Fourier analysis

The Fourier transform, named after Joseph Fourier, is a reversible integral transform of one function into another. The second function, which is called a Fourier transform, gives the coefficients of sinusoidal basis functions (vs. their frequencies) whose linear combination (summation or integral) produces the original function. That recombination of sinusoidal basis functions is called an inverse Fourier transform.

The Fourier transform has several specific variations, depending upon the type of function being transformed. These are described below. See also: List of Fourier-related transforms.

Applications

Fourier transforms have many scientific applications — in physics, number theory, combinatorics, signal processing, probability theory, statistics, cryptography, acoustics, oceanography, optics and diffraction, geometry, and other areas. (In signal processing and related fields, the Fourier transform is typically thought of as decomposing a signal into its component frequencies and their amplitudes.) This wide applicability stems from several useful properties of the transforms:

• The transforms are linear operators and, with proper normalization, are unitary as well (a property known as Parseval's theorem or, more generally, as the Plancherel theorem, and most generally via Pontryagin duality).
• The transforms are invertible, and in fact the inverse transform has almost the same form as the forward transform.
• The exponential basis functions are eigenfunctions of differentiation, which means that this representation transforms linear differential equations with constant coefficients into ordinary algebraic ones. (For example, in a linear time-invariant physical system, frequency is a conserved quantity, so the behavior at each frequency can be solved independently.)
• By the convolution theorem, Fourier transforms turn the complicated convolution operation into simple multiplication, which means that they provide an efficient way to compute convolution-based operations such as polynomial multiplication and multiplying large numbers.
• The discrete version of the Fourier transform (see below) can be evaluated quickly on computers using fast Fourier transform (FFT) algorithms.

Variants of the Fourier transform

Continuous Fourier transform

Most often, the unqualified term "Fourier transform" refers to the continuous Fourier transform, representing any square-integrable function $s\left(t\right)$ as a linear combination of complex exponentials with frequencies $\omega \,$:

$s\left(t\right)={\frac {1}{\sqrt {2\pi }}}\int \limits _{-\infty }^{\infty }S\left(\omega \right)e^{i\omega t}\,d\omega .$

The quantity, $S(\omega )\,$, provides both the amplitude and initial phase (as a complex number) of basis function: $e^{i\omega t}\,$.

The function, $S(\omega )\,$, is the Fourier transform of $s(t)\,$, denoted by the operator ${\mathcal {F}}\,$:

$S(\omega )=\left({\mathcal {F}}s\right)(\omega )={\mathcal {F}}\{s\}(\omega )\,$

And the inverse transform (shown above) is written:

$s(t)=\left({\mathcal {F}}^{-1}S\right)(t)={\mathcal {F}}^{-1}\{S\}(t)\,$

Together the two functions are referred to as a transform pair. See continuous Fourier transform for more information, including:

• formula for the forward transform
• tabulated transforms of specific functions
• discussion of the transform properties
• various conventions for amplitude normalization and frequency scaling/units

Multi-dimensional version

The formulation for the Fourier transform given above applies in one dimension. The Fourier transform, however, can be expanded to arbitrary dimension $n$. The more generalised version of this transform in dimension $n$, notated by ${\mathcal {F}}_{n}$ is:

$s\left(\mathbf {x} \right)=\left({\mathcal {F}}_{n}^{-1}S\right)\left(\mathbf {x} \right)={\frac {1}{(2\pi )^{n/2}}}\int S\left({\boldsymbol {\omega }}\right)e^{i\left\langle {\boldsymbol {\omega }},\mathbf {x} \right\rangle }\,d{\boldsymbol {\omega }},$

where $\mathbf {x}$ and ${\boldsymbol {\omega }}$ are $n$-dimensional vectors, $\left\langle {\boldsymbol {\omega }},\mathbf {x} \right\rangle$ is the inner product of these two vectors, and the integration is performed over all $n$ dimensions.

