Time FFT function calculates change of signal’s frequency components through time. The signal
is divided in smaller equally spaced segments (possibly overlapped), an FFT for each segment is
calculated and all FFT sequences are used to build a 3D graphic. The same calculation method is
used for Spectrogram view. FFT calculation parameters depend on your current Spectral
analysis defaults . You can change
the parameters for the calculated Time-FFT anytime by choosing Edit/Properties option from the
main menu or Properties from the context menu.
By choosing 3D-Tools/Time-FFT option from the main menu, you can set Time FFT parameters,
calculate it and display 3D graphics.
Time FFT parameters
FFT segment length in samples: Length of a signal segment for the FFT calculation
Number of FFT segments: Number of signal segments. It can be calculated from the segment
length and offset parameters.
Offset between segments: Distance (in samples) between beginnings of consecutive signal
segments. It can be calculated from FFT segment length and the number of FFT segments.
Frequency range: Frequency range you would like to see in your graphics.
Special case of Time-FFT analysis is Spectrogram – it is calculated in a same way as Time-FFT,
except the 3D-graphics is shown from above and colored so that the amplitude differences are
color-coded (same amplitude always has the same color). Choosing 3D tools/Spectrogram option
from the main menu brings up the same dialog as in the case of Time-FFT, except that resulting
graphics is shown as Spectrogram.
Time axis is always shown from left to right (X-axis). The direction of the frequency axis (Y-axis)
can be changed by using main menu option "3D Tools/Spectrogram frequency direction".
Editing Time-FFT/Spectrogram properties
After creating spectrogram or Time-FFT window, you can change its parameters (FFT length,
number of segments, etc.) by choosing "Time-FFT properties..." option from its context-menu.
There is even a faster way to change properties of the Time-FFT/Spectrogram window by using a
Ctrl + mouse wheel:
· Moving it to the front (up) will increase time resolution and decrease frequency resolution by a factor of 2
· Moving it back (down) will decrease time resolution and increase frequency resolution
by a factor of 2