Re: Rational resample before FFT, and FFT rate

[John Ackermann N8UR]

On 4/30/24 18:35, Gary Schafer wrote:

> "I need data points at convenient intervals for time series plotting, 
> e.g., 512 samples/second going into a 512 bin FFT to provide one maximum 
> amplitude value per second."
>
> Let me answer that one at the same time as "Ultimately I want to plot 
> both the amplitude and any frequency change to sub-Hz resolution."
>
> Okay, you've hit a contradiction. You can NOT get "sub-Hz" resolution 
> using a sample rate, in Hz, that equals your time record size, in 
> samples.

Hi Gary -- sorry if my message was confusing.  I do understand the 
relationships between sample rate, FFT depth, time resolution, and 
frequency resolution.  I used 512 samples/512 bins/1 second as a simple 
test case for the problem that I'm not getting one data point per frame, 
regardless of the frame rate.  Given how noisy this data is, I may well 
end up with something like one frame per 10 seconds, but before I can 
play with that I need to understand what is wrong with my processing to 
get the amplitude of the maximum bin per frame.

John