Spectral analysis defaults |
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Most of spectral analysis tools in *SIGVIEW* are based on the FFT algorithm. There are many
different parameters which can be applied to the signal before the FFT analysis is performed, to
the FFT calculation itself or its result.

If you want to change the default settings for spectral analysis calculation in *SIGVIEW*, use *Signal
tools/Spectral analysis* defaults option from the main menu. These settings will be applied to all
new FFT-based calculations, including FFT, Time-FFT, cross spectral analysis, etc.

Once the FFT or Time FFT is already calculated according to your current spectral analysis
defaults, you can edit those anytime by choosing *Edit/Properties* menu option for calculated
window (FFT, Time FFT,…). The same option is also available in the window’s
context menu.

These settings will apply only to that single window.

The following parameters can be changed for each FFT based spectral analysis operation in *SIGVIEW*:

· **Never** use zero padding: FFT or slower DFT algorithm will be applied on your signal
without altering or extending it with zeros. This is the slowest calculation method, but
the advantage is that your signal is not changed in any way. For prime number signal
lengths the calculation will be performed by using very slow DFT algorithm.

· **Optimal** method: Your signal will be expanded with zeros to the next possible length
allowing the usage of the FFT algorithm instead of very slow DFT.

· **Next power of 2**: Your signal will be expanded with zeros until the next power-of-2
length. This will enable the usage of fastest FFT algorithm.

To increase the precision of the spectral analysis without taking longer signal segments, you can
also expand existing signal segment with zeros, even beyond the next power-of-2 length. That
will not add essentially new information in the FFT result, but will increase its precision, i.e. will
reduce the size of the one frequency bin. You will simply get FFT result with more points - from
the same signal segment. (If signal has 1024 samples and you choose expanding by factor 8,
resulting spectrum will have 4096 pt. instead of 512 without using option). The result will be
comparable to interpolation of the normal FFT result. To use this feature, you can choose **2x, 4x
or 8x** zero padding option.

· **Instantaneous spectrum (no averaging)**: Each change in the signal will cause the
FFT to recalculate. This is the default behavior.

· **Average last X spectrum results**: This option is useful only for the FFT of the live
input signal or other fast changing signals. Last X complex FFTs will be averaged to
calculate the final result. That will decrease the influence of the noise in the signal
and is usually recommended for all spectral analysis functions on the live signal.

· *Magnitude*: value generally regarded as “spectrum”; calculated as sqrt(R^2 + I^2)

· *Power spectrum*: Magnitude squared.

· *Power spectral density (PSD)*: Power spectrum is calculated first and its values are divided
with the width of the frequency bin. This “normalization” makes comparisons between FFT
sequences from different signals with different sampling rates possible.

· *Phase:* show signal phase angle (in degrees)

· *Real & Imaginary part*: show only real or only imaginary part of the spectrum