Fourier series

The continuous transform is itself actually a generalization of an earlier concept, a Fourier series, which was specific to periodic (or finite-domain) functions $s\left(t\right)$ (with period $\tau \,$), and represents these functions as a series of sinusoids:

$s(t)=\sum _{k=-\infty }^{\infty }S_{k}\cdot e^{i\omega _{k}t}\,$,

where $\omega _{k}=2\pi k/\tau \,$, and $S_{k}\,$ is a (complex) amplitude.

For real-valued $s(t)\,$, an equivalent variation is:

$s(t)={\frac {1}{2}}a_{0}+\sum _{k=1}^{\infty }\left[a_{k}\cdot \cos(\omega _{k}t)+b_{k}\cdot \sin(\omega _{k}t)\right],$

where $a_{k}=2\cdot \operatorname {Re} \{S_{k}\}\,$   and   $b_{k}=-2\cdot \operatorname {Im} \{S_{k}\}\,$.

Discrete-time Fourier transform

For use on computers, both for scientific computation and digital signal processing, one must have functions, x[n], that are defined for discrete instead of continuous domains, again finite or periodic. A useful "discrete-time" function can be obtained by sampling a "continuous-time" function, x(t). And similar to the continuous Fourier transform, the function can be represented as a sum of complex sinusoids:

$x[n]={\frac {1}{2\pi }}\int _{-\pi }^{\pi }X(\omega )\cdot e^{i\omega n}\,d\omega$

But in this case, the limits of integration need only span one period of the periodic function, $X(\omega )\,$, which is derived from the samples by the discrete-time Fourier transform (DTFT):

$X(\omega )=\sum _{n=-\infty }^{\infty }x[n]\,e^{-i\omega n}\,$

Discrete Fourier transform

The DTFT is defined on a continuous domain. So despite its periodicity, it still cannot be numerically evaluated for every unique frequency. But a very useful approximation can be made by evaluating it at regularly-spaced intervals, with arbitrarily small spacing. Due to periodicity, the number of unique coefficients (N) to be evaluated is always finite, leading to this simplification:

$X[k]=X\left({\frac {2\pi }{N}}k\right)=\sum _{n=-\infty }^{\infty }x[n]\,e^{-i2\pi {\frac {k}{N}}n}$,     for $k=0,1,\dots ,N-1\,$

When the portion of x[n] between n=0 and n=N-1 is a good (or exact) representation of the entire x[n] sequence, it is useful to compute:

$X[k]=\sum _{n=0}^{N-1}x[n]\,e^{-i2\pi {\frac {k}{N}}n}$,

which is called discrete Fourier transform (DFT). The inverse DFT represents x[n] as the sum of complex sinusoids:

$x[n]={\frac {1}{N}}\sum _{k=0}^{N-1}X[k]e^{i2\pi {\frac {k}{N}}n}\quad \quad n=0,1,\dots ,N-1\,$

The table below will note that this actually produces a periodic x[n]. If the original sequence was not periodic to begin with, this phenomenon is the time-domain consequence of approximating the continuous-domain DTFT function with the discrete-domain DFT function.

Fast Fourier transform

Computing a DFT directly requires O(N²) operations (see Big O notation). But it can be computed in only O(N log N) operations using a fast Fourier transform (FFT) algorithm, which makes the FFT a practical and important transformation on computers.

Other variants

The Fourier variants can also be generalized to Fourier transforms on arbitrary locally compact abelian topological groups, which are studied in harmonic analysis; there, the Fourier transform takes functions on a group to functions on the dual group. This treatment also allows a general formulation of the convolution theorem, which relates Fourier transforms and convolutions. See also the Pontryagin duality for the generalized underpinnings of the Fourier transform.

Time-frequency transforms such as the short-time Fourier transform, wavelet transforms, chirplet transforms, and the fractional Fourier transform try to obtain frequency information from a signal as a function of time (or whatever the independent variable is), although the ability to simultaneously resolve frequency and time is limited by an (mathematical) uncertainty principle.

