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Chapitre 1-2 : Digital signal, impulse response, convolution and z-transform [french]
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Chapitre 3 : Sampling theorem [english]
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Chapitre 4 : Discrete fourier transform and fast fourier transform [french] [english]
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Chapitre 5 : Random signals [french]
Misc :
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Interactive fourier series (from falstad.com) : [link]