[Discuss-gnuradio] am reception examples

[Matteo Campanella]

In some of the examples, I have noticed certain approaches I am not able to 
explain on my own, so I am asking for help to the list...I am talking for 
example about am_rcv.py - it uses an offset_freq of 30kHz to recalculate the 
correct offset afterwards. I have tried modifying the python in order NOT to 
use the initial offtset, and everything works correctly - does anybody as an 
answer? To make things clearer:

       offset_freq = 30e3
#        IF_freq = offset_freq - station            ;original line
       IF_freq = -station                                ;modified line

and some lines later:

       src.set_rx_freq (0, IF_freq)
       actual_IF_freq =src.rx_freq(0)
       actual_offset = actual_IF_freq + station

Also, as I could notice printing the values returned, it does look like the 
usrp cannot be simply put on a frequency with the rx_freq method - the 
strategy used in this script is indeed to go as close as possible to the 
needed frequency and then use the software dds/fir to do the last 
translation of the spectrum.

In another script, usrp_wfm_rcv.pl, another method has been used, the 
usrp.tune, that, according to the tests, does not need any successive 
adjustment - is there any reason why the am receiver is not written using 
this method?

Another doubt is about optfir.low_pass versus gr.firdes.low_pass - from the 
tests I have run, the first one is much faster of the second - is there any 
reason to use the second instead of the faster optfir?

regards
Matteo