FFT Playback

GNU Radio
1

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

2

Marcus:

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

Looking at the configuration for my FFT, and changing ‘Refresh Rate’
from
the default ‘15’ to ‘1M’ gives me some extra playback. My datafile was
recorded for about 20 seconds at a sample rate of 50Msps through a
low-pass
filter to only capture 0-30MHz. 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!

3

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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:

wrote:
Greetings, Marcus

Thanks.
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4

There must be something with my record, because my 20 seconds of running
only produces about a 180MB file. I am not seeing any indications of
dropped data, though. I am recording to a RAMDisk, but I get the same
results recording to my HDD.

Does anyone have recommendations for how to ensure I get my data
recorded?
I really think that if I cam record the data, playback will be more what
I
am looking for.

Paul B. Huter

5

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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 :slight_smile:

Greetings,
Marcus

On 03.12.2013 21:45, Paul B. Huter wrote:

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6

Without the low pass filter, I was getting a sequence of D’s. The low
pass
filter uses a Volk machine, and downsamples the 50M to 30M with a 5M
transition size. I am using the GUI companion on Linux, but I vaguely
recall that I may have had better luck recording when I just created a
Python script on a PC. I will try the script (on Linux) this afternoon,
and
I will also try running at 25M downsampled to 10M.

In the meantime, any additional comments/suggestions would be
appreciated.

Thanks.

Paul B. Huter

7

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With most hardware interfaces, you get notifications when your
driver/interface starts dropping samples – with UHD you’d see loads
of ‘O’ indicating overflows.
If you don’t see that, there is something really wrong. How does your
data acquisition take place? 50Msps is really a lot for an average PC
if you do filtering; I don’t know how your low pass filter looks, but
I think it’s probably causing to much computational load for your
computer to process the incoming samples in real time.

Greetings,
Marcus

On 04.12.2013 12:39, Paul B. Huter wrote:

[email protected] wrote:

I like the way the waterfall looks, but I’m having the same
at which the GUI updates its visualization.
No. “Autoscale” is (as far as I know and can remember right now)

Thanks!
You could zeropad your data generously to notice the

However, with repeat ON, I get playback, but I can’t

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8

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How do you downsample 50 to 30 Msps? With a fractional resampler?
That’s highly demanding of your CPU! Do it with the recorded data, not
before, if you can afford the memory.

On 04.12.2013 13:47, Paul B. Huter wrote:

don’t know how your low pass filter looks, but I think it’s

the ease of reply:
rate of one million frames per second… doesn’t even
with 1 billion samples going in, there shouldn’t really be
samples are internally processed prior to being transformed
available at run time, you can’t do that before running the

On Tue, Dec 3, 2013 at 3:01 PM, Marcus M.
describing you might be better off with a waterfall

starts back over. Is there a way for me to know
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9

On Wed, Dec 4, 2013 at 6:39 AM, Paul B. Huter [email protected]
wrote:

There must be something with my record, because my 20 seconds of running
only produces about a 180MB file. I am not seeing any indications of dropped
data, though. I am recording to a RAMDisk, but I get the same results
recording to my HDD.

Does anyone have recommendations for how to ensure I get my data recorded? I
really think that if I cam record the data, playback will be more what I am
looking for.

Paul B. Huter

You can also use the static plotting tools that come with GNU Radio,
which can be better for short files than trying to run them “live” in
GRC. Look into using gr_time_plot_c, gr_psd_plot_c, and
gr_spectrogram_plot_c. Use the “-h” flag to bring up the help screen
to set various parameters.

Tom