[Discuss-gnuradio] Re: FM stereo problem

[Patrick Strasser]

schrieb Bernardo Gonçalves am 2011-01-25 10:15:
> Hello everyone,
> 
> As Euripedes Rocha told few days ago in the email "Who actually *does*
> use GNU Radio?", we're starting to use it here, but we do not currently
> have daughterboards to use with USRP2.
> 
> Due to this, specially in my case, I'm building some simple diagrams
> using GRC, and one of them is a FM stereo transmitter/receiver in the
> same flow graph.
> 
> Basically, the problem is that it works just as mono, not stereo...
> 
> Here is a screenshot of the flow graph:
> http://dl.dropbox.com/u/8898915/fmstereotxrx.png
> 
> The grc's file is here:
> http://dl.dropbox.com/u/8898915/FMstereotxrx.grc

First, I like this very much. It's a nice example of how easy it is to
implement a system like FM mod/demod in GRC. Did you consider donating
this to GNU Radio as a GRC example? I did something similar, just needs
the proper docs.

Second, I looked at your flowgraph. You shift the difference part from
center frequency 38kHz to 0 with the Xlating FIR filter, and then use a
complex-to-float block. This will not result in the positive
frequencies. You'd rather have to filter out the negative frequencies
with a Hilbert transformer, that is a special FIR filter keeping only
positive frequencies, before converting to float again.

A different way would be to do it analogue to analog (;-)): first
bandpass filtering for 38kHz+-15Khz, mixing down with 38kHz to 0 and
filtering out the mirror image > 38kHz.

The first one should be working much better in a digital complex-valued
system.

A sample file for testing the graph would be very helpful, with
distinguishable left-right channels.

Patrick
-- 
Engineers motto: cheap, good, fast: choose any two
Patrick Strasser <patrick dot strasser at student dot tugraz dot at>
Student of Telemati_cs_, Techn. University Graz, Austria