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Re: [Discuss-gnuradio] Splitting a vector at the output of the FFT

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Splitting a vector at the output of the FFT
Date: Wed, 18 Mar 2015 16:52:51 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

Hi Alejandro,

are you sure you want to use the FFT sinks?
They basically "just" internally put the sample stream into vectors of fftlen length, then calculate the dB(abs(fft(these vectors))), and display the result.
What you're describing sounds more like you'd want to use the standalone, not-graphical-sink FFT block.

The FFT is defined as most of the time in DSP: each bin corresponds to a specific exponent in the complex exponential function, ie. e^(j*n_bin*omega); compare [1].


[1] http://www.fftw.org/doc/The-1d-Discrete-Fourier-Transform-_0028DFT_0029.html#The-1d-Discrete-Fourier-Transform-_0028DFT_0029

On 03/18/2015 04:31 PM, Alejandro Pascual Laguna wrote:
Hello GNUR's!

I am a newbie in this community so I may pose a very simple question...
I am trying to split via GUI the output vector of the FFT block in two halves, one with the upper part and one with the lower part -i.e. I don't want a duplicate of the output vector-. Say I have a FFT of 1024 points, how do I split the output into the first 512 points and the final 512 points? This leads me to the related question of how is the FFT implemented: is the DC component coming first and then the positive part of the spectrum with fftSize/2 samples to be finally followed by fftSize/2-1 of the negative part? Or is it some other way? Moreover, is it normalized by the number of points?

Thank you very much.


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