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

[Eric Blossom]

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