Family of Fourier transforms

The following table summarizes the family of Fourier transforms. We see

• that discreteness in one domain implies periodicity in the opposite transformed domain and the converse is true.
• that continuity in one domain implies aperiodicity in the transformed domain and the converse is true.
• pure reality in one domain (i.e. imaginary part is zero everywhere) implies conjugate symmetry in the transformed domain and the converse is true.

Transform	Time domain	Frequency domain
Continuous Fourier transform	Continuous, Aperiodic	Continuous, Aperiodic
Fourier series	Continuous, Periodic	Discrete, Aperiodic
Discrete-time Fourier transform	Discrete, Aperiodic	Continuous, Periodic
Discrete Fourier transform	Discrete, Periodic	Discrete, Periodic

Interpretation in terms of time and frequency

In terms of signal processing, the transform takes a time series representation of a signal function and maps it into a frequency spectrum, where ω is angular frequency. That is, it takes a function in the time domain into the frequency domain; it is a decomposition of a function into harmonics of different frequencies.

When the function f is a function of time and represents a physical signal, the transform has a standard interpretation as the frequency spectrum of the signal. The magnitude of the resulting complex-valued function F at frequency ω represents the amplitude of a frequency component whose initial phase is given by: arctan (imaginary part/real part).

However, it is important to realize that Fourier transforms are not limited to functions of time, and temporal frequencies. They can equally be applied to analyze spatial frequencies, and indeed for nearly any function domain.

Applications in signal processing

In signal processing, Fourier transformation can isolate individual components of a complex signal, concentrating them for easier detection and/or removal. A large family of signal processing techniques consist of Fourier-transforming a signal (such as a clip of audio or an image), manipulating the Fourier-transformed data in a simple way, and reversing the transformation. Some examples include:

• Removal of unwanted frequencies from an audio recording (used to eliminate hum from leakage of AC power into the signal, to eliminate the stereo subcarrier from FM radio recordings, or to create karaoke tracks with the vocals removed);
• Noise gating of audio recordings to remove quiet background noise by eliminating Fourier components that do not exceed a preset amplitude;
• Equalization of audio recordings with a series of bandpass filters;
• Digital radio reception with no superheterodyne circuit, as in a modern cell phone or radio scanner;
• Image processing to remove periodic or anisotropic artifacts such as jaggies from interlaced video, stripe artifacts from strip aerial photography, or wave patterns from radio frequency interference in a digital camera;
• Cross correlation of similar images for co-alignment;
• X-ray crystallography to reconstruct a protein's structure from its diffraction pattern;
• Fourier transform ion cyclotron resonance mass spectrometry to determine the mass of ions from the frequency of cyclotron motion in a magnetic field.

Fourier transformation is also useful as a compact representation of a signal. For example, JPEG compression uses Fourier transformation of small square pieces of a digital image. The Fourier components of each square are rounded to lower arithmetic precision, and weak components are eliminated entirely, so that the remaining components can be stored very compactly. In image reconstruction, each Fourier-transformed image square is reassembled from the preserved approximate components, and then inverse-transformed to produce an approximation of the original image.

References

• E. M. Stein, G. Weiss, "Introduction to Fourier Analysis on Euclidean Spaces", Princeton University Press, 1971. ISBN: 0-691-08078-X
• A. D. Polyanin and A. V. Manzhirov, Handbook of Integral Equations, CRC Press, Boca Raton, 1998. ISBN 0-8493-2876-4
• Smith, Steven W. The Scientist and Engineer's Guide to Digital Signal Processing, 2nd edition. San Diego: California Technical Publishing, 1999. ISBN 0-9660176-3-3. (also available online: [1])

See also

• fractional Fourier transform
• Bispectrum
• Characteristic function (probability theory)
• Chirplet
• Number-theoretic transform
• Laplace transform
• Mellin transform
• Orthogonal functions
• Pontryagin duality
• Schwartz space
• Two-sided Laplace transform
• Wavelet

External links

• Tables of Integral Transforms at EqWorld: The World of Mathematical Equations.
• An Intuitive Explanation of Fourier Theory by Steven Lehar.