Re: Trying to understand FFT
- Fine Wolf , understand.
I keep away from minus then.
--- In SpectrumLabUsers@yahoogroups.com, wolf_dl4yhf <dl4yhf@...> wrote:
> Hi Sven,
> (aargh.. the stupid 'Send' button should be disabled as long as no
> text has been typed - sorry for my 'empty response')
> Negative overlap indeed makes little sense, and (as a percentage) is a
> bit hard to explain.
> Actually this is just the result of a calculation.
> "Zero percent overlap" means that the time between two FFT
> calculations is exactly the same as the time required to acquire enough
> samples (to fill the input block for one FFT).
> Positive overlap means the FFT is calculated more often than the time
> required to acquire enough samples. Thus the samples used at the end of
> the previous FFT are used *again* at the beginning of the next FFT.
> Negative overlap actually means "no overlap", PLUS a longer gap between
> two FFTs. Example:
> 1024 points per FFT, 11025 samples per second -> 92 milliseconds to
> collect enough samples for one FFT.
> If the screen update interval (or "waterfall scroll interval") is larger
> than 92 ms, there will be samples taken from the soundcard (or similar)
> which do NOT enter the FFT at all, i.e. there's a gap, and you may lose
> information for the display.
> Hope this explains it, half way.
> All the best,
> Wolf .
> Am 21.07.2011 10:43, schrieb sm5le:
> > Hi Wolf
> > Q. abt " overlap % " :
> > I understand that I schall try to have about 50% overlap
> > when using waterfall.
> > But what happens when the overlap goes minus ( -% ) ?
> > Is -50% the same as +50% ?
> > I still have readings on waterfall when the overlap is minus.
> > Tnx Sven