gqrx narrow band fm parameters

[Hoang Nguyen Tran]

Hi all,

For past few weeks, I tried Narrowband FM receiver in GNU radio, you can see the flow graph below. This is Narrow band FM receiver was build by Alexandru Csete OZ9AEC.
I tested it with USRP 1 and then B210. Finally I got the signal and then I tried to decode it. I was successful in decoding signal with 1200 bps, however not any 9600 bps signal that I received, has been decoded even the signal was very strong and clear. I though the problem was from my set up with soundmodem UZ7HO. Then I send my record file to another ham expert and ask him for decoding. Then he told me that :
"There is a audio band filter active on your output that is causing the limit the bandwidth and therefor you are unable to decode.

Your audio bandwidth cutoff at 3khz

My Audio bandwidth no cutoff"

I don't really know what is audio bandwidth cut off, but I guess have some problem in my recording process lead to lack of sample to decode in high baud rate packet.

After that,I try GQRX build from GNU radio 3.7.8 and try to receive and decode STRAND -1 signal. And yes, I was able to decode the signal from STRaND - 1 9600 bps.
Because GQRX was build base on GNU radio and also from Alexandru Csete , So I think they have common points.
So could you please take a look and see what was wrong in my flow graph that lead to unable to decode 9600bps signal ?

Thank for your time,
Hoang

p/s : the grc file of flow graph : https://drive.google.com/file/d/0B2vFfxYydj06bTkwaFpycUpZYXM/view?usp=sharing

[AndreaVi]

The nbfm demodulator block has a deemphasis filter active (tau=75); try to set it to 0 and see what happens.

[Hoang Nguyen Tran]

Thank for replying AndreaVi,

I will try with Tau = 0. Btw, do you know where I can find some other basic parameter of in NBFM receiver of GQRX. I just want to see what's difference.

Vào 06:39:58 UTC-4 Thứ Hai, ngày 19 tháng 10 năm 2015, AndreaVi đã viết:

[Hoang Nguyen Tran]

Hi AdreaVi,

I think Deemphasis filter (tau) is 75e-6 as the default value.

I change to Zero and the error occurred:

ZeroDivisionError: interger division or modulo by zero

Hoang

Vào 06:39:58 UTC-4 Thứ Hai, ngày 19 tháng 10 năm 2015, AndreaVi đã viết:

[Robin Gape]

Hoang,

1) Carson's rule (https://en.wikipedia.org/wiki/Carson_bandwidth_rule) tells one the rough bandwidth occupied by an FM signal. For 4.8 kHz audio (9.6kHz data rate) & 5 kHz deviation, that would give a bandwidth of (at least) 20kHz, not the 15kHz that the GRC flowgraph would seem to give.

2) However, there is ±10kHz of Doppler shift at 70cms, so the bandwidth needs to be widened to allow for the signal to be received without retuning. That would give a bandwidth of at least 20+10+10 = 40kHz for satisfactory reception.

3) the 9.6kb/s modulation is direct FM, with no pre-emphasis applied, as Andrea indicated. You could try setting the de-emphasis (τ) to 10µS or 1µS for the NBFM block. 75µS ≡ 2.2kHz turnover, 10µS ≡ 16kHz turnover. (Alternatively, if one is confident enough, one could program one's own pure FM demodulator—but not recommended for a newcomer!)

4) the article at http://www.amsat.org/amsat/articles/g3ruh/109.html is worth reading. The receiver should have an audio 6-8kHz low-pass filter, and linear phase characteristics, which an FIR filter readily provides.

Happy experimenting, and enjoy the satellites,

Robin, G8DQX

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[Hoang Nguyen Tran]

Thank you for your replying, Robin,

I set an variable for low pass filter, and I always set 24kHz for the band width.

For the doppler problem, I use doppler correction block, there fore the signal alway in the center frequency. so we can remove the +/- 10kHz part.

I tried with gqrx, bandwidth wide (20kHz I think) and in NBFM mode with Tau =75uS, I was able to decode the signal.

Thus, GQRX has build from GNU radio, so do you think there is something else difference  between the NBFM receiver above and the GQRX set up ? 

Another strange part I dont understand that is I use UDP sink then try to replay it with VLC, everything worked very well in GQRX but I got the choppy sound and discontinuous when I use UDP sink in GNU radio. 

What a magic was done by Alexandru Csete !?!?

 

Added UDP sink: https://drive.google.com/file/d/0B2vFfxYydj06UkFQTE5vbmUwWUE/view?usp=sharing
Vào 13:43:21 UTC-4 Thứ Hai, ngày 19 tháng 10 năm 2015, Robin, G8DQX đã viết: