### Rectangular Window Side-Lobes

From Fig.3.3 and Eq. (3.4), we see that the main-lobe width is radian, and the side-lobe level is 13 dB down.

Since the DTFT of the rectangular window approximates the sinc function (see (3.4)), which has an amplitude envelope proportional to (see (3.7)), it should ``roll off'' at approximately 6 dB per octave (since ). This is verified in the log-log plot of Fig.3.6.

As the sampling rate approaches infinity, the rectangular window
transform (
) converges exactly to the
sinc
function.
Therefore, the departure of the roll-off from that of the
sinc
function can be ascribed to *aliasing* in the frequency domain,
due to sampling in the time domain (hence the name ``
'').

Note that each side lobe has width
, as
measured between zero crossings.^{4.3} The main lobe, on the other hand, is
width
. Thus, in principle, we should never confuse
side-lobe peaks with main-lobe peaks, because a peak must be at least
wide in order to be considered ``real''. However, in
complicated real-world scenarios, side-lobes can still cause
estimation errors (``bias''). Furthermore, two sinusoids at closely
spaced frequencies and opposite phase can partially cancel each
other's main lobes, making them appear to be narrower than
.

In summary, the DTFT of the -sample rectangular window is proportional to the `aliased sinc function':

Thus, it has zero crossings at integer multiples of

(4.11) |

Its main-lobe width is and its first side-lobe is 13 dB down from the main-lobe peak. As gets bigger, the main-lobe narrows, giving better

*frequency resolution*(as discussed in the next section). Note that the window-length has

*no effect*on side-lobe level (ignoring aliasing). The side-lobe height is instead a result of the abruptness of the window's transition from 1 to 0 in the time domain. This is the same thing as the so-called

*Gibbs phenomenon*seen in truncated Fourier series expansions of periodic waveforms. The abruptness of the window discontinuity in the time domain is also what determines the side-lobe roll-off rate (approximately 6 dB per octave). The relation of roll-off rate to the smoothness of the window at its endpoints is discussed in §B.18.

**Next Section:**

Rectangular Window Summary

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Zero-Phase Zero Padding