Re: Waterfall display slightly offset from actual signal strength display
Neil Smith G4DBN
On my main waterfall screen, I have this, showing 11.7 Hz per point (what I called FFT bin size):
and on the IF screen on my SW2, is says 2.9 Hz/point
The fast fourier transform calculation takes the full bandwidth of the digital stream coming from the FPGA front end and makes a calculation of the amount of energy in a specific frequency range (the "bin size" I mentioned) over a specified time period. The math(s) behind it is one of the wonders of the world. There are subtleties around the overlap with adjacent bins.
Usingf MATLAB or Octave, you can play with the parameters, but a simple example is if you generate a couple of tones at 50 Hz and 120 Hz, then add noise, like this:
Fs = 1000; % Sampling frequency
you get a signal like this:
and it is hard to see the two frequencies from that time-domain signal.
However, if you run a fast fourier transform on the signal at the sampling rate, you can calculate the amount of signal in each increment of frequency:
Y = fft(X);
and you end up with a spectrum that you can use to create the intensities of the display on one line of a waterfall. The size of each bin depends on how fast you are taking the samples. The faster you sample, the wider each bin becomes.
The whole subject area is fascinating, and the math(s) gets intricate, although the solutions in sotware have probably had more academic effort expended on improving the speed and efficiency of processing than almost any other set of algorithms.
I'm sure someone who knows this stuff better could offer a more
cogent explanation. There is a nice fun read at
if you have a spare couple of hours!
On 23/05/2021 16:41, Ron Hunsicker via groups.io wrote:
Neil, thanks, an explanation that I found interesting--"Oh, that's why it does that!"
-- Neil http://g4dbn.uk