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Converting From A Normalized To Digital LowPass Filter
Syntax fn2dlp(cutoffdtnumindeninnumoutdenout)
See Also fnbut , fncheb , fn2clp

Description
Converts a normalized filter to a digital lowpass filter. The real or double-precision scalar cutoff specifies the cutoff frequency for the lowpass filter. The scalar dt specifies the time between samples and has the same type as cutoff. The column vector numin has the same type as cutoff and specifies the numerator polynomial for the normalized filter. The column vector denin has the same type as cutoff and specifies the denominator polynomial for the normalized filter.

The input values of numout and denout have no effect. The output value numout is set to the column vector representing the numerator polynomial for the digital lowpass filter. The output value denout is set to the column vector representing the denominator polynomial for the digital lowpass filter. The output values of numout and denout have the same type as cutoff.

The response of the normalized filter is
               2
     |numin[s]|
     |--------|
     |denin[s]|

and it is near 1 for s in the interval
           __
     [0, \/-1]

and near 0 for the rest of the positive imaginary axis. The response of the digital lowpass filter is

     |                __           |2
     | numout[ exp( \/-1 dt w ) ]  |
     -------------------------------
     |                __           |2
     | denout[ exp( \/-1 dt w ) ]  |

and it is near 1 for w in the interval [0, cutoff(2)] and near 0 for other w in the interval [0, pi / dt].

Example
     numin   = 1.
     denin   = {1., sqrt(2.), 1.}
     cutoff  = 2.
     dt      = .2
     numout  = novalue
     denout  = novalue
     fn2dlp(cutoff, dt, numin, denin, numout, denout)
     
     ymin = 1e-5
     ymax = 1e+1
     fdplot(dt, ymin, ymax, cutoff, numout, denout)
     
returns the following plot: