Transforms to and from the Frequency Domain - Digital Video and Audio Broadcasting Technology - page 107

ple of the period within the window of observation; in between that it will
become smaller and assume the exact value time and again. This is called
the “picket fence” effect (Fig. 6.19.).
The fluctuation in the amplitude of the spectral line is caused by a dis-
persal of the energy due to a widening of the main lobe and by the appear-
ance of sidelobes (Fig. 6.20.).
In addition, aliasing products may appear if the measurement signal is
not properly band-limited; moreover quantization noise becomes visible
and will limit the dynamic range.
These systematic errors can be prevented or suppressed by program-
ming an observation time of corresponding length, by good suppression of
aliasing products and by using A/D converters having a correspondingly
high resolution. In the next section, “windowing” will be discussed as a
further aid in suppressing DFT system errors.

u(t)
original signal

u(t)
Original signal

k(t)
window function
(e.g. Hanning)

k(t)
Window function
(e.g. Hanning)

u'(t) = u(t)*k(t);
Windowed
time signal

u`(t) = k(t)u(t);
windowed time
domain signal

Fig. 6.21. Multiplying a signal by a window function