Digital Signal Processing Reference
In-Depth Information
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Illustration 51:
Analysis of a long-lasting, non-periodic signal
The non-periodicity is achieved here by using a noisy signal. This noisy signal is now filtered through a
high quality lowpass (with steep edges) with the cut-off frequency 50 Hz. This does not mean that this filter
does not let anything through above 50 Hz. These frequencies are damped to a greater or lesser degree
depending on the filter quality.
Here we are looking at the "blocking state region" above 50 Hz, beginning at 70 Hz. The upper four signal
segments contain considerably more or rather "stronger" frequency shares in this area than the overall
signal (bottom). Thus, "cutting out" partial segments produces frequencies which were not contained in the
original signal! And - the shorter the time segment the more uncertain the frequency band is. This can be
clearly seen by comparing the spectra of the last but one signal segment - which lasts longer - with the
four upper signal segments. Incidentally, here too the overall signal is analysed via the signal length
(= length of measurement) 1s.
In addition the "link" between the individual segments of the signal which have been
separated arbitrarily is lost. This may mean that information has been cut up. Information
consists of certain "agreed" patterns - see Chapter l - and therefore lasts a certain time. In
order to capture this information in its entirety the signal segments would as a precaution
have to overlap.
