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Discrete fourier Transforms
Introduction The Discrete Fourier Transform is a periodic and the inverse of the
Disrete Fourier Transform is also periodic. Considering an non periodic
time domain function and its related nonperiodic continuous Fourier Tranform.
If the time domain function is sampled a set of discrete values results.
If this data is transformed then a periodic frequency domain function results.
This data is again in discrete form and the inverse transform of this is a periodic
function. The figure below show a typical aperiodic waveform and its Fourier transform. The physical sampling function is mathematical equivalent to mutliplying the function with a Dirac Comb  III_{T}( i ) function . Dirac comb. This function and its Fourier Transform is shown below. The sampling interval is T and the sampled function results as follows. The graphical result of this product operation in the time domain and the resulting convolution operation in the frequency domain is shown below. It should be noted the selection of T relates to the alias effect where, in the frequency domain, the adjacent periodic waveforms overlap and signal information is corrupted an lost. Ideally T should be selected such that at a k value of 1/2T the amplitude is effectively zero. The sampling operation does not involve an infinite number of samples. It results from taking N samples over a sample period T_{0} = N.T. This is mathematically equivalent to truncating the samples using a top hat function. Top hat with a width of T_{0} and a height 1. This is also called a pulse function. This operation is mathematically represented as shown below The graphical result of this operations is shown below. It is clear that the larger the value T_{0} the narrower the frequency domain function and the less the corruption of the signal. Ideally, stating the obvious, selecting an infinite T_{0} and a zero T would clearly result in a Discrete Fourier Transform which would be exactly equal to a continuous Fourier transform In practice sample values comprising the discretised time domain function are tranformed to Discrete Fourier Transforms not Continuous Fourier transforms as shown above. The Discrete Fourier transform is simply the product of the Continuous Fourier Transform and Dirac Comb  IIIa( 1/T_{0} ) function as illustrated below. The product of the two transforms equates to the convolution of the related time domain functions transforming the original aperiodic function to a periodic function as shown below . The time domain function can be expressed as below Now to derive the frequency domain function from the periodic time domain function. For a periodic time domain function the frequency domain function is a series of pulses. The factor c_{n} is developed as follows The integration is completed over one period and therefore the equation can be simplified to. Because T_{0} = N.T .The equation can be written The resulting fourier Transform is It can be proved that there are only N distint values computable from this and the equation can be therefore developed into the form. 
To be continued....
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Last Updated 14/03/2009