Re: Need help with demodulating Baofeng NBFM using a RTL-SDR

[Christophe Seguinot]

Hi

If I clearly understand, your data (herck_src.txt) is following this path:

BPSK Modulation,[ NBFM mod, NBFM Demod], BPSK Demod

It seems your BPSK Symbol rate is SR=16 KBauds, so your BPSK spectrum (*) is 16000(1+excesswb)=SR(1+excesswb) close to 20 Khz.

(*) at constellation modulator output

This spectrum is not symetric [-10 Khz, +10 Khz] ,  it cannot be used to be send to an audio port (real signal) .. So you do you send the BPSK?

It Looks like on your flowgraph you only send the real part to the Baofeng ith 48 Khz audio_rate:

• sending only the real part is equivalent to sending half the binary information contained in the BPSK
• As Baofeng (I don't know what it is) dedicated to voice, it audio bandwith is probably limited to 4 kHz
  

What you intend to do is probably impossible. It would required at least:

• to transmist the whole BPSK  (real+imag part), you need to modulate it with a very low carrier frequency close to Fmax/2 (Fmax being the maximum audio range of BAofeng)
• The symbol rate must be lowered not to excess (Fmax/2) :  SR(1+excesswb)< (Fmax/2)
• BUT the NBFM demodulated signal is probably affected by the NBFM mod/demod and direct demodulation may be not so straightforward.
  

Hope this help.

On 09/02/2021 05:35, Elijah Fry wrote:

Hello,

For the last few days, I've been banging my head into a wall trying to figure out how to demodulate a Baofeng radio's NBFM signal.

The reason I would like to do this is because I'm sending BPSK Modulated packets through an audio port into the Baofeng radio, but then the Baofeng adds its own NBFM modulator (Because the Baofeng is a radio meant for voice) on top of my BPSK Modulation. This means I can't decode my BPSK data unless I can figure out how to demodulate the Baofeng's NBFM modulation that is covering my BPSK modulated packets.

I'll post images of the float below. Anyways, I've tried WBFM receivers, FM demodulators, and NBFM receivers. Nothing seems to work. and if I attach a GUI Sink straight to the NBFM receive output, GNUradio gets all laggy. I've also tried using rational resamplers. Nothing seems to work for demodulating the Baofeng's NBFM Modulation. Can anyone help me? I can't find hardly any information on this.

I'm transmitting 2w on the Baofeng (Lowest it'll do), and in SDR#, my RTL-SDR is not being overloaded.

TX flow (currently):

RX flow (currently):

Kind regards, Elijah.