Re: [Discuss-gnuradio] FFT Playback

[Marcus Müller]

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Hi Paul!

I'm gonna go ahead and rearrange parts of your message for the ease of
reply:
> My datafile was recorded for about 20 seconds at a sample rate of 
> 50Msps through a low-pass filter to only capture 0-30MHz.
50Msps * 20s = 1Gs (you're sure your file is 8GB in size?) should be
plenty enough, so my guess was totally astray.

> Looking at the configuration for my FFT, and changing 'Refresh
> Rate' from the default '15' to '1M' gives me some extra playback.
what gui toolkit are you using? wx or qt?
However, a refresh rate of one million frames per second... doesn't
even remotely make sense. Since there is no way that your file source
can supply 1024Msps for the GUI sink, even without throttling, I don't
know what the (wx or qt or whatever) FFT sink does in that case. I'd
have to read the source code...

> I like the way the waterfall looks, but I'm having the same issue
> as the FFT.

Ok, this is confusing. The waterfall sink should basically be able to
have one pixel of display per sample going in. And with 1 billion
samples going in, there shouldn't really be repitions visible, unless
your data is repetitive.

> Is there some correlation between data rate on the record and FFT 
> refresh rate?
No. Refresh rate is just the display refresh rate, i.e. the speed at
which the GUI updates its visualization.

> How is 'FFT Size' used (right now I have that at '1024')?
Um this is kinda awkward - it's the size of the FFT; how the samples
are internally processed prior to being transformed depends on the
actual FFT sink implementation.

> On a related note - is there a way to get the FFT to start out in
> 'Autoscale', or do I need to hit the button every time?

No. "Autoscale" is (as far as I know and can remember right now) not a
/mode/ for the qt nor the wx gui fft sink, but scales the display to
fit the data that is display /the moment the button is clicked/. So,
since data can only be available at run time, you can't do that before
running the flow graph. However, at least for the qt implementation,
you can set y max/y min, if you know it beforehand.

So: Verify your data is really about 1 billion samples long and not
only a few thousand; if your waterfall looks periodic, your recorded
signal is periodic.

Greetings,
Marcus

On 03.12.2013 22:45, Paul B. Huter wrote:
> 
> Is there some correlation between data rate on the record and FFT
> refresh rate? How is 'FFT Size' used (right now I have that at
> '1024')? On a related note - is there a way to get the FFT to start
> out in 'Autoscale', or do I need to hit the button every time?
> 
> Thanks!
> 
> 
> On Tue, Dec 3, 2013 at 3:01 PM, Marcus Müller <address@hidden>
> wrote:
> 
> Most probably your data is simply to short in relation to the fft 
> length and the amount of samples your specific graphical FFT
> amplitude sink drops for display. Please review you fft length,
> update rate, and try out different fft GUIs (qt/wx).
> 
> You could zeropad your data generously to notice the repitition. 
> However, I think with what you're describing you might be better
> off with a waterfall graph; try them, they're fun :)
> 
> Greetings, Marcus
> 
> On 03.12.2013 21:45, Paul B. Huter wrote:
>>>> When I play my data file back through a throttle and
>>>> frequency translating FIR filter to an FFT sink with repeat
>>>> OFF, it seems to just show a static plot. However, with
>>>> repeat ON, I get playback, but I can't tell when data ends
>>>> and starts back over. Is there a way for me to know when it
>>>> repeats? Or, is there a way to get playback with repeat
>>>> turned off?
>>>> 
>>>> Thanks.
>>>> 
>>>> Paul B. Huter
>>>> 
>>>> 
>>>> 
>>>> _______________________________________________
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>>>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>>> 
>> 
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