[Discuss-gnuradio] wfm_pll decimation factors possibly wrong?

[Martin Dvh]

Hi all,
I am working with  the wfm_pll code for the usrp_wfm_rcv_pll example.
What I don't get is how it can work with the current decimation factors.
The usrp_decim=200 and the audio_decim=10
This results in all blocks outputting at 32 kS/s
Which results in -16, +16 Khz bandwith/niquist freqs.

But how can the stereo_carrier_recovery work.
It used the output of
self.stereo_carrier_filter = gr.fir_filter_fcc(audio_decimation, 
stereo_carrier_filter_coeffs)
Which is a signal with a samplerate of 32 kS/s

The 19 kHz pilot_tone and the 38 kHz recovered carrier are outside the niquist 
freq.(-16, +16 Khz)

I would expect that the audio_decimation should be much lower then 10 for this 
to work
See code snippets below,

Greetings,
Martin

in usrp_wfm_rcv_pll.py:
        adc_rate = self.u.adc_rate()                # 64 MS/s
        usrp_decim = 200
        self.u.set_decim_rate(usrp_decim)
        usrp_rate = adc_rate / usrp_decim           # 320 kS/s
        chanfilt_decim = 1
        demod_rate = usrp_rate / chanfilt_decim
        audio_decimation = 10
        audio_rate = demod_rate / audio_decimation  # 32 kHz

in blksimpl/wfm_pll.py:

audio_rate = demod_rate / audio_decimation

self.stereo_carrier_filter = gr.fir_filter_fcc(audio_decimation, 
stereo_carrier_filter_coeffs)
self.rds_signal_filter = gr.fir_filter_fcc(audio_decimation, 
stereo_rds_filter_coeffs)
self.stereo_dsbsc_filter = gr.fir_filter_fcc(audio_decimation, 
stereo_dsbsc_filter_coeffs)


more snippets from blksimpl/wfm_pll.py:
            # Pick off the stereo carrier/2 with this filter. It attenuated 10 
dB so apply 10 dB gain
            # We pick off the negative frequency half because we want to base 
band by it!
            ##  NOTE  THIS WAS HACKED TO OFFSET INSERTION LOSS DUE TO DEEMPHASIS

            stereo_carrier_filter_coeffs = gr.firdes.complex_band_pass(10.0,
                                                                   demod_rate,
                                                                   -19020,
                                                                   -18980,
                                                                   
width_of_transition_band,
                                                                   
gr.firdes.WIN_HAMMING)

            #print "len stereo carrier filter = 
",len(stereo_carrier_filter_coeffs)
            #print "stereo carrier filter ", stereo_carrier_filter_coeffs
            #print "width of transition band = ",width_of_transition_band, " 
audio rate = ", audio_rate

            # Pick off the double side band suppressed carrier Left-Right 
audio. It is attenuated 10 dB so apply 10 dB gain

            stereo_dsbsc_filter_coeffs = gr.firdes.complex_band_pass(20.0,
                                                                     demod_rate,
                                                                     
38000-15000/2,
                                                                     
38000+15000/2,
                                                                     
width_of_transition_band,
                                                                     
gr.firdes.WIN_HAMMING)
            #print "len stereo dsbsc filter = ",len(stereo_dsbsc_filter_coeffs)
            #print "stereo dsbsc filter ", stereo_dsbsc_filter_coeffs
            # construct overlap add filter system from coefficients for stereo 
carrier

            self.stereo_carrier_filter = gr.fir_filter_fcc(audio_decimation, 
stereo_carrier_filter_coeffs)

            self.rds_carrier_generator = gr.multiply_cc();
            self.rds_signal_generator = gr.multiply_cc();
            self_rds_signal_processor = gr.null_sink(gr.sizeof_gr_complex);



            alpha = 5 * 0.25 * math.pi / (audio_rate)
            beta = alpha * alpha / 4.0
            max_freq = -2.0*math.pi*18990/audio_rate;
            min_freq = -2.0*math.pi*19010/audio_rate;

            self.stereo_carrier_pll_recovery = 
gr.pll_carriertracking_cc(alpha,beta,max_freq,min_freq);
            self.stereo_carrier_pll_recovery.squelch_enable(False);


            # set up mixer (multiplier) to get the L-R signal at baseband

            self.stereo_basebander = gr.multiply_cc();

            # pick off the real component of the basebanded L-R signal.  The 
imaginary SHOULD be zero

            self.LmR_real = gr.complex_to_real();
            self.Make_Left = gr.add_ff();
            self.Make_Right = gr.sub_ff();

            self.stereo_dsbsc_filter = gr.fir_filter_fcc(audio_decimation, 
stereo_dsbsc_filter_coeffs)