What is "Decimation"?

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NoelC
Posts: 64
Joined: Sun Nov 27, 2016 11:50 pm

What is "Decimation"?

Postby NoelC » Thu Jan 05, 2017 4:48 pm

Besides being yet another setting to fool with and find that it reduces bandwidth (and apparently increases SNR in some cases), what does the DEC (Decimation) setting actually do?

Please try to explain it in terms of someone who's not familiar with the design of the RSP1 hardware.

Thanks.

-Noel

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W9KJO
Posts: 52
Joined: Thu Oct 06, 2016 3:38 pm

Re: What is "Decimation"?

Postby W9KJO » Thu Jan 12, 2017 12:14 am

Interesting that there is no response to this question after all these months. I just downloaded the latest version, the previous one did not have DEC.

I would like to know what DEC is and does as well?

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NoelC
Posts: 64
Joined: Sun Nov 27, 2016 11:50 pm

Re: What is "Decimation"?

Postby NoelC » Thu Jan 12, 2017 2:35 am

I've found some more online... It seems to be a way of reducing the sampling rate to "1 in N" samples, presumably to lower the load on the digital hardware.

I'm not yet sure what practical advantages it has, though one person on the forum mentioned that it seemed to increase SNR under some conditions.

Personally besides seeing it lower the displayable bandwidth in the MAIN SP window, I see that changing the DEC value affects the frequencies of the "ghost" signals.

-Noel

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W9KJO
Posts: 52
Joined: Thu Oct 06, 2016 3:38 pm

Re: What is "Decimation"?

Postby W9KJO » Thu Jan 12, 2017 8:25 am

my experience has been that version 1.12.2 does not work as well as version 1.04 as far as drop outs or stutter is concerned. I can run 5 MHZ bandwidth on 1.04 with clear reception while on 1.12.2 I get stutter and drop outs at 3 mhz.

I would like to hear from the SDR UNO folks at SDR Play but they don't seem to respond to the forum anymore.

W9KJO

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F1BJB
Posts: 99
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: What is "Decimation"?

Postby F1BJB » Thu Jan 12, 2017 9:42 am

Hi
Decimation is a way to reduce the number of samples delivered to the software.
I believe that the minimum sample rate of the hardware is 2M samples per second witch is huge for a software designed to use
a top grade sound card limited to 192 KHz sampling but with at least 16 bits resolution.
Here we have only 12 bits resolution so to reduce the rate it is simple to add two consecutive samples and the result is only 13 bits
so it can be used directly .
Doing that you gain in dynamic range but only if you reduce the IF bandwidth accordingly and that seems possible with the hardware
of the RSP.
HTH
PS : in fact decimation is also used on top range sound cards if you use them at 48KHz
But they don't tell :)
And they don't adjust anti aliasing filters accordingly .

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NoelC
Posts: 64
Joined: Sun Nov 27, 2016 11:50 pm

Re: What is "Decimation"?

Postby NoelC » Thu Jan 12, 2017 11:45 pm

Forgive me, but it's soaking in more slowly than I'd like. I'm fairly technical but have no background in radio and especially in the concepts of SDR. I have this feeling it will fall into place if I can get just a few more pieces of the puzzle in place...

You're saying Decimation causes multiple successive samples to be added/averaged, I get that, but samples of... What, exactly?

Assume, as I mentioned in the original post, that I don't know how an SDR works internally.

I'm fairly technical but have little background in radio and none in the concepts of SDR. Computers I know intimately.

At this point I know that (part of) the radio spectrum is being sampled in the RSP box, a lot of data is being transferred to the computer, and software is analyzing (selected parts of) the data and turning it into something that can be seen and heard. But please understand that terms like "IF Bandwidth" mean nothing to me - yet.

I realize that a lot of things are being managed by the software, so that operation seems almost seamless to the user. For example, I can be tuned to a station and change the sampling rate and/or Decimation and/or whether you're using "Zero IF" or "Low IF" and still the station remains tuned.

That's great, but it really hides any ability to derive what's going on by trial and error / observation. At this point I can't even imagine trying to get familiar with more than one software package to use the thing, so let's focus on SDRuno...

Perhaps we can talk about one thing at a time (and I very much appreciate any and all patience; I'm a noob but I want to learn)...

What I see is that with various settings of SR and DEC the available bandwidth when fully zoomed out in the MAIN SP is changed. Why?

-Noel

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W9KJO
Posts: 52
Joined: Thu Oct 06, 2016 3:38 pm

Re: What is "Decimation"?

Postby W9KJO » Fri Jan 13, 2017 8:04 pm

When I adjust Decimation to anything but 1 I get what appear to be noise bars all across the spectrum. I am set on 5 MHZ on my bandwidth as I desire to monitor multiple freqs on both 80m and 40m at the same time for use with Fldigi for digital modes.

How would I effectively use Decimation in my situation.

W9KJO

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F1BJB
Posts: 99
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: What is "Decimation"?

Postby F1BJB » Fri Jan 13, 2017 8:29 pm

NoelC wrote:You're saying Decimation causes multiple successive samples to be added/averaged, I get that, but samples of... What, exactly?
l


In fact SDR is a mix of analog and digital technology.
The bridge between those two worlds is the Shannon theorem
https://en.wikipedia.org/wiki/Nyquist%E ... ng_theorem
Basically the RSP uses a local oscillator to convert a radio signal to a frequency that
can be used by its analogue to digital converter.
It also tries to filters out any frequencies incompatible with the sampling rate used.
That is the role of the IF bandwidth.

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F1BJB
Posts: 99
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: What is "Decimation"?

Postby F1BJB » Fri Jan 13, 2017 8:56 pm

W9KJO wrote:How would I effectively use Decimation in my situation.

I believe there is very little gain to use decimation with a 5 MHz bandwidth.
It would require a sampling frequency of 10MHz and this is pretty close to the 480 Mbps limit of USB2
10MHz x 32 bits(I/Q) =320Mbps and USB2 is half duplex and there is some overhead.

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