Re: [Discuss-gnuradio] usage? Retuning w/o rebuilding

[Eric Blossom]

On Tue, Jan 11, 2005 at 03:21:10PM -0500, cswiger wrote:
> Gang - in the freq_xlating_fir_filter_XXX.cc.t is a comment:
> 
>     // rebuild composite FIR if the center freq has changed
> 
> How is that done? How do you retune w/o tearing down and rebuilding
> a graph?
> 
> Tks
> 
> --Chuck

You can set the frequency on the freq_xlating_fir_filter at any time.
Just call set_center_freq.

It's also possible to change the user specified filter taps on the fly
too.  Use set_taps, though it isn't often required.

Behind the scenes, when you change the center frequecy or the user
provided taps, we construct a set of complex filter taps from the real
taps and center freq you provide.  The complex taps have a portion of
the frequency translation worked into them.  This allows us to do the
freq translation after the decimation, which is really good for saving
CPU cycles.  We run it at the lower sample rate.  You can find a good
explanation of the math in http://tns-www.lcs.mit.edu/~vanu/JSAC98.ps

Eric