Why is there a discrepancy between Spectrum channel calculations and Data Pad calculations?

When viewing a spectrum value channel calculation and comparing it to the Data Pad spectrum calculation for a selection, there may appear to be a discrepancy between the two values. This discrepancy is due to the FFT size and selection made. 

The spectrum channel calculation takes a certain number of data points at a time, starting from the very beginning of the recording. For example, for an FFT size of 4k, the program will take the first 4096 data points in the data file and perform the FFT analysis on those data points. Then it will move to the next 4096 data points, and so on. If you have selected an FFT overlap option, such as 50%, each subsequent FFT selection block will begin about 2000 data points after the previous block, so the channel calculation will give a new spectral value about every 2000 data points. The image below includes boxes showing what one FFT analysis duration might be for a 4k FFT size. This file was recorded at 1k/s, so one FFT block would be roughly 4 seconds. With 50% overlap, the next FFT block would begin 2 seconds later:


In other words, the first data point in channel 2 in the screenshot above would be an analysis of samples 0-4096 in channel 1. The next block would use samples 2048-6144. The third block would use 4096-8192, and so on.

In contrast, the Data Pad spectrum calculation starts its analysis at the first point in the selection made. As an example, for a selection started at 3:26 as shown above, the data point at 3:26 would be the first data point in the FFT analysis block analyzed in the Data Pad spectral analysis. However, for the Spectral analysis performed in channel 2, the value at 3:26 includes some of an FFT analysis block that began at 3:24. These data points would not be a part of the Data Pad analysis because they precede the selection.

Note that the Data Pad Spectrum calculation settings, such as FFT size and Overlap, are configured in the Window > Spectrum View panel.