Free Books

DFT Sinusoids

The sampled sinusoids generated by integer powers of the $ N$ roots of unity are plotted in Fig.6.2. These are the sampled sinusoids $ (W_N^k)^n = e^{j 2 \pi k n / N} = e^{j\omega_k nT}$ used by the DFT. Note that taking successively higher integer powers of the point $ W_N^k$ on the unit circle generates samples of the $ k$th DFT sinusoid, giving $ [W_N^k]^n$, $ n=0,1,2,\ldots,N-1$. The $ k$th sinusoid generator $ W_N^k$ is in turn the $ k$th $ N$th root of unity ($ k$th power of the primitive $ N$th root of unity $ W_N$).

Figure 6.2: Complex sinusoids used by the DFT for $ N=8$.

Note that in Fig.6.2 the range of $ k$ is taken to be $ [-N/2,N/2-1] = [-4,3]$ instead of $ [0,N-1]=[0,7]$. This is the most ``physical'' choice since it corresponds with our notion of ``negative frequencies.'' However, we may add any integer multiple of $ N$ to $ k$ without changing the sinusoid indexed by $ k$. In other words, $ k\pm
mN$ refers to the same sinusoid $ \exp(j\omega_k nT)$ for all integers $ m$.

Next Section:
Notation and Terminology
Previous Section:
Nth Roots of Unity