On Mon, Jan 14, 2008 at 12:04:47AM +0100, ChoJin wrote: > Hello, > > I'm still getting my hands on the gnuradio's code by walking though the > examples and so on, and I'm looking at the differences between > usrp_wfm_rcv.py and usrp_nbfm_rcv.py (from the gnuradio-example) > and I would like to understand why in the latter we use the following > code: > > # Create filter to get actual channel we want > chan_coeffs = gr.firdes.low_pass (1.0, # gain > self.if_rate, # sampling rate > 13e3, # low pass cutoff > freq > 4e3, # width of trans. > band > gr.firdes.WIN_HANN) # filter type > > print "len(rx_chan_coeffs) =", len(chan_coeffs) > > # Decimating Channel filter with frequency translation > # complex in and out, float taps > self.ddc = gr.freq_xlating_fir_filter_ccf(if_decim, # decimation > rate > chan_coeffs, # taps > 0, # frequency > translation amount > self.if_rate) # input > sample rate > > whereas in the former we use this code: > > chan_filt_coeffs = optfir.low_pass (1, # gain > usrp_rate, # sampling rate > 80e3, # passband cutoff > 115e3, # stopband cutoff > 0.1, # passband ripple > 60) # stopband attenuation > #print len(chan_filt_coeffs) > chan_filt = gr.fir_filter_ccf (chanfilt_decim, chan_filt_coeffs) > > What are the differences (apart from the cutoff values of course)? > So basically why using gr.firdes.low_pass instead of optfir.low_pass > and why using gr.freq_xlating_fir_filter_ccf instead of > gr.fir_filter_ccf > what difference does it make in these two examples? > > Could someone enlighten me? >
Hi ChoJin, In taking a look at the code, it looks like usrp_nbfm_rcv is using gr.freq_xlating_fir_filter_ccf because it wanted to do a fine adjustment on the center freqency. See line 367... However, there's really no reason to use it since the residual returned by usrp.tune is something on the order of a 0.01 Hz. It wasn't always this way, but was fixed sometime in the last year or so. Thus, the code should probably be changed to use gr.fir_filter_ccf since it will be faster. Thanks for the question. Eric _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
