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[Discuss-gnuradio] gr.fir_filter_ccf vs. gr.freq_xlating_fir_filter_ccf


From: ChoJin
Subject: [Discuss-gnuradio] gr.fir_filter_ccf vs. gr.freq_xlating_fir_filter_ccf
Date: Mon, 14 Jan 2008 00:04:47 +0100

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?

-- 
Best Regards,
ChoJin




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