# DFT from Fourier transform

I always studied the DFT starting from his formula, but for some reasons I need to do comparison between the FT and the DFT. I found the pdf in this link very useful http://www.robots.ox.ac.uk/~sjrob/Teaching/SP/l7.pdf because it explains how the DFT can be obtained starting from the FT. After the first formula it says " We could regard each sample f[k] as an impulse having area f[k]". What I don't understand is how it possible that the sample, being an impulse, has an area f[k]. Shouldn't t have an area f[k] multiplied by a very low number? This passage for me is crucial because I cannot understand why, passing from the FT to the DFT, the time increment disappears.

• riemann integration. then replace each skinny rectangle with an impulse having the same area. – robert bristow-johnson Jan 5 at 15:37
• I am not sure to have understood. So, the time increament disappears beacuse it is inside f[k]? If the area is known then how can I calculate the height of the impulse? – Ashish Bhigah Jan 5 at 15:56
• @AshishBhigah: Well, the scaling in that answer is correct, just check the plot comparing the FT with its DFT approximation. – Matt L. Jan 5 at 17:07
• @AshishBhigah: You have to decide what you want. If you want to approximate the CT Fourier transform by the DFT you need to choose the scaling used in the answer I linked to. When you look at the plot of that answer, do the values at DC coincide or don't they? Did you follow the derivation? It is assumed that the DFT is defined without any scaling (and the corresponding inverse DFT would have a scaling of $1/N$). – Matt L. Jan 5 at 17:32
• @AshishBhigah: The DC value of the CT Fourier transform is just the integral $\int x(t)dt$ (which if the integration limits are finite is a scaled average). Now approximate that by a Riemann sum and you'll see that you get a $\Delta t$ (which just corresponds to the $dt$ in the integral). – Matt L. Jan 5 at 18:29