The (zero-centered) rectangular window may be defined by
To see what happens in the frequency domain, we need to look at the DTFT of the window:
where the last line was derived using the closed form of a geometric series:
The term ``aliased sinc function'' refers to the fact that it may be simply obtained by sampling the length- continuous-time rectangular window, which has Fourier transform sinc (given amplitude in the time domain). Sampling at intervals of seconds in the time domain corresponds to aliasing in the frequency domain over the interval Hz, and by direct derivation, we have found the result. It is interesting to consider what happens as the window duration increases continuously in the time domain: the magnitude spectrum can only change in discrete jumps as new samples are included, even though it is continuously parametrized in .
As the sampling rate goes to infinity, the aliased sinc function therefore approaches the sinc function
Figure 3.2 illustrates for . Note that this is the complete window transform, not just its real part. We obtain real window transforms like this only for zero-centered, symmetric windows. Note that the phase of rectangular-window transform is zero for , which is the width of the main lobe. This is why zero-centered windows are often called zero-phase windows; while the phase actually alternates between 0 and radians, the values occur only within side-lobes which are routinely neglected (in fact, the window is normally designed to ensure that all side-lobes can be neglected).
More generally, we may plot both the magnitude and phase of the window versus frequency, as shown in Figures 3.4 and 3.5 below. In audio work, we more typically plot the window transform magnitude on a decibel (dB) scale, as shown in Fig.3.3 below. It is common to normalize the peak of the dB magnitude to 0 dB, as we have done here.