
Sampling Rate: For the spectrum plot, the "Software Sampling Rate" is necessary. Sometimes the hardware offers
sampling rates not very suitable for software applications. For instance, DAB demodulations needs I/Q data having been sampled
with 2.048Msps, which not every hardware is able to deliver. The Airspy can send data sampled with 4.096Msps, which have to
be "decimated" by a factor of two before processing. "Decimation" means filtering and downsampling. You find the
two sampling rates in use on the "Connection" tab of the Frontend section on the GUI.
If you are in doubt whether a certain sampling rate can be used in your scenario, please counsult the
Setup Dialog on this website.

FFT Length: "FFT" means "Fast Fourier Transform" and indicates the algorithm used to transform
the I/Q data from the time domain to the frequency domain. The frquency spectrum shows the result of the FFT.
"FFT Length" indicates the number of I/Q data used to perform the FFT. This number determines  together with
the Sampling Rate  the frequency width of a single point of the FFT outcome (usually called a "bin"). The simple formula is:
Frequency Resolution fRes = Sampling Rate / FFT Length.
In our highres example with the Doppler Shifts we get 2,048,000[1/sec]/2,097,152[bin] = 0.977/sec/bin ~ 1Hz/bin.
For convenience, this calculation is also performed in the software and the result is indicated as an inset into the spectrum plot.

FFT Length Selection: The FFT length can be selected from the corresponding dropdown box.
There is no such thing as a free lunch. Usually, one does not want to set the frequency display to an extremely high resolution like the one
above in the Doppler shift example. The reason is simple: The higher the FFT length, the longer it takes to collect the I/Q data
to fill the memory buffer for the FFT. In our above example
it already takes one second to fill the FFT buffer once. This results in some disadvantages:

Slow Display: The spectrum display is slowed down. Of course, this holds also for the waterfall display.

No Realtime: As a result, one loses the responsiveness of the spectrum. The longer the FFT length, the worse
is the realtime behaviour of the display.
The tradeoff between high timely resolution (responsiveness) and high frequency resolution (high FFT length) is a wellknown
fundamental property of every Fourier Transform.