I am trying to understand how to use the output of FFT vectors for computation, rather than just displaying spectra (FFT Sink). I have assembled a pseudo-random generators (linear feedback shift registers) which output two identical pseudo-random sequences (PRN) when clocked by the sound card output at different rates. This all works very well. My objective is to demonstrate fine time stamping by cross-correlating these two streams.
In http://jmfriedt.sequanux.org/xcorr_grc.png, recording the PRN sequence (Audio source) as the two PRNs are generated at slightly different clock frequencies (here 20.0 and 20.2 kHz) and multiplying one sequence with the other (ie cross-correlating) generates the periodic cross correlation peaks, all is fine (http://jmfriedt.sequanux.org/xcorr1.png or http://jmfriedt.sequanux.org/xcorr2.png bottom chart). The frequency offset between the two 7-bit long PRN is 200 Hz, so the delay between cross correlation peaks is 127*5 ms=635 ms as observed, all is good. However I'd like to demonstrate the same effect when going through the Fourier domain. I convert the stream to vector (4096 sample long is much more than the about 256-sample long PRN), multiply one vector with the complex conjugate of the other, iFFT the result, making sure I don't swap quadrants (I checked that FFT -> iFFT without swapping creates the same original signal), back to stream and display the result -> same pictures as above, top graphs. 1. is this the correct way of processing Fourier transforms of incoming signals 2. why are the cross-correlation peaks generated in the Fourier domain not located at the exact same time than those generated in the time domain ? I could understand some lag due to the low pass filter or accumulating the data in the vector, but here I get a bit of jitter of cross correlation peaks generated through the Fourier transform wrt those generated in the time domain. Obvisouly this should all end up in a passive radar measurement: I can live with time-domain cross-correlation calculation, but this is the first time I am using the stream -> vector functionality so I'd like to understand the consequences. Thanks, JM -- JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 Besancon, France _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
