Hi, The following are some code segments of gsm-scan.py basically used for gsm data reception. I have a few questions about the difference between input_rate and sps,
clock_rate = 64e6 #set the default input rate, we will check with the USRP if it is being used input_rate = clock_rate / options.decim gsm_symb_rate = 1625000.0 / 6.0 sps = input_rate/gsm_symb_rate # configure the processing blocks # configure channel filter filter_cutoff = 145e3 #135,417Hz is GSM bandwidth filter_t_width = 10e3 #Only DSP adjust for offset on datafile, adjust tuner for USRP #TODO: see if we can change this offset at runtime based on freq detection if options.inputfile: offset = self.offset else: offset = 0.0 filter_taps = gr.firdes.low_pass(1.0, input_rate, filter_cutoff, filter_t_width, gr.firdes.WIN_HAMMING) self.filter = gr.freq_xlating_fir_filter_ccf(1, filter_taps, offset, input_rate) # configure demodulator # adjust the phase gain for sampling rate self.demod = gr.quadrature_demod_cf(sps); #configure clock recovery gain_mu = 0.01 gain_omega = .25 * gain_mu * gain_mu # critically damped self.clocker = gr.clock_recovery_mm_ff( sps, gain_omega, 0.5, #mu gain_mu, 0.3) #omega_relative_limit, self.burst = gsm.burst_ff() self.connect(self.filter, self.demod, self.clocker, self.burst) . Can anyone please give me an idea why " gr.freq_xlating_fir_filter_ccf " is used here and 'input_rate' is used as an argument in stead of 'sps'. Whereas, in quadrature demod and Clock_recovery_mm 'sps' is used for sampling rate and why clock recovery is used after demodulation block ? Are I and Q samples are flowing out of clocker block or are they the encoded bits ? Thanks, M.Awais
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