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Re: Working Narrowband FM examples?

From: Gary Schafer
Subject: Re: Working Narrowband FM examples?
Date: Thu, 5 Aug 2021 12:58:31 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

What is the actual hardware connected as the Osmocom source? It might not like the 
specific sample rate you're trying to set (384k). I tried your graph with a RTL-SDR and 
got the exact, same thing with a 384k sample rate: "mwah mwah mwah". Change 
your sample rate to 1.536 MHz (an integer multiple of 48 kHz), set the quad rate of your 
NBFM Receive block to 1/2 of this (768000, due to the decimation by 2 in your FFT Filter 
block), and give it a whirl. I tried it on mine and it worked fine.
Let us know if this fixes it! Gary **************************************

Hi Mike,

That's exactly what I was looking for, thanks.

However:  I swapped out the USRP source for an osmocom source, and it seems to be "mostly 
working" but the audio comes through like the adults from the old Charlie Brown show from when 
I was a kid.  Instead of clearly-intelligible speech like I can hear through GQRX, using gnuradio I 
can only get speech that sounds like "mwah mwah mwah mwah"

Attached is a .png of my flowgraph, hope it's not too large for the list (about 

Appreciate any advice anyone might have.


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