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2-norm or infinity-norm of digital filter

`L = filternorm(b,a)L = filternorm(b,a,pnorm)`

A typical use for filter norms is in digital filter scaling to reduce quantization effects. Scaling often improves the signal-to-noise ratio of the filter without resulting in data overflow. You also can use the 2-norm to compute the energy of the impulse response of a filter.

`L = filternorm(b,a)` computes
the 2-norm of the digital filter defined by the numerator coefficients
in `b` and denominator coefficients in `a`.

`L = filternorm(b,a,pnorm)` computes
the 2- or infinity-norm (inf-norm) of the digital filter, where `pnorm` is
either 2 or `inf`.

` L = filternorm(b,a,2,tol)` computes
the 2-norm of an IIR filter with the specified tolerance,

[1] Jackson, L. B. *Digital Filters
and Signal Processing: with MATLAB Exercises*. 3rd Ed.
Hingham, MA: Kluwer Academic Publishers, 1996, Chapter 11.

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