Linear FFT amplitude is incorrect

[philippitts]

Problem

The linear FFT amplitude is calculated incorrectly in the FFT Plot widget.

Expected

In the above example, the time series peak to peak value is measured at +/- 1500uV. The plot indicates a peak at 800uV, but the peak should be at 1500uV.

Operating System and Version

Windows 10

GUI Version

5.2.2

Are you running the downloaded app or from Processing/VSCode?

Downloaded app

Type of OpenBCI Board

Ganglion

[bobthefairycat]

The DFT computed is likely using the exponential representation of the Fourier series, the coefficients on the negative values of frequency mirror those on the positive side, e.g.: https://www.thefouriertransform.com/series/cosineExample.php

You would expect the positive side to be half the actual amplitude, since the original signal is constructed as a sum including the component from the negative side.