Getting back to my question about adjusting decimation on the fly, and having the FFT sink adjust to show the spectrum correctly, I have an example of what I am doing on my site at:
www.nd.edu/~ematlis/z.gnuradio
There's a jpg and the grc file there. If anybody would care to take a look and comment, that would be great. Basically, I want to dynamically adjust the software decimation going on in the AM Demod block, and have the result shown correctly in the FFT sink. What I am seeing is the x-axis of the FFT changes, but not the position of the spectrum. A variable slider is used to control the decimation value in the AM Demod block, and the Sample Rate in the FFT sink is dependent on the same variable, but something is not correct.
The AM demodulator block does not support changing decimation during runtime. You can check:
gnuradio-core/src/python/gnuradio/blks2impl/am_demod.py
It has no methods for changing any parameters.
However, you can implement your own AM demod (it's very simple) and change the decimation of the low pass filter - that should work.
Alex