wrote:

I have some flash tutorials on the DFT, digital convolution and FIRGood stuff. What's the advantage of using an inverse FFT compared to

filters here:

http://www.fourier-series.com/fourierseries2/DSP.html

Perhaps this will help your understanding.

Brent

just calculating the sinc directly?

I think there is an advantage of using the sinc function directly.

When computing the inverse FFT you need to pick a certain length IFFT

to use. The longer you pick , the more true the coefficients will be,

but the more difficult it is to compute it. Going straight to the

sinc function gives you ideal coefficients with little computational

difficulty.

The sinc you so admire is infinitely long, requiring even more coefficients than what you get from an IFFT. :-) Simply truncating the sinc results in lousy stop-band attenuation. So you window however many coefficients you decide to use and end up with "Windowed sinc" design. That is good, but there are other methods that are often more effective.

Jerry

--

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

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.

**References**:**Relation between delay in FIR filter and the cutoff frequency...***From:*ronycsdu

**Re: Relation between delay in FIR filter and the cutoff frequency...***From:*brent

**Re: Relation between delay in FIR filter and the cutoff frequency...***From:*Fully Half Baked

**Re: Relation between delay in FIR filter and the cutoff frequency...***From:*brent

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