Thanks Marcus.
Time to put this to the test:
https://imgur.com/a/hRsYAFu
I need a way to test that the values I detect in java are the same
values that I get in GR. I tried a Message Debug connected to the
output of "Complex to Mag" but that gave me a red arrow so not
allowed.
Any suggestions?
Thanks
A
On Tue, Nov 28, 2023 at 2:07 AM Marcus Müller wrote:
>
> Hi Al,
>
> On 27.11.23 06:44, Al Grant wrote:
> > Thanks for the detailed reply Marcus.
>
> you're most welcome!
>
> > *M=800*
> > Where is M=800 in my rational resampler? I see looking at the link to the
> > picture I posted I had Decimation=100 (is M shorthand for decimation?)
>
> https://imgur.com/a/B2HqCKc shows "Rational Resampler" with "Decimation:
> 800", not 100.
>
> > *PICTURE*
> > To your picture:
> >
> > [image: image.png]
> >
> > But my sample only has a repeating tone on 1 frequency, but in the picture
> > above with frequency on the x axis (and power on the y?), and 3 peaks I
> > have circled in red, at first glance I would have said this shows a signal
> > on the 3 different frequencies?
>
> No, it doesn't have 3 different frequencies! You missed my point there: this
> is *one*
> signal. The frequency axis extends to - infinity and to + infinity; there's
> infinitely
> many (light green) repetitions, not 3, in a discrete-time signal. (I just
> couldn't draw
> infinitely wide pictures!)
> We just conveniently decide that, to look at the signal, it suffices to look
> at the
> baseband one repetition, the dark green one.
>
> It does suffice to describe all the information in the signal! But: it breaks
> down when
> you do things like decimating or interpolating, because suddenly you get the
> effects of
> these repetitions you "just tried very hard to forget about" :D
>
> > I am assuming you didn't mean what I have written above, but I
> > can't reconcile the picture with the concept you are trying to convey.
>
> Hope the above helps!
>
>
> > *FILTER TYPE*
> > I see you have suggested a series of resampling filters, instead of 1 big
> > one, is that for computing efficiency or because it wont work the way I
> > expect the way I have done it?
> >
> > What is the difference between just a straight decimating block and
> > rational resampler?
>
> If you choose to use a filter that cuts out the lower 1/M of the original
> bandwidth, it's
> the same. (assuming the rational resampler is set to "Interpolation: 1")
>
> > *BASEBAND*
> > Since my original post I have a slightly better understanding of a baseband
> > file. Correct me if I get it wrong, but for example a RTL-SDR can capture
> > baseband at 2.4Mhz (i.e. spectral width).
>
> Sounds right!
>
> > To my example I am interested in
> > 160.100Mhz to 160.200 with 100 channels spaced at 160.100, 160.110Mhz etc
> > etc.
>
> So, a bandwidth of 100 · 0.01 MHz, if I get you correctly, or 1 MHz. Add a
> bit "left and
> right", because the analog filters aren't perfect, so maybe 1.2 MHz, just
> because we can
> trivially afford to be so generous.
>
> > So in one baseband file I can capture all 100 channels. Cool.
>
> Exactly!
>
> > For the moment I just want to focus on 1 disaster (channel) at a time, and
> > am interested in getting the file into Java and doing the processing there.
>
> And you seem to be doing the right thing in principle (I didn't look at the
> numbers being
> sensible here): you're selecting a single channel, say "OK, because that
> channel is only
> 1/M of the overall bandwidth, I decimate by M".
>
> The thing I'm not understanding about your flow graph is then why the
> following low-pass
> filter at all?
> (it's also incorrectly parameterized, as far as I can tell, because your
> input sampling
> rate wasn't 32 kHz (as specified in the low-pass filter) times 800 (=25.6
> MHz) (or 100,
> assuming the 800 was a typo, so 100·32 kHz = 3.2 MHz), but probably a
> bandwidth that your
> RTL dongle actually supports.)
>
>
> Best regards,
> Marcus
>
--
"Beat it punk!"
- Clint Eastwood
