Walsh function

In mathematical analysis, the set of Walsh functions form an orthogonal basis of the square-integrable functions on the unit interval. The functions take the values -1 and 1 only, on sub-intervals defined by dyadic fractions. They are useful in electronics, and other engineering applications.

The orthogonal Walsh functions are used to perform the Hadamard transform, which is very similar to the way the orthogonal sinusoids are used to perform the Fourier transform.

The Walsh functions are related to the Haar functions; both form a complete orthogonal system. The Haar function system may on the one hand be preferablebecause of its wavelet properties (e.g. localization), on theother hand the Walsh functions are bounded (in fact of modulus1 everywhere).

The order of the function is 2^s, where "s" is an integer, meaning that there is 2^s (time-)intervals in which the value is -1 or 1.

A list of the 2^"s" Walsh functions make a Hadamard matrix.

One way to define Walsh functions is using the binary digit representations of reals and integers. For an integer "k" consider the binary digit representation

:"k" = "k"₀ + "k"₁2+...+"k"_"m"2^"m",

for some integer "m", and with "k"_"i" equal to 0 or 1. Then if "k" is the Gray code transform of "j-1",the "j"-th Walsh function at a point "x", with 0 &le; x < 1, is

:"wal" _"j"(x) = (-1)^{(k₀x₀+...k_mx_m),}

if

:"x" = "x"₀/2+ "x"₁/2² + "x"₂/2³+...,

where again "x"_"i" is 0 or 1 (only finitely often 1, if "x" is a rational number).

Walsh functions can be interpreted as the characters of

:(Z₂)^N,

the group of sequences over Z₂; using this viewpoint, several generalizations have been defined.

Applications (in mathematics) can be found wherever digit representations are used, e.g. in the analysis of digital quasi-Monte Carlo methods.

Walsh functions are used in Radio Astronomy to reduce the effects of electrical crosstalk between antenna signals.

External links

* [http://mathworld.wolfram.com/WalshFunction.html Walsh functions at MathWorld]
* [http://sepwww.stanford.edu/public/docs/sep70/carlos1/paper_html/node5.html Walsh functions at Stanford Exploration Project]
*Joseph Leonard Walsh