Hello Gullik, The freq offset FFT is performed on the product of the received FDM signal and a local copy of the BPSK pilot. This product is not simply a frequency shifted version of the received FDM signal, so sampling the fft bins won't recover the demodulated data.
Re sampling the waterfall FFT it's not an optimal way of demodulating the FDMDV signal - for example it doesn't apply the receive filter. This means the BER for a given SNR would be poorer than the current design. Using ffts works for OFDM, but that's a different modem design. - David On Wed, 2012-06-13 at 13:06 +0200, Gullik Webjörn wrote: > On Wed, 2012-06-13 at 09:42 +1000, David Rowe wrote: > > Hello Gullik, > > > > The FFT is not used to differentiate between carriers. It is used to > > estimate the frequency of the "down converted" pilot tone. Some other > > modems (e.g. OFDM) use FFTs and inverse FFTs for the demod and mod > > processes, but FDMDV modem is more like 15 parallel 50 baud PSK Modems. > > > > > Assuming result / cpu cycles: Since the FFT *is* done (for finding the > pilot), isn't the result already available in that real+imag for each > bin describe amplitude and phase of each component? > > Assuming the waterfall FFT, isn't the total phase + amplitude of each > carrier available (even if just the magnitude is used for the wfall)? > > Also, I realised that trying to match the bins to the carriers is > not such a good idea, since lowest bin is only half as wide as > the rest ( i.e. 0 Hz - spacing/2) > > Best Regards, > > Gullik /SM6FBD > ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
