Re: [Discuss-gnuradio] am reception examples

[Eric Blossom]

On Fri, Mar 03, 2006 at 12:06:01AM +0100, Matteo Campanella wrote:
> 
> 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?

usrp_wfm_rcv was written later in the game, and uses the prefered
method (works with all daughterboards).  Whether or not you need to
the final software DDS depends on how precisely you need to be on
frequency.  The WFM signal is > 100 kHz wide, thus an offset of a few
kHz makes no difference.  For narrow band FM, or AM, etc, you probably
want to be closer.  Take a look at usrp_nbfm_rcv.py, it uses the
preferred technique.  Note that if you really are trying to handle AM
properly you're probably better off building a synchronous receiver or
some other method that tracks the carrier.

> 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?

optfir uses the Remez / Parks-McClellan design method.
gr.firdes.low_pass designs filters using the window method.  It all
depends on what you want.  See any DSP book for details.

Eric