Re: Rational resample before FFT, and FFT rate

Okey-doke! Good luck!

Gary

On 5/3/24 10:54, John Ackermann N8UR wrote:
> (Off list)
>
> Hi Gary --
>
> I didn't want you to think I've given up on this.  I've decided that it might be simpler just to use Python and am working through the really excellent PySDR tutorials to get started building the program up a step at a time.
>
> Those appear to use numpy for all the processing.  I don't know if I'll need to get into scipy for some of the fancier stuff.
>
> Thanks for your help!
>
> John
> ----
> On 4/30/24 21:40, Gary Schafer wrote:
>> "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