Narrow Band FM Demodulation in PMR446 Band using GNU Radio and RTL2832U

When I first started using the RTL2832U, one of the first things that struck me was the "quietness" of the rf spectrum. Using the SDR# application, I searched the spectrum for "interesting phenomena". Aside from some strange and unknown signals, I couldn't seem to find anything interesting in the lower VHF spectrum. After hours of manually digging into the lower VHF spectrum , I decided to take a peek in the FM Broadcast band (89 MHz - 108 MHz) hoping to see something pop up in the waterfall spectrum. And indeed, as expected, some strong FM radio stations popped up (see image below)

It seemed that the RTL SDR module was working, but my appetite for more "exotic signals" remained. I almost immediately felt the need for transmitting my own signals and demodulate them LIVE using GNU Radio. How could I do that? I figured, the fastest way to start transmitting fast was to buy a very low budget walkie talkie. I ordered a Tectalk Fun orange 29690 PMR, for only 29€ (40$). It operates in the Private Mobile Radio band at 446 MHz. It uses 8 channels, separated by 12,5 KHz and uses Narrow Band Frequency Modulation. (Note: You do not need a license for the PMR band in Europe. The band is however licensed in North America)

The First thing I did was trying to locate and demodulate the PMR signal using SDR#. It didn't take long to find the signal and  demodulate it using NBFM with a 6 kHz bandwidth.

The next logic step would be to demodulate the signal using GRC.

It was a little bit more difficult to capture the signal in GRC, this due to the small bandwidth of the signal of interest. After trying out several frequencies, I managed to capture the signal.

Below you can find the flow graph.

As depicted, you can see I used a sample rate of 2.4 Msps. Because of the narrow bandwidth, you might think it is a ridiculous high sample rate for the low frequency audio. I tried using a lower sample rate of 500 kHz, but strangely enough the audio began to stutter.