[sdiy] Digital filtering question

[Thomas Strathmann]

On 8/24/10 13:15 , cheater cheater wrote:
>> It should be the first (two) zero crossing(s) because for a rectangle which
>> corresponds to a lowpass filter with cutoff frequency f_c in the frequency
>> domain you get a sinc function whose width (distance from zero crossing in
>> the fourth quadrant to z.c. in the first quadrant)
>
> OK, I guess what you mean is: the two zero crossings x_1, x_2 such
> that |x_1| =<  |x_2|<  x_n for any other zero crossings x_n.

Yes, and in this case we can be more specific and additionally write
x_1 = -x_2.

To come back to what hase been said by Richie Burnett about the 
practical side of digital filtering: Another way to design a digital 
(lowpass) filter would be to take an analogue filter design and apply 
the bilinear transform to its transfer function (i.e. substituting 
(z-1)/(z+1) for the Laplace variable s) and then taking it from there. 
This yields an IIR filter instead of a FIR filter. An IIR filter has the 
advantage of needing very few coefficients (i.e. multiplications, 
additions, and memory) compared to FIR filters which makes them more 
suitable for resource contrained environments.
For FIR filter design there's support in Matlab (e.g. the fir1 function) 
and maybe in free packages like Octave or Scilab, too.

	Thomas