Introduction to the Convolution
http://www.youtube.com/watch?v=IW4Reburjpc&feature=player_embedded
The Fourier Transform
http://www.youtube.com/watch?v=ObklYbQaX24&feature=player_embedded
Discrete LTI system
http://s-mat-pcs.oulu.fi/~ssa/ESignals/sig3_2.htm