OAT Spectral Map Unexpected Output

Issue Details

I use the OAT Spectral Map in my VI. The input signal is the same each time I run the code but I always get different outputted data when I change the "Window length" input.


I can't understand the reason, because the signal is the same and it should have the same specter all the time.

Solution

The OAT Spectral Map VI is one of the functions of the Sound and Vibration Toolkit. 
It computes a spectral map of the scaled signal [EU] as a function of time or rotational speed. 
The output of this VI can differ dependent on the input data. One of the inputs that plays a role in the outputted data is the "Window length". 
"Window length" specifies the length of the window to apply the colormap. It must be a power of two greater than or equal to 128 and less than or equal to 2048. Increasing the value of "Window length" can improve the frequency or order resolution but decreases the time or speed resolution. The default value is 2048 in order to provide the best possible resolution of time or speed. 
When the selected plot type is Frequency-Time, Time-Frequency, Frequency-RPM, or RPM-Frequency, "Window length" equals half of the frequency bins. If the plot type is Time-Order, Order-Time, RPM-Order, or Order-RPM, "Window length" equals half of the order bins. 
When we change the "Window length" we change the amount of data to be monitored, although the signal is the same its spectrum can differ as there is changed the time of monitoring.