It is sometimes useful to extract only a ‘good’
parts of the signal and perform your spectral
analysis only on those while ignoring the rest. The manual extraction of those parts, analysis on
each of them and combination of the results can be a very demanding job. SIGVIEW includes
one option to make this job easier, enabling you to select ‘good’ segments from the signal and to
perform FFT analysis on them.
Choosing segments from the signal can be controlled through the options in signal’s context
menu (right-click menu), under "Segments" section:
Start segment here: marks the beginning of the new segment on the current signal ruler position
End segment here: marks the end of the new segment on the current signal ruler position
Create segment from selection: Creates a new segment directly from the current selection
Delete this segment: Deletes the segment currently under a signal ruler
Clear all segments: clears all selected segments from the current signal
FFT on selected segments: performs FFT on selected segments. This specific FFT
implementation performs FFT on overlapping segments inside each segment, calculates average
of all FFT results and displays the result in a new window.
Selected segments will be marked with green color behind the signal display. The beginning of a
new (not finished) segment will be shown as a thin green line.
Each signal with segments can be saved in file and reloaded with full segments information by
using “Save workspace…” or “Save as tool…” SIGVIEW option.
If a new segment overlaps with existing segments, SIGVIEW will simply merge both segments
into a one bigger segment. The same behavior is implemented if newly selected segment
contains several existing segments.
It is not possible to edit segment boundaries. You have to delete the segment and to define a new
segment with new boundaries.
Setting FFT parameters and performing FFT
SIGVIEW performs FFT on selected segments by calculating averaged FFT from smaller
overlapping blocks in each segment, and finally averaging all results from all segments into one
resulting FFT sequence.
After selecting all segments you would like to include in your analysis and choosing “FFT on
selected segments” option from context menu, dialog will appear with following fields:
FFT length: length for overlapping FFTs
Overlapping: 25%, 50%, 75%, None
FFT length field will include all possible FFT lengths considering the length of a smallest selected
segment in a signal (up to 131072 samples). Please note that FFT will be calculated for all
samples from each of selected segments, even if overlapping needs to be slightly adjusted. It
means that if you select Overlapping=50%, it could be changed to 48% or 45% for certain
segment to fit integer number of FFTs into the segments.
FFT window created with this option behaves exactly as any other SIGVIEW FFT window. You
can open its properties, set all spectral analysis parameters, or save it to by using “Save
workspace…” or “Save as tool…” options.
Each time you change segment selection in a signal, FFT will be recalculated and new graphics
will be shown.
You can create many FFT windows from the same signal segments at the same time but with
different FFT settings. This can be useful to test how different parameters affect the result.