Re: [Discuss-gnuradio] FM Stereo decoder, frequency doubler, IIR issues

[Tom Rondeau]

On Wed, Apr 24, 2013 at 8:56 AM, Chuck Ritola <address@hidden> wrote:
> Hi list,
>
> I just got started with GNU Radio Companion for use with my FCDPP and it's
> been great, already am decoding FM stereo and it sounds very good. However
> it probably isn't the most optimal way to do it:

There are FM receivers/demod blocks already that you can play and
experiment with. In 3.6, they are located in
gnuradio-core/src/python/gnuradio/blks2impl, look for wfm_rcv blocks.

Obviously, though, nothing stopping you from building and
experimenting on your own.

> My filters are the inbuilt bandpass - they're a bit hard on the CPU and was
> hoping to use the IIR filters as biquads instead but I can't seem to get
> them to work. Filter was designed with this applet (
> http://www.earlevel.com/main/2010/12/20/biquad-calculator/ ) and I fed the A
> values into Feed-Forward of the IIR and B values to the Feed-Backward and it
> would not pass signal so I swapped A and B values and it apparently just
> passed the signal unaffected. Any ideas what I'm doing wrong?

I think this has already been addressed in another email thread, but
try negating the A vector. And try using the gr_filter_design tool.

> What is the internal resolution for the IIRs? I am asking because some of my
> filter Q's are in the 10-60 range and am concerned about blowing them up.

The taps are passed in as double-precision floating point.

> Also, what would be the most efficient way (in GNU Radio) to get the
> absolute value of a signal? The best I've come up with so far was a
> Float->Complex feeding a Complex->Magnitude.

Well, you could always write a block to do that yourself. There's also
the blocks::transcendental block that you can use. Should be able to
pass it "fabsf" and "float" as the arguments.

> Thanks!
>
> Chuck

Tom