Re: Rational resample before FFT, and FFT rate

[Gary Schafer]

"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."

Sorry about my misunderstanding. I recreated a portion of your flowgraph just 
to see what it would do. I left the 16 kHz sample rate but with a 2^15 time 
record size. Once I ran the flowgraph, it was 17.5 seconds before the Number 
Sink updated, and 34 seconds before the spectral display (vector sink) updated. 
However, after that time, each update roughly twice per second. Is that 
different from what you're seeing?

Gary

On 4/30/24 20:17, John Ackermann N8UR wrote:
> 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